Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Physiological and pharmacological studies of the feline thalamus Marshall, Kenneth Christie 1971

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
831-UBC_1971_A1 M37.pdf [ 8.81MB ]
Metadata
JSON: 831-1.0093283.json
JSON-LD: 831-1.0093283-ld.json
RDF/XML (Pretty): 831-1.0093283-rdf.xml
RDF/JSON: 831-1.0093283-rdf.json
Turtle: 831-1.0093283-turtle.txt
N-Triples: 831-1.0093283-rdf-ntriples.txt
Original Record: 831-1.0093283-source.json
Full Text
831-1.0093283-fulltext.txt
Citation
831-1.0093283.ris

Full Text

PHYSIOLOGICAL AND PHARMACOLOGICAL STUDIES OF THE FELINE THALAMUS by KENNETH CHRISTIE MARSHALL B . S c , U n i v e r s i t y o f Toronto, 1961 M.Sc, U n i v e r s i t y o f Toronto, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In the Department of PHYSIOLOGY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard. THE UNIVERSITY OF BRITISH COLUMBIA February, 1971 In present ing th i s thes i s in pa r t i a l f u l f i lmen t of the requirements fo r an advanced degree at the Un iver s i t y of B r i t i s h Columbia, I agree that the L ib ra ry sha l l make i t f r e e l y ava i l ab le for reference and study. I fu r ther agree that permission for extensive copying of th i s thes i s for s cho la r l y purposes may be granted by the Head of my Department or by h i s representat ives ; It is understood that copying or pub l i ca t i on of th i s thes i s fo r f i nanc i a l gain sha l l not be allowed without my wr i t ten permiss ion. Department of s/,o t o <*• y The Un ivers i ty of B r i t i s h Columbia Vancouver 8, Canada PHYSIOLOGICAL.AND PHARMACOLOGICAL STUDIES OF THE FELINE THALAMUS ABSTRACT The drug s e n s i t i v i t y o f neurones of the Nucleus v e n t r a l i s l a t e r a l i s (VL) of the thalamus, and t h e i r s y n a p t i c a c t i v a t i o n by e l e c t r i c a l s t i m u l a t i o n of brachium conjunctivum (BC), p r e c r u c i a t e c o r t e x and entopenduncular nucleus, (EN) has been s t u d i e d i n a n e s t h e t i z e d and i n decerebrate c a t s . C e l l s evoked w i t h s h o r t l a t e n c y by BC s t i m u l a t i o n were p a r t i c u l a r l y s e n s i t i v e t o e x c i t a t i o n by i o n t o p h o r e t i c a l l y a p p l i e d a c e t y l c h o l i n e (ACh) and L-glutamate (LG) when compared with c e l l s of more d o r s a l t h a l a m i c n u c l e i . The VL neurones d i d not e x h i b i t such an enhanced s e n s i t i v i t y t o DL-homocysteic a c i d and N-methyl a s p a r t i c a c i d . The a-methyl d e r i v a t i v e of g l utamic a c i d (a-MG) was found i n many cases t o depress or b l o c k the e x c i t a t i o n of tha l a m i c neurones by LG, but had no e f f e c t on ACh e x c i t a t i o n . a MG sometimes depressed the e f f e c t s of other e x c i t a t o r y amino a c i d s , but to a l e s s e r degree than those produced by LG. Short l a t e n c y s i n g l e a c t i o n p o t e n t i a l and l a t e b u r s t responses evoked i n VL by BC or c o r t i c a l s t i m u l a t i o n have been r e p o r t e d by other workers and were confirmed i n t h i s study. However, i t was found t h a t both c o r t i c a l and BC s t i m u l u s evoked e a r l y b u r s t responses which were observed only i n a n e s t h e t i z e d animals. EN s t i m u l a t i o n evoked b u r s t response i n VL neurones wi t h l a t e n c i e s of 4-22 msec. Both the e a r l y b u r s t and the EN evoked responses could be converted to s h o r t e r l a t e n c y s i n g l e s p i k e s by i o n t o p h o r e t i c a l l y a p p l i e d amino a c i d s and ACh. ACh f a c i l i t a t e d s y n a p t i c a c t i v a t i o n by c o r t i c a l and BC s t i m u l i but c o u l d e i t h e r e x c i t e or depress the responses t o EN s t i m u l a t i o n . I o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e and d i h y d r o - 3 - e r y t h r o i d i n e blocked ACh e x c i t a t i o n of VL c e l l s but d i d not a l t e r t h e i r s y n a p t i c a c t i v a t i o n , although a t r o p i n e c o u l d r e v e r s e the ACh d e p r e s s i o n of EN evoked responses. Intravenous a t r o p i n e i n doses of 0.5-1.0 mgm/kgm a l s o blocked these ACh e f f e c t s , but i n a d d i t i o n markedly reduced the BC evoked f i e l d response i n VL without a f f e c t i n g the response to c o r t i c a l s t i m u l a t i o n . I t was concluded t h a t the pathways from EN and motor cortex to VL are u n l i k e l y t o be c h o l i n e r g i -c a l l y mediated, but t h a t ACh may be the s y n a p t i c t r a n s m i t t e r f o r a t l e a s t p a r t of the c e r e b e l l o - t h a l a m i c pathway. P e n t o b a r b i t a l and a - c h l o r a l o s e were potent b l o c k e r s of ACh e x c i t a t i o n i n VL neurones.. I t was shown t h a t neurones of EN g i v e r i s e t o c o l l a t e r a l axon branches which p r o j e c t to VL and N. centrum-medianum. S t i m u l a t i o n of sensori-motor c o r t e x evoked c e l l and f i e l d responses i n the l a t e r a l , but not the medial p a r t s of the centrum medianum-Parafascicular comple TABLE OF CONTENTS Page I n t r o d u c t i o n Anatomy - Nucleus V e n t r a l i s L a t e r a l i s 1 - Nucleus Centrum Medianum 14 E l e c t r o p h y s i o l o g y - Nucleus V e n t r a l i s L a t e r a l i s 17 Roles of VL and CM 37 Ion t o p h o r e s i s and the pharmacology of thalamic neurones * ...... 40 Parkinson's Disease ; 48 C h o l i n e s t e r a s e 52 Methods P r e p a r a t i o n o f animals 55 S t e r e o t a x i c procedure w ?. i 59 Electrodes...... 61 Equipment i ^ 66 H i s t o l o g y i 7 0 Re s u l t s Nucleus V e n t r a l i s L a t e r a l i s - E l e c t r o p h y s i o l o g y 7 5 - . E f f e c t o f drugs 8 5 Nucleus Centrum Medianum 118 Entopeduncular Nucleus 122 D i s c u s s i o n . i 126 Summary and C o n c l u s i o n s . * 144 B i b l i o g r a p h y * ...... ...... . ; 147 LIST OF TABLES Page I. Drug S o l u t i o n s Used f o r I o n t o p h o r e s i s 65 I I . E f f e c t s of C h o l i n e r g i c B l o c k i n g Agents on EN-Evoked C e l l s 109 i i . LIST OF FIGURES Fi g u r e Page 1. Spontaneous b u r s t i n g of thalamic neurones 20 2. Model f o r t h a l a m i c b u r s t i n g a c t i v i t y . . . 20 3. D e c e r e b r a t i n g e l e c t r o d e s 61a 4. C o n c e n t r i c b i p o l a r s t i m u l a t i n g e l e c t r o d e 61a 5. F i v e - b a r r e l l e d i o n t o p h o r e s i s e l e c t r o d e 68 6. Block diagram f o r i o n t o p h o r e s i s and r e c o r d - -i n g equipment 68 7. Spot marking t i p of s t e e l r e c o r d i n g e l e c t r o d e . . 71 8. Spot marking t i p of g l a s s e l e c t r o d e 71 9. Photograph of f r o z e n b r a i n s e c t i o n 7 3 10. Spontaneous b u r s t i n g o f VL neurones 78 11. BC-evoked responses i n VL 78 12. BC-induced b u r s t i n g i n VL 80 13. C o r t i c a l l y - e v o k e d responses i n VL 80 14. C o r t i c a l - i n d u c e d b u r s t i n g i n VL 8 2 15. A n t i d r o m i c a l l y a c t i v a t e d VL neurone 8 2 16. EN-evoked p r e s y n a p t i c f i e l d response i n VL 8 4 17. EN-evoked s p i k e responses i n VL 8 4 18. Mean l a t e n c i e s of EN-evoked VL neurones. 86 19. ACh e x c i t a t i o n of VL neurones 86 20. ACh d e p r e s s i o n of b u r s t i n g thalamic neurones... 88 21. BC sti m u l u s d e p r e s s i o n of ACh-induced f i r i n g . . . 89 22. EN s t i m u l u s d e p r e s s i o n of ACh-induced f i r i n g . . . 89 23. ACh f a c i l i t a t i o n of BC-evoked f i r i n g 91 i i i . F i g u r e Page 24. ACh c o n v e r s i o n of c o r t i c a l l y - e v o k e d b u r s t to s i n g l e s p i k e response 91 25. ACh c o n v e r s i o n o f EN-evoked b u r s t to s i n g l e s p i k e response 93 26. E f f e c t of ACh and DLH on EN-evoked s p i k e response 93 27. S e n s i t i v i t y of t h a l a m i c neurones to ACh and amino a c i d s 97 28. E x c i t a t i o n of t h a l a m i c neurones by ACh and amino a c i d s , 9 9 29. E x c i t a t i o n of t h a l a m i c neurones by DG. and LG... 99 30. E f f e c t of i o n t o p h o r e t i c a t r o p i n e and DH£E on ACh-induced f i r i n g ; 100 31. E f f e c t o f i o n t o p h o r e t i c a t r o p i n e on BC- and c o r t i c a l l y - e v o k e d neurone 10 3 32. E f f e c t of i o n t o p h o r e t i c DH$E on BC-evoked s p i k e 105 33. E f f e c t of I.V. a t r o p i n e on BC-evoked response!. 105 34. E f f e c t of I.V. a t r o p i n e on BC- and c o r t i c a l l y -evoked response. 105a 35. E f f e c t of I.V. a t r o p i n e on EN-evoked neurone d i s c h a r g e •. 107 36. E f f e c t of i o n t o p h o r e t i c a t r o p i n e and DHftE on EN-evoked neurone d i s c h a r g e 108 37. E f f e c t of a-MG and NMG on LG-induced f i r i n g . . . . I l l 38. E f f e c t o f a-MG on drug-induced f i r i n g I l l 39. E f f e c t o f s t r y c h n i n e on BC-evoked d e p r e s s i o n of drug-induced f i r i n g 113 40. E f f e c t of a - c h l o r a l o s e on drug-induced f i r i n g . . 113' 41. E f f e c t o f p e n t o b a r b i t a l on drug-induced f i r i n g . . 114 i v . F i g u r e P a g e 42. E f f e c t o f t h i o p e n t a l o n d r u g - i n d u c e d f i r i n g . . . . 116 43. E f f e c t o f t h i o p e n t a l o n l a t e n c y o f d r u g -i n d u c e d f i r i n g . . 117 44. E f f e c t o f p e n t o b a r b i t a l o n B C - e v o k e d c e l l s 117 45. C o r t i c a l l y - e v o k e d f i e l d r e s p o n s e s i n CM... 120 46. C o r t i c a l l y - e v o k e d c e l l r e s p o n s e i n CM 123 47. C o l l i s i o n o f VL- and CM-evoked a n t i d r o m i c a c t i v a t i o n o f EN n e u r o n e e 123 LIST OF ABBREVIATIONS ACh a c e t y l c h o l i n e AL ansa l e n t i c u l a r i s BC brachium conjunctivum CL Nucleus c e n t r a l i s l a t e r a l i s CM Nucleus centrum medianum CNS c e n t r a l nervous system DA dopamine DG D-glutamic a c i d DH(3E d i h y d r o - 3 - e r y t h r o i d i n e DLH D,L homocysteic a c i d EN entopeduncular nucleus EPSP e x c i t a t o r y p o s t s y n a p t i c p o t e n t i a l GABA y-amino b u t y r i c a c i d HC1. h y d r o c h l o r i c a c i d 5HT 5-hydroxy-tryptamine IPSP i n h i b i t o r y p o s t s y n a p t i c p o t e n t i a l I.V. i n t r a v e n o u s kg k i l o g r a m L. l i t r e L-dopa L - d i h y d r o x y - p h e n y l a l a n i n e LG L-glutamate or L-glutamic a c i d L - G l u t . L-glutamate ( i n f i g u r e s 27,28, 29, LGN l a t e r a l g e n i c u l a t e nucleus LP Nucleus l a t e r a l i s p o s t e r i o r LSD l y s e r g i c a c i d d i e t h y l a m i d e M metre MA m i l l i a m p e r e MD Nucleus m e d i a l i s d o r s a l i s mg m i l l i g r a m min minute ml m i l l i l i t r e mm m i l l i m e t r e MRF mesencephalic r e t i c u l a r f ormation msec m i l l i s e c o n d mV m i l l i v o l t MS megohm N nucleus NA n o r a d r e n a l i n e _^ nA nanoampere (10 ampere) NMA N-methyl-D,L-aspartic a c i d NMG N-methyl-D,L-glutamic a c i d NR red nucleus N . r e t i c Nucleus r e t i c u l a r i s t h a l a m i P p r o b a b i l i t y PAE post-anodal e x a l t a t i o n Pf Nucleus p a r a f a s c i c u l a r i s PSP p o s t s y n a p t i c p o t e n t i a l RF r e t i c u l a r f ormation sec second v i . SN s u b s t a n t i a n i g r a V v o l t VA Nucleus v e n t r a l i s a n t e r i o r VBC v e n t r o b a s a l complex VL Nucleus v e n t r a l i s l a t e r a l i s VM Nucleus v e n t r a l i s m e d i a l i s VPL Nucleus v e n t r a l i s p o s t e r o - l a t e r a l i s aMG a-methyl-D,L-glutamic a c i d u micron (10~6 metre) yA microampere (10~6 ampere) v i i . ACKNOWLEDGMENTS The w r i t e r wishes to acknowledge the help o f many persons who a s s i s t e d him d u r i n g the experimental work f o r and p r e p a r a t i o n o f t h i s t h e s i s . Mrs. Y. Heap, Miss E. P e t r a l i , Mr. R. Walker, Mr. R. As s i n n a , Mr. K. Henze and Mrs. A.M. F o r s y t h a l l p r o v i d e d v a l u a b l e a s s i s t a n c e . Mr. P. Graystone and Dr. R.D. Huffman were generous i n t h e i r s h a r i n g of s c i -e n t i f i c and t e c h n i c a l knowledge. The sound and c o n s t r u c t i v e a d v i c e p r o v i d e d by the can d i d a t e ' s committee, Dr. F.D. G a r r e t t , Dr. R. K e e l e r , Dr. C.N. Loeser and Dr. H.B. Sanders i s very much a p p r e c i a t e d . The a s s i s t a n c e and en-couragement of I l o n a M a r s h a l l i n a l l phases of t h i s work was very b e n e f i c i a l . The c o n t r i b u t i o n s o f Dr. Hugh McLennan t o the author's e d u c a t i o n i n r e s e a r c h techniques, i n t e r p r e t a t i o n of data and concepts of n e u r a l f u n c t i o n have p r o v i d e d the. most v a l u a b l e and l a s t i n g b e n e f i t s of t h i s endeavour and are s i n c e r e l y a p p r e c i a t e d . CHAPTER I INTRODUCTION ANATOMY Nucleus V e n t r a l i s L a t e r a l i s  D e f i n i t i o n The n u c l e i of the d o r s a l thalamus have posed, -and continue t o pose many problems of d i f f e r e n t i a t i o n and nomenclature. Much of the d i f f i c u l t y has a r i s e n because e a r l i e r anatomists used o n l y c y t o a r c h i t e c t u r a l d i f f e r e n t i a t i o n t o separate and d e f i n e the v a r i o u s p a r t s of the thalamus. T h i s c r i t e r i o n gave r i s e t o s e p a r a t i o n of r e g i o n s which have no r e c o g n i z e d d i f f e r e n c e s i n f u n c t i o n or i n f i b r e c o n n e c t i o n s , and c o n v e r s e l y l e d to f u s i o n o f r e g i o n s which have s i n c e been found t o d i f f e r i n f i b r e connections and f u n c t i o n . Because of s p e c i e s d i f f e r e n c e s . i n thalamic O r g a n i z a t i o n , s t u d i e s i n d i f f e r e n t animals sometimes gave r i s e t o whole new systems o f : n o m e n c l a t u r e . Nucleus v e n t r a l i s l a t e r a l i s (VL) has been exposed to these problems as much as any nucleus. Walker's [1938] e x t e n s i v e s t u d i e s o f . t h e primate thalamus helped to achieve some measure of agreement on thalamic terminology. His d e s c r i p t i o n s were based p a r t l y 2 on c y t o a r c h i t e c t u r e , but a l s o on f i b r e connections between the thalamus and c o r t e x . Most North American authors have used Walker's d e f i n i t i o n of VL, and i t i s now g e n e r a l l y c o n s i d e r e d as t h a t p a r t of the v e n t r a l t i e r n u c l e i which r e c e i v e s monosynaptic connections from the deep c e r e b e l l a r n u c l e i and which has r e c i p r o c a l monosynaptic connections with the motor p o r t i o n of the c e r e b r a l c o r t e x . The nucleus i s q u i t e l a r g e and is.bounded m e d i a l l y and d o r s o m e d i a l l y by the i n t e r n a l medullary lamina and N. v e n t r a l i s m e d i a l i s (VM), d o r s o l a t e r a l l y by N. v e n t r a l i s a n t e r i o r (VA), v e n t r o l a t e r a l l y by N . . r e t i c u l a r i s thalami (N. R e t i c . ) and N. v e n t r a l i s p o s t e r o - l a t e r a l i s (VPL) and, at i t s more p o s t e r i o r l e v e l s , d o r s a l l y by N. l a t e r a l i s p o s t e r i o r (LP). The c e l l s of VL are v a r i e d i n s i z e and d e n s i t y , and s e v e r a l authors [e.g., Sheps, 1945;.Dekaban, 1953] have noted d i f f e r e n c e s i n these c h a r a c t e r i s t i c s i n d i f f e r e n t p a r t s of the n u cleus. Some anatomical d e s c r i p t i o n s have used d i f f e r e n t d e f i n i t i o n s of VL., For example, Olzewski [1951] i n c l u d e d i n VL a dorso-ca.udal p a r t which does not r e c e i v e a d i r e c t -i n p u t from the.cerebellum, but which does have c y t o a r c h i t e c - . t u r a l s i m i l a r i t i e s to other p a r t s of VL. The work of H a s s l e r [1959] has given r i s e t o a system of nomenclature which d i f f e r s markedly from t h a t of Walker. He d i v i d e d the v e n t r a l p a r t s of VL and VA i n t o N. v e n t r a l i s o r a l i s p o s t e r i o r (Vop) and N. v e n t r a l i s o r a l i s a n t e r i o r (Voa) r e s p e c t i v e l y . 3 More d o r s a l p a r t s of VL and VA were given d i f f e r e n t names, because H a s s l e r has found t h a t only the v e n t r a l p a r t of VL r e c e i v e s f i b r e s from the cerebellum v i a the brachium conjunctivum (BC). Though there appears t o be some chemical s p e c i f i c i t y . to the v e n t r a l p a r t of VL [McCance, P h i l l i s and Westerman, 196 8a], e l e c t r o p h y s i o l o g i c a l , and other anatomical s t u d i e s [Appelberg, 1961; McCance, P h i l l i s and Westerman, 1968b; Cohen, Chambers and Sprague, 1958] do not support H a s s l e r ' s f i n d i n g s . The system o f - H a s s l e r i s used more f r e q u e n t l y i n human surgery than i n s t u d i e s on experimental animals. Walker's d e s c r i p t i o n of VL i n Macaca mulatta i s s i m i l a r to those used i n s t e r o t a x i c c a t b r a i n a t l a s e s r e f e r r e d t o i n t h i s study . [Jasper and Ajmone-Marsan, 1954; S n i d e r and Niemer, 1961; Reinoso-Suarez, 1961] and w i l l be the one f o l l o w e d i n t h i s t h e s i s . Some p a r t s of the border between VL and VA are very d i f f i c u l t to d i s t i n g u i s h , and v a r i o u s a f f e r e n t systems appear t o o v e r l a p p a r t s of both these n u c l e i . However, because of the r e l a t i v e l y poor d e f i n i t i o n of a f f e r e n t s and e f f e r e n t s of VA, i t has not been s t u d i e d i n t h i s t h e s i s . C e r e b e l l o - T h a l a m i c Pathway The major, and even the d e f i n i n g a f f e r e n t pathway-. to.VL a r i s e s from the deep n u c l e i of the cerebellum. T h i s pathway has been s t u d i e d e x t e n s i v e l y and a l a r g e measure of agreement on i t s . c h a r a c t e r i s t i c s has now been achieved. The • 4 r e s u l t s c i t e d i n the f o l l o w i n g d e s c r i p t i o n are d e r i v e d from s t u d i e s on c a t s and monkeys, i n which the c e r e b e l l o - t h a l a m i c pathways appear t o be s i m i l a r l y organized.. The deep c e r e b e l l a r n u c l e i are u s u a l l y separated i n t o t h r e e c e l l groups,.the f a s t i g i a l or medial nucleus, the i n t e r p o s i t u s or i n t e r m e d i a t e nucleus and the dentate or l a t e r a l n u c leus. N. i n t e r p o s i t u s i s sometimes f u r t h e r d i v i d e d i n t o a n t e r i o r , and p o s t e r i o r p a r t s . In the absence of d e f i n i t e i n f o r m a t i o n on d i f f e r e n c e s of connections of the e f f e r e n t f i b r e s of these two p a r t s , N. i n t e r p o s i t u s w i l l be t r e a t e d as a u n i t here. The major p a r t of the e f f e r e n t s from the c e r e b e l l a r n u c l e i form the brachium conjunctivum (BC) or s u p e r i o r c e r e b e l l a r peduncle. F l o o d and Jansen [1966] u s i n g the Gudden method to determine r e t r o g r a d e degeneration i n c e r e b e l l a r n u c l e i , have shown t h a t most and perhaps a l l c e l l s of the^ i n t e r -p o s i t u s and dentate n u c l e i send t h e i r axons v i a the i p s i l a t e r a l BC t o b r a i n stem s t r u c t u r e s . The e f f e r e n t s of the f a s t i g i a l nucleus f o l l o w v a r i o u s pathways. A p o r t i o n of t h i s f i b r e group c r o s s e s the m i d l i n e w i t h i n the c e r e b e l l a r white matter as the hook bundle or f a s c i c u l u s u n c i n a t u s . A l a r g e p a r t of the hook bundle then j o i n s the c o n t r a l a t e r a l i n f e r i o r c e r e b e l l a r peduncle, w h i l e a p a r t , j o i n s the con-t r a l a t e r a l BC, or forms a separate ascending bundle j u s t d o r s a l to the BC [ C a r p e n t e r , . B r i t t i n and P i n e s 1 9 5 8 ] . These two groups of c r o s s e d f i b r e s o r i g i n a t e p r i n c i p a l l y from the c audal p a r t of the f a s t i g i a l n u c l e u s . Most c e l l s i n the r o s t r a l p o r t i o n c o n t r i b u t e f i b r e s to the i p s i l a t e r a l i n f e r i o r 5 c e r e b e l l a r peduncle. I t appears l i k e l y t h a t the f a s t i g i a l nucleus does not c o n t r i b u t e f i b r e s t o the i p s i l a t e r a l BC. One of Flood,and Jansen's experiments allowed study of the c e r e b e l l a r n u c l e i a f t e r s e c t i o n of t h e . d e c u s s a t i n g BC f i b r e s i n the midbrain. The r e s u l t i n g degeneration p a t t e r n s i n d i c a t e d t h a t the f i b r e s of BC o r i g i n a t i n g from the f a s t i g i a l nucleus do not c r o s s the m i d l i n e i n t h i s r e g i o n , whereas most, but probably not a l l of the f i b r e s from the dentate and i n t e r p o s i t u s n u c l e i do. The l a t t e r o b s e r v a t i o n s are a t v a r i a n c e w i t h other s t u d i e s , anatomical and e l e c t r o -p h y s i o l o g i c a l , which i n d i c a t e an ascending b i l a t e r a l p r o j e c t i o n of f i b r e s o r i g i n a t i n g i n the f a s t i g i a l n u c l e u s . The d i s -crepancy may be r e s o l v a b l e by r e f e r e n c e t o the degeneration study of Cohen, Chambers and Sprague [1958]. These authors noted t h a t although the f i b r e s from the f a s t i g i a l nucleus do not e n t e r the d e c u s s a t i o n of BC, they do c r o s s i n a more d i s p e r s e d manner at l e v e l s r o s t r a l t o the d e c u s s a t i o n . F l o o d and Jansen's o b s e r v a t i o n t h a t some c e l l s of the i n t e r p o s i t u s and dentate n u c l e i f a i l e d t o show degeneration f o l l o w i n g s e c t i o n of the d e c u s s a t i n g BC l e d them to the c o n c l u s i o n t h a t a p o r t i o n of the f i b r e s from the n u c l e i ascend uncrossed to l e v e l s r o s t r a l t o the d e c u s s a t i o n , a c o n c l u s i o n c o n t r a r y to the f i n d i n g s of most other workers. The.observation can be e x p l a i n e d i n another way. Cohen, Chambers and Sprague, u s i n g the Nauta technique f o r t e r m i n a l degeneration showed t h a t l e s i o n s i n e i t h e r i n t e r p o s i t u s or dentate n u c l e i r e -6 s u i t i n f i b r e d e g eneration i n the i p s i l a t e r a l v e s t i b u l a r and r e t i c u l a r n u c l e i . Thus c e l l s of c e r e b e l l a r n u c l e i which send t h e i r axons, or even axon c o l l a t e r a l s t o these r e g i o n s may.be expected not to. degenerate f o l l o w i n g s e c t i o n of the BC a t h i g h e r l e v e l s , e.g., a t the d e c u s s a t i o n . The s i t e s of t e r m i n a t i o n of f i b r e s ascending i n the BC have.been m o s t l y . e l u c i d a t e d by t e r m i n a l degeneration s t u d i e s f o l l o w i n g l e s i o n s t o BC or c e r e b e l l a r n u c l e i , a l -though e l e c t r o p h y s i o l o g i c a l methods have been e f f e c t i v e l y used i n answering some questions.- Most workers have found t h a t the f a s t i g i a l . nucleus p r o j e c t s b i l a t e r a l l y upon the thalamus [e.g., Thomas, Kaufman, Sprague and Chambers, 1956], r e a c h i n g VA, VL, i n t r a l a m i n a r n u c l e i and perhaps p a r t of VPL.-The f a s t i g i a l nucleus does not appear to c o n t r i b u t e to the c e r e b e l l o - r u b r a l pathway [Thomas, e t a l . , 1956], F i b r e s from the i n t e r p o s i t u s and dentate n u c l e i which ascend r o s t r a l t o the d e c u s s a t i o n of BC probably c r o s s completely i n the d e c u s s a t i o n . Some of these f i b r e s , a f t e r c r o s s i n g , e i t h e r t erminate i n or send c o l l a t e r a l s t o the red nucleus (NR), w h i l e many continue i n a r o s t r a l d i r e c t i o n t o terminate i n VL, though some end i n other thalamic n u c l e i [Cohen, e t a l . , 1958]. The p r o j e c t i o n s from the dentate and i n t e r p o s i t u s n u c l e i t o the red nucleus appear to have a d e f i n i t e p a t t e r n , i n t h a t i n t e r p o s i t u s p r o j e c t s to the more caudal m a g n o c e l l u l a r p a r t of NR,_. whereas the dentate nucleus sends f i b r e s t o the 7 more r o s t r a l p a r v o c e l l u l a r p a r t . T h i s p a t t e r n h a s b e e n d e m o n s t r a t e d i n t h e a n a t o m i c a l s t u d i e s o f C o u r v i l l e [1966] and A n g a u t and B o w s h e r [1965]. T h a t a t l e a s t some o f t h e e n d i n g s i n NR a r e c o l l a t e r a l s o f f i b r e s p r o j e c t i n g t o VL h a s b e e n shown by T o y a m i , T s u k a h a r a , K o s a k a and M a t s u n a m i , [ 1 9 7 0 ] , who w e r e a b l e t o a c t i v a t e i n t e r p o s i t u s c e l l s a n t i d r o m i c a l l y and NR c e l l s m a n o s y n a p t i c a l l y by s t i m u l a t i n g VL. The a c t i v a t i o n o f NR c e l l s was shown t o be v i a a x o n c o l l a t e r a l s o f t h e i n t e r p o s i t u s — V L f i b r e s . The p r e c i s e d i s t r i b u t i o n o f c e r e b e l l o - t h a l a m i c t e r m i n a l s i n t h e t h a l a m u s i s s t i l l i n d o u b t . Many a n a t o m i c a l and e l e c t r o p h y s i o l o g i c a l s t u d i e s h a v e i n d i c a t e d p r o j e c t i o n s t o t h e i n t r a l a m i n a r n u c l e i ( N . c e n t r u m medianum (CM), p a r a f a s c i c u l a r i s ( P f ) and c e n t r a l i s l a t e r a l i s ( C L ) ) a s , w e l l as t o VL and p a r t s o f VA a n d VPL [ e . g . , Cohen e t a l . , 1958? Thomas, e t a l . , 1956]. However, . M e h l e r h a s v i g o r o u s l y d e n i e d t h a t "BC f i b r e s t e r m i n a t e i n CM o r P f . [ M e h l e r , 1966, 1969]. I t w i l l s u f f i c e h e r e t o c o n c l u d e t h a t a t l e a s t a l a r g e p o r t i o n o f t h e a s c e n d i n g BC p r o j e c t s t o t h e c o n t r a l a t e r a l VL. R u b r o - T h a l a m i c P a t h w a y The a s c e n d i n g f i b r e p r o j e c t i o n s o f t h e r e d n u c l e u s h a v e b e e n d i f f i c u l t t o d e t e r m i n e b e c a u s e o f t h e i n e v i t a b l e d e s t r u c t i o n o r a c t i v a t i o n o f o t h e r f i b r e s y s t e m s , p a r t i c u l a r l y BC, when l e s i o n i n g o r s t i m u l a t i n g r e s p e c t i v e l y t h a t n u c l e u s . R e a s o n a b l e e v i d e n c e f o r t h e e x i s t e n c e o f a r u b r o - t h a l a m i c 8 p a t h w a y i n t h e c a t may be f o u n d i n t h e w o r k o f P o m p e i a n o and B r o d a l [1957] who f o u n d r e t r o g r a d e d e g e n e r a t i o n i n t h e a n t e r i o r p a r t o f NR a f t e r l e s i o n s a t more r o s t r a l l e v e l s , a n d o f B o w s h e r and A n g a u t [1968] who a b l a t e d t h e c e r e b e l l u m a n d , a f t e r a l l o w i n g t i m e f o r d e g e n e r a t i o n o f BC f i b r e s , made l e s i o n s i n NR and s t u d i e d t h e r e s u l t a n t t e r m i n a l d e g e n e r a t i o n . The l a t t e r a u t h o r s f o u n d no a s c e n d i n g d e g e n e r -a t i n g f i b r e s a f t e r l e s i o n s t o t h e c a u d a l p a r t o f NR, b u t w e r e a b l e t o d e m o n s t r a t e p r e t e r m i n a l d e g e n e r a t i o n i n z o n a i n c e r t a , c a u d a l CM, VA and VL a f t e r l e s i o n s t o t h e r o s t r a l p a r t . T hey c o n c l u d e d t h a t t h e a s c e n d i n g p a t h w a y was s i m i l a r i n d i s t r i -b u t i o n t o t h e d e n t a t o - t h a l a m i c p a t h w a y , e x c e p t t h a t t h e p r o p o r t i o n s o f f i b r e s t o VA and VL w e r e r e v e r s e d , i . e . , more o f t h e r u b r o - t h a l a m i c f i b r e s e n d e d i n VA t h a n i n VL. The d e g e n e r a t i o n s e e n was s p a r s e c o m p a r e d t o t h a t r e s u l t i n g f r o m l e s i o n s t o t h e d e n t a t e n u c l e u s , a n d t h e d e g e n e r a t i n g r u b r o -t h a l a m i c f i b r e s w e r e o f f i n e r c a l i b r e t h a n d e n t a t o - t h a l a m i c a x o n s . C o r t i c o - VL Pathway-A m a j o r p a t h w a y o r i g i n a t i n g i n m o t o r c o r t e x a l s o t e r m i n a t e s i n V L . I n p r i m a t e s t h e s e f i b r e s a r i s e f r o m c e l l s i n a r e a 4 o f t h e m o t o r c o r t e x [ P e t r a s , 1 9 6 9 ]. I n c a t s ; t h e p a t h w a y h a s b e e n d e t e r m i n e d b y many a u t h o r s [ e . g . , . A u e r \ 1956 ; Kusama, O t a n i and Kawana, 1966 and P e t r a s , 1969] t o o r i g i n a t e i n t h e p r e c r u c i a t e c o r t e x . R i n v i k [1968] h a s f o u n d t e r m i n a l 9 d e g e n e r a t i o n i n VL a f t e r l e s i o n s t o p e r i c r u c i a t e g y r u s as w e l l as n e i g h b o u r i n g r e g i o n s s u c h a s c o r o n a l and a n t e r i o r e c t o s y l v i a n g y r u s . The p a t h w a y a p p e a r s t o r e a c h a l l p a r t s o f V L , b u t d o e s h a v e a s o m a t o t o p i c a l l y o r g a n i z e d p r o j e c t i o n . [Kusama, e t a l . , 1966] w h i c h w i l l be r e f e r r e d t o a t a l a t e r s t a g e . P a l l i d o - T h a l a m i c Pathway-The e x i s t e n c e o f a p a t h w a y f r o m t h e g l o b u s p a l l i d u s (GP) t o t h e t h a l a m u s was f i r s t d e m o n s t r a t e d i n t h e monkey by R a n s o n and R a n s o n [19 3 9 ] , and R a n s o n , R a n s o n and R a n s o n [1941] u s i n g b o t h r e t r o g r a d e d e g e n e r a t i o n a n d M a r c h i . t e c h n i q u e s . The p a t h w a y h a s s i n c e b e e n s t u d i e d b y t e r m i n a l d e g e n e r a t i o n t e c h n i q u e s [ N a u t a and M e h l e r , 1 9 6 1 , 1966; M e h l e r , 1966; C a r p e n t e r and S t r o m i n g e r , . 1 9 6 7 ] and h a s b e e n f o u n d t o o r i g i n a t e o n l y f r o m t h e i n t e r n a l segment o f GP o f p r i m a t e s and i t s h o m o l o g u e , t h e e n t o p e d u n c u l a r n u c l e u s (EN) o f c a r n i v o r e s [ F o x , H i l l m a n , S i e g e s m u n d and S e t h e r , 1 9 6 6 ] . The f i b r e s o f t h i s p a t h w a y t e r m i n a t e i n VA, VL and CM, w i t h t h e d i s t r i b u t i o n p a t t e r n i n VA and VL o v e r l a p p i n g t h a t o f t h e c e r e b e l l o -t h a l a m i c p a t h w a y [ S z a b o , 1 9 6 5 ] . The p r o j e c t i o n f r o m EN i s much l e s s d e n s e t h a n t h a t a r i s i n g f r o m t h e c e r e b e l l u m . N e r v e f i b r e s f r o m t h r e e o t h e r c e n t r e s h a v e b e e n p r o p o s e d t o end i n V L . Th o u g h e v i d e n c e s u p p o r t i n g t h e e x i s -t e n c e o f e a c h o f t h e s e p a t h w a y s h a s b e e n f o u n d , t h e i r c h a r a c - * t e r i s t i c s a r e n o t w e l l d e f i n e d . 10 Nigro-Thalamic Pathway C o l e , Nauta and Mehler [1964] f i r s t presented evidence of a n i g r o - t h a l a m i c pathway i n c a t s and monkeys by showing t h a t l e s i o n s i n the s u b s t a n t i a n i g r a (SN) r e s u l t e d i n , q u i t e heavy t e r m i n a l degeneration i n and around VL. T h i s f i n d i n g i s now supported by the work of Carpenter and Strominger [1967] on monkeys and F a u l l and Carman ' (1968) on. r a t s . M e t t l e r [1970], as the r e s u l t o f r e t r o g r a d e degener-a t i o n s t u d i e s of SN i n monkeys, does not deny the e x i s t e n c e of a n i g r o - t h a l a m i c pathway, but f e e l s i t may be secondary t o the n i g r o - p a l l i d a l pathway. A b l a t i o n of the l e n t i f o r m nucleus by M e t t l e r r e s u l t e d i n almost t o t a l d egeneration of SN neurons, l e a d i n g him to conclude t h a t the n i g r o - t h a l a m i c p r o j e c t i o n i s e i t h e r s m a l l or composed of c o l l a t e r a l s of the main f i b r e s p r o j e c t i n g to the l e n t i f o r m nucleus.. E l e c t r o - ' p h y s i o l o g i c a l c o n f i r m a t i o n of a n i g r o - t h a l a m i c pathway has r e c e n t l y . b e e n presented by F r i g y e s i and Machek [1970], I n t r a l a m i n a r - VL Pathway-A n e u r a l pathway from CM to VL has been proposed on anatomical grounds., Nauta.and Whitlock [1954] and Kaelber and M i t c h e l l [1967] found degeneration i n VL a f t e r l e s i o n s to i n t r a l a m i n a r n u c l e i . In a d d i t i o n , P u r p u r a / S c a r f f and McMurtry [1965] have found s h o r t l a t e n c y e x c i t a t o r y p o s t -11 s y n a p t i c p o t e n t i a l s i n c e l l s o f VL a f t e r s t i m u l a t i o n o f m e d i a l n o n - s p e c i f i c n u c l e i . The c o n c l u s i o n s d r a w n f r o m t h e s e e x p e r i m e n t s a r e , h o w e v e r , . l i a b l e t o c r i t i c i s m . - The i n t e r n a l m e d u l l a r y l a m i n a w h i c h i s c l o s e l y a s s o c i a t e d , w i t h t h e i n t r a l a m i n a r n u c l e i , a p p e a r s t o c o n s i s t m a i n l y o f a s c e n d i n g f i b r e s f r o m t h e n o n - s p e c i f i c n u c l e i and o t h e r c e n t r e s . I t i s p o s s i b l e , t h e r e f o r e , t h a t r e s u l t s o f s t i m u l i o r l e s i o n s t o t h e i n t r a l a m i n a r n u c l e i c o u l d be c o n f u s e d by e f f e c t s on f i b r e s o f p a s s a g e i n t h e i n t e r n a l m e d u l l a r y l a m i n a . No e v i d e n c e i s y e t a v a i l a b l e f o r t h e . C M - VL p a t h w a y f r o m s t u d i e s o f r e t r o g r a d e d e g e n e r a t i o n o r a n t i d r o m i c a c t i v a t i o n o f CM n e u r o n e s a s a r e s u l t o f , l e s i o n s i n o r s t i m u l i a p p l i e d t o V L . E v e n t h i s k i n d o f e v i d e n c e w o u l d be s u b j e c t t o s i m i l a r o b j e c t i o n s , s i n c e i n t r a l a m i n a r - s t r i a t a l p r o j e c t i o n s c o u r s e t h r o u g h t h e r o s t r a l t h a l a m u s on t h e i r way t o . t h e s t r i a t u m . I t i s p r e d i c t a b l e t h e r e f o r e , t h a t t h e n a t u r e o f t h e CM - VL p a t h w a y may r e m a i n i n d i s p u t e f o r some t i m e . R e t i c u l o - T h a l a m i c P a t h w a y Some a n a t o m i c a l e v i d e n c e f o r a d i r e c t p a t h w a y f r o m t h e b r a i n s t e m r e t i c u l a r f o r m a t i o n (RF) t o t h e t h a l a m u s i s a v a i l a b l e . The e v i d e n c e d o e s n o t i n d i c a t e t h a t t h e r e i s a s p e c i f i c o r p a r t i c u l a r l y h e a v y p r o j e c t i o n t o V L . S h u t e and L e w i s [1963] f o u n d t h a t c h d l i n e s t e r a s e s t a i n i n g i n t h e t h a l a m u s was r e d u c e d a f t e r l e s i o n s ; - t o t h e RF, and t h e r e f o r e p o s t u l a t e d 12 the e x i s t e n c e of c h o l i n e r g i c r e t i c u l o - t h a l a m i c connections. Although e l e c t r o p h y s i o l o g i c a l e f f e c t s o f RF s t i m u l a t i o n have been seen i n thalamic r e l a y n u c l e i , the r e s u l t s are not s t r o n g l y suggestive of d i r e c t connections, as the responses are v a r i e d and of r e l a t i v e l y long l a t e n c y [e.g., McCance, P h i l l i s and Westerman, 196 8a]. I t might be expected t h a t r e t r o g r a d e degeneration s t u d i e s a f t e r l e s i o n s i n VL would c l a r i f y the p i c t u r e , but as the ascending f i b r e may send o f f c o l l a t e r a l s t o v a r i o u s r e g i o n s , n e g a t i v e r e s u l t s would not be of any v a l u e . From the preceding d e s c r i p t i o n , . i t i s apparent t h a t many regions of the b r a i n which are re c o g n i z e d as having some r o l e i n c o n t r o l and c o - o r d i n a t i o n of movement have the means to a l t e r a c t i v i t y of c e l l s i n VL. E f f e r e n t Pathways By f a r the h e a v i e s t p r o j e c t i o n of f i b r e s from VL i s to the c e r e b r a l c o r t e x , p a r t i c u l a r l y t o the motor area. T h i s pathway has been i d e n t i f i e d on the b a s i s of r e t r o g r a d e and orthograde degeneration s t u d i e s [e.g., Walker, 1938; S t r i c k , 197 0] , and has been found by Walker t o be s o m a t o t o p i c a l l y o r g a n i z e d . Thus the l a t e r a l p o r t i o n s of VL p r o j e c t l a r g e l y to the medial p a r t s or l e g areas of the motor c o r t e x , the medial p a r t s p r o j e c t t o the l a t e r a l motor c o r t e x , or head r e g i o n w i t h the i n t e r m e d i a t e r e g i o n s sending f i b r e s t o the arm and trunk c o r t i c a l areas. 13 A few w o r k e r s h a v e s t u d i e d o t h e r e f f e r e n t s o f VL. J o h n s o n [1961] and Smaha, K a e l b e r and M a h a r r y [1969] h a v e f o u n d s u b - c o r t i c a l d e g e n e r a t i o n a f t e r l e s i o n i n g VL.. T h e s e a u t h o r s a g r e e t h a t d e g e n e r a t i o n i s s e e n i n CL, CM and N. r e t i c . J o h n s o n a l s o saw d e g e n e r a t i o n i n N m e d i a l i s d o r s a l i s (MD) w h i l e Smaha e t a l . f o u n d i t i n N, l a t e r a l i s I n t e r m e d i u s , N. l a t e r a l i s p o s t e r i o r ( L P ) , VA, s u p e r i o r c o l l i c u l u s and c a u d a t e n u c l e u s . I t i s a g a i n d i f f i c u l t t o a s s e s s t h e v a l i d i t y o f t h e s e r e s u l t s . P e t r a s [1969] h a s shown i n t h e monkey t h a t c o r t i c o - t h a l a m i c f i b r e s d i r e c t e d t o w a r d CM and CL c o u r s e i n p a r t t h r o u g h VA-and VL en r o u t e t o t h e i r t a r g e t n u c l e i . I t i s p o s s i b l e t h e r e f o r e t h a t t h e d e g e n e r a t i o n s e e n by t h e a b o v e a u t h o r s i n CM and CL r e s u l t e d f r o m i n t e r r u p t i o n o f t h e s e p a t h w a y s , r a t h e r t h a n f r o m damage t o VL c e l l s . D e g e n e r a t i o n i n t h e c a u d a t e n u c l e u s c o u l d be s i m i l a r l y e x p l a i n e d by damage t o i n t r a l a m i n a r - s t r i a t a l f i b r e s o f p a s s a g e . I t seems r e a s o n a b l e t h a t VL c e l l s p r o j e c t t o N r e t i c -w h i c h h a s b e e n p r o p o s e d t o p r o v i d e a n e g a t i v e f e e d - b a c k s y s t e m f o r VL [ M a s s i o n , 1 9 6 8 ] , and t o LP w h i c h i s t h o u g h t t o f u n c t i o n i n p a r t as a VL a s s o c i a t i o n n u c l e u s . No a l t e r n a t e e x p l a n a t i o n o f t h e d e g e n e r a t i o n i n t h e s u p e r i o r c o l l i c u l u s c a n be s u g g e s t e d . The f a c t r e m a i n s , h o w e v e r , t h a t t h e o n l y e f f e r e n t p a t h w a y w h i c h i s r e a s o n a b l y d o c u m e n t e d a n d w h i c h h a s t h e o p p o r t u n i t y t o p l a y a m a j o r r o l e i n m o t o r c o n t r o l i s t h e V L -c o r t i c a l p a t h w a y . 14 I t would seem t h e r e f o r e t h a t VL i s i n a f a v o u r a b l e p o s i t i o n t o a c t as an i n t e g r a t i o n centre f o r s i g n a l s from s u b c o r t i c a l s t r u c t u r e s i n v o l v e d i n motor c o n t r o l , and then t o pass the i n t e g r a t e d i n f o r m a t i o n on to the motor c o r t e x . I t i s a l s o p o s s i b l e t h a t VL may i n t h i s way p r o v i d e i n p a r t a " f i n a l common pathway" to pyramidal t r a c t c e l l s of the motor c o r t e x . That such c e l l s are s t r o n g l y i n f l u e n c e d by. VL a c t i v a t i o n has been shown by Yoshida, Yajima and Uno [1966] and L e b l a n c and Cordeau [ 1 9 6 9 ]. F u r t h e r i n t e g r a t i o n undoubtedly.occurs at the c o r t i c a l l e v e l , but,other pathways which s e l e c t i v e l y i n f l u e n c e the motor c o r t e x are not recog-n i z e d a t p r e s e n t . Nucleus Centrum Medianum The i n c l u s i o n of the nucleus centr e median of Luys (centrum medianum - CM) i n a d e s c r i p t i o n of s t r u c t u r e s con-cerned w i t h motor f u n c t i o n s r e q u i r e s some j u s t i f i c a t i o n , as t h i s nucleus has not g e n e r a l l y been assigned any major r o l e , i n motor mechanisms. The reasons f o r i t s i n c l u s i o n w i l l become c l e a r i n the f o l l o w i n g d i s c u s s i o n of experimental work on t h i s n ucleus. The nucleus.centrum medianun was f i r s t d e f i n e d by Luys i n 1865 i n the human b r a i n . Because of s i g n i f i c a n t s p e c i e s v a r i a t i o n s i n t h i s n u c l e u s , and because of i t s t r a d i t i o n a l i n c l u s i o n i n the " i n t r a l a m i n a r n u c l e i " which 15 h a v e o f t e n b e e n c o n s i d e r e d as a f u n c t i o n a l u n i t , d i f f e r e n c e s i n n o m e n c l a t u r e and i n p r o p o s e d a n a t o m i c a l c o n n e c t i o n s h a v e a r i s e n . I n an a t t e m p t t o r e c t i f y t h i s s i t u a t i o n M e h l e r [1966] h a s p u b l i s h e d an i m p o r t a n t summary o f t h e c y t o a r c h i t e c -t u r e and a n a t o m i c a l c o n n e c t i o n s o f t h e n u c l e u s . . I n p r i m a t e s , t h e n u c l e u s c e n t r u m medianum h a s b e e n s u b d i v i d e d i n t o two p a r t s b y v a r i o u s a n a t o m i s t s on the b a s i s o f c y t o a r c h i t e c t u r a l s t u d i e s . C r o u c h [1934], O l z e w s k i [1952] and V o g t and V o g t [1941] h a v e d e s c r i b e d a m e d i a l p o r t i o n o f t h e n u c l e u s h a v i n g m o d e r a t e - s i z e d c e l l s , and a l a t e r a l p a r t composed o f s m a l l e r , l e s s d e n s e l y a r r a n g e d c e l l s . I n l o w e r mammals, t h e s m a l l - c e l l e d c e n t r u m medianum i s v a r i o u s l y r e g a r d e d as a b s e n t , o r a t b e s t p o o r l y d e v e l o p e d . The c o m p r e h e n s i v e p h y l o g e n e t i c a n d human o n t o g e n e t i c s t u d i e s o f N i i m i , K a t a y a m a , K a n a s e k i and M o r i m o t o [1960] and t h e i r d e s c r i p t i o n o f Uemera's w o r k on t h e n u t r i a i n d i c a t e t h a t t h e s m a l l - c e l l e d p a r t i s p o o r l y d e v e l o p e d i n t h e m o l e and r a t , b u t a c h i e v e s a h i g h e r d e g r e e o f d i f f e r e n t i a t i o n i n t h e g u i n e a p i g , r a b b i t , s q u i r r e l and n u t r i a . T h e i r c o m p a r i s o n o f t h e s e d a t a w i t h t h o s e g a i n e d f r o m human f e t u s e s a t d i f f e r e n t s t a g e s h a s p r o m p t e d t h e m t o c o n s i d e r t h e s m a l l - c e l l e d p o r t i o n o f t h e n u c l e u s a s N c e n t r u m medianum p r o p e r , and t h e l a r g e r -c e l l e d d o r s o m e d i a l p o r t i o n a s a l a t e r a l s u b - g r o u p o f t h e n u c l e u s p a r a f a s c i c u l a r i s . M e h l e r 1 s 1966 d e s c r i p t i o n o f CM, b a s e d on f i b r e c o n n e c t i o n s u n i q u e t o t h e s m a l l - c e l l e d l a t e r a l p a r t a g r e e s w i t h t h a t o f N i i m i e t a l . I t d o e s seem r e a s o n a b l e , 16 t h e r e f o r e , t o c o n s i d e r t h a t t h e p h y s i o l o g i s t ' s CM - P f c o m p l e x c o m p r i s e s a t l e a s t t h r e e - s u b g r o u p s - N p a r a f a s c i c u l a r i s , t h e s m a l l - c e l l e d l a t e r a l CM, and a t h i r d p a r t b e t w e e n t h e s e two w h i c h may be c o n s i d e r e d a m e d i a l p a r t o f CM, b u t i s p r o b a b l y b e t t e r r e f e r r e d t o as N p a r a f a s c i c u l a r i s l a t e r a l i s . I n t h e e n s u i n g d e s c r i p t i o n t h e r e f o r e , r e f e r e n c e s t o CM. w i l l d e s i g n a t e t h a t a r e a d e f i n e d by - M e h l e r [1966] and N i i m i e t a l . [1960] as t h e CM and r e f e r e n c e s t o P f s h o u l d be t a k e n t o i n c l u d e b o t h t h e m e d i a l a n d l a t e r a l p a r t s o f t h a t n u c l e u s . • N a u t a and M e h l e r [19 66] h a v e r e c e n t l y shown t e r m i n a l d e g e n e r a t i o n i n CM f o l l o w i n g l e s i o n s i n t h e i n t e r n a l g l o b u s p a l l i d u s o f t h e monkey o r t h e N e n t o p e d u n c u l a r i s o f t h e c a t . P f , h o w e v e r , was f r e e f r o m d e g e n e r a t i o n . F u r t h e r , d e g e n e r a t i o n o f c o r t i c o - t h a l a m i c p a t h w a y s h a s i n d i c a t e d t h a t t h e r e a r e p r o j e c t i o n s i n t h e c a t f r o m t h e p r e c r u c i a t e c o r t e x t o CM [Aue r , 1956 ; N i i m i , K i s h i , M i k i and F u j i t a , .1963] . I n t h e monkey and c h i m p a n z e e i n w h i c h t h e s e n s o r y , m o t o r and p r e m o t o r c o r t e x a r e more d i s c r e t e l y l o c a l i z e d t h a n i n c a r n i v o r e s , s e l e c t i v e l e s i o n s d e m o n s t r a t e t h a t m o t o r c o r t e x ( a r e a 4) s e n d s f i b r e s t o CM and t h e p r e m o t o r c o r t e x ( a r e a 6 and 8) p r o j e c t s t o P f and N c e n t r a l i s l a t e r a l i s (CL) [ A s t r u c , 1964; P e t r a s , 1964,1969]. An a d d i t i o n a l d i f f e r e n c e i n t h e c o n n e c t i o n s o f t h e s e n u c l e i i s s e e n when t h e i r e f f e r e n t f i b r e s a r e c o n s i d e r e d . B e c a u s e o f t h e p r o x i m i t y o f t h e s e 17. n u c l e i to the i n t e r n a l medullary lamina, great d i f f i c u l t y i s encountered i n t r y i n g t o i n t e r p r e t the r e s u l t s of orthograde degeneration s t u d i e s a f t e r l e s i o n s , i n t h i s area. Retrograde degeneration s t u d i e s , however,.have shown t h a t the c e l l s of CM e x h i b i t marked degeneration a f t e r l e s i o n s of ^the putamen but not of the caudate N. [Powell and Cowan, 1 9 5 6 ] . C e l l u l a r degeneration i n Pf was more pronounced a f t e r l e s i o n s to the caudate nucleus. The q u e s t i o n of whether these n u c l e i p r o j e c t i n p a r t d i r e c t l y t o the c e r e b r a l c o r t e x i s s t i l l undecided. ELECTROPHYSIOLOGY Nucleus V e n t r a l i s L a t e r a l i s I n t r i n s i c A c t i v i t y The t h a l a m i c r e l a y n u c l e i have been found t o e x h i b i t spontaneous rhythmic e l e c t r i c a l a c t i v i t y . The experimental work on, and p o s s i b l e s i g n i f i c a n c e of t h i s phenomenon has been reviewed by Andersen and Andersson [ 1 9 6 8 ]. Rhythmic a c t i v i t y r e p r e s e n t a t i v e of t h a l a m i c r e g i o n s was f i r s t observed by A d r i a n [1941] and the p r e c i s e nature of t h i s a c t i v i t y has been pursued and c l a r i f i e d , p a r t i c u l a r l y w i t h the a i d of i n t a c e l l u l a r recordings^, by Andersen, E c c l e s and co-workers. Though the work by Andersen and h i s c o l l e a g u e s has been con-c e n t r a t e d i n the v e n t r o b a s a l complex (VBC) of the thalamus 18 (N. v e n t r a l i s p o s t e r o l a t e r a l i s and N. v e n t r a l i s p o s t e r o m e d i a l i s ) t h e i r o b s e r v a t i o n s a p p e a r t o m a t c h c l o s e l y t h o s e s e e n i n VL by v a r i o u s w o r k e r s ' (.also A n d e r s e n e t a _ l . , 1 9 6 9 ) . The r h y t h m i c a c t i v i t y c o n s i s t s o f g r o u p s o f p a t t e r n e d c e l l d i s c h a r g e s ( s p i n d l e s ) w h i c h r e c u r w i t h a minimum f r e q u e n c y o f a b o u t 2 p e r m i n u t e b u t more, commonly a t 4 -6 p e r m i n u t e . The s p i n d l e s g e n e r a l l y h a v e a d u r a t i o n o f a b o u t one s e c o n d , b u t t h i s v a r i e s c o n s i d e r a b l y . W i t h i n e a c h s p i n d l e , u s i n g e x t r a c e l l u l a r r e c o r d i n g , p o s i t i v e waves may be s e e n t o o c c u r a t a r a t e o f a b o u t 6 -12 p e r s e c o n d . W i t h s u i t a b l e r e c o r d i n g t e c h n i q u e s s i n g l e u n i t a c t i o n p o t e n t i a l s a r e s e e n j u s t b e f o r e e a c h o f t h e p o s i t i v e w a v e s . I f t h e a c t i v i t y o f more t h a n one c e l l i s r e c o r d e d by an e l e c t r o d e , i t i s s e e n t h a t a l t h o u g h e a c h c e l l may n o t f i r e w i t h e a c h wave o f s p i n d l e , t h e d i s c h a r g e s w h i c h a r e o b s e r v e d a r e g e n e r a l l y s y n c h r o n i z e d w i t h t h e p o s i t i v e wave p a t t e r n . ( F i g u r e l a ) . I n t r a c e l l u l a r r e c o r d i n g h a s shown t h a t t h e o n s e t o f t h e s p i n d l e i s n o r m a l l y s i g n a l l e d b y a s e r i e s o f h y p e r p o l a r i z i n g p o t e n t i a l s o f i n c r e a s i n g a m p l i t u d e . A f t e r a few o f t h e s e r e c u r r i n g h y p e r -p o l a r i z a t i o n s a ' d e p o l a r i z i n g p o t e n t i a l may o c c u r , r e s u l t i n g i n g e n e r a t i o n o f an a c t i o n p o t e n t i a l , o r more f r e q u e n t l y a b u r s t o f a c t i o n p o t e n t i a l s . S u c h d i s c h a r g e s a r e r a p i d l y , f o l l o w e d b y a f u r t h e r h y p e r p o l a r i z a t i o n and r e p e t i t i o n o f t h e p a t t e r n , u n t i l t h e s i z e o f t h e h y p e r p o l a r i z a t i o n becomes r e d u c e d , t h e d e p o l a r i z i n g p o t e n t i a l s c e a s e , and t h e h y p e r -p o l a r i z a t i o n s a l s o e n d ( F i g u r e l b ) . The h y p e r p o l a r i z i n g p o t e n t i a l s h a v e b e e n r e l a t e d t o i n h i b i t o r y p o s t - s y n a p t i c p o t e n t i a l s by A n d e r s e n a nd S e a r s [1964] on g r o u n d s t h a t t h e y c o u l d be r e d u c e d o r i n v e r t e d by h y p e r p o l a r i z a t i o n o f t h e c e l l membrane, o r i n t r a c e l l u l a r i n j e c t i o n o f c h l o r i d e i o n s . Thus t h e h y p e r p o l a r i z a t i o n s a r e g e n e r a t e d by a c t i v i t y o f i n h i b i t o r y , n e u r o n e s a c t i n g u p o n t h e c e l l s . The n a t u r e o f t h e d e p o l a r i z a t i o n h a s n o t b e e n a s c e r t a i n e d , b u t i t i s t h o u g h t b y A n d e r s e n t o be due t o a p o s t - a n o d a l e x a l t a t i o n ( P A E ) , a nd t o be a c h a r a c t e r i s t i c o f t h e t h a l a m i c c e l l s , t h e g e n e r a t i o n o f w h i c h i s f a c i l i t a t e d b y t h e h y p e r p o l a r i -z a t i o n s . I t i s c o n c e d e d t h a t t h e d e p o l a r i z a t i o n c o u l d be c a u s e d b y s y n a p t i c a c t i v a t i o n f r o m e x c i t a t o r y c e l l s w h i c h h o w e v e r r e m a i n u n i d e n t i f i e d . By r e c o r d i n g w i t h m u l t i p l e i n d e p e n d e n t l y c o n t r o l l e d m i c r o e l e c t r o d e s , e v i d e n c e h a s b e e n p r e s e n t e d [ A n d e r s e n , A n d e r s s o n and L0mo, 19.6 4] t h a t n e i g h b o u r i n g v e n t r o b a s a l t h a l a m i c c e l l s p e r f o r m i n f u l l s y n c h r o n y w i t h e a c h o t h e r and w i t h c e l l s i n a r e l a t e d c o r t i c a l a r e a . The s i z e o f t h e t h a l a m i c f i e l d w i t h i n w h i c h f u l l s y n c h r o n y may be o b s e r v e d h a s b e e n j u d g e d t o be as s m a l l as 150-200 ]i. To e x p l a i n t h e s y n c h r o n o u s b u r s t i n g a c t i v i t y A n d e r s e n and E c c l e s , [ 1 9 6 2 ] h a v e p r o p o s e d a m o d e l i n v o l v i n g a c t i v a t i o n o f i n h i b i t o r y i n t e r n e u r o n e s l o c a t e d w i t h i n t h e same t h a l a m i c n u c l e u s b y a x o n c o l l a t e r a l s o f t h a l a m o - c o r t i c a l r e l a y c e l l s . I n h i b i t o r y i n t e r n e u r o n e s a c t i v a t e d b y a - r e l a y c e l l d i s c h a r g e w o u l d p r o v i d e a i n h i b i t o r y f e e d - b a c k t o n e a r b y r e l a y c e l l s 20 F i g u r e 1 (a). Spontaneous b u r s t i n g " s p i n d l e " o f VPL neurones-e x t r a c e l l u l a r r e c o r d i n g . . . «, V IOmV\ i oo msec F i g u r e 1(b) 1. Spontaneous b u r s t - o f ' s i n g l e VPL neurone, f o l l o w e d by h y p e r p o l a r i z a t i o n — i n t r a c e l l u l a r r e c o r d i n g . CORTEX THALAMUS I *I I C E"I I MEDIAL LEMNISCUS F i g u r e j 2, Axon ' e o l l a t e r a ' r - l n h x b l t o r y interneurone model. of< s u b s t r a t e f o r r h y t h m i c bur sting., .in--.^t-h^'Lamic; r e l a y n u c l e i . • •-»_•>..•» r-v. (Figure 2 ) . By t h i s means, a pattern o r i g i n a t i n g i n one or a few neurones could be spread to a neighbouring group of c e l l s . The involvement of axon c o l l a t e r a l s rather than independent afferent f i b r e s i s indicated from experiments which showed that stimulation of cerebral cortex after degeneration of cortico-thalamic f i b r e s evoked.a powerful post-stimulus series of discharges, s i m i l a r to the spindles occurring spontaneously, but commencing with large amplitude hyperpolarizations, rather than small ones [Andersen and Andersson, 1 9 6 8 ]. In agreement with t h i s theory i s the 1.2 msec latency of the hyperpolarization following antidromic a c t i v a t i o n of the c e l l s — a b o u t the appropriate time for synaptic a c t i v a t i o n f i r s t of an i n h i b i t o r y interneurone, then the thalamic relay c e l l . A c tivation of relay c e l l s by afferents, e,.g., from stimulation of the medial lemniscus i s also followed by a series of bursts, as would be required by the axon c o l l a t e r a l theory. That an anatomical substrate for such a mechanism does e x i s t has been demonstrated, p a r t i c u l a r l y by the work of Tombol [ 1 9 6 7 ] . She has shown, using the Golgi technique, that axon c o l l a t e r a l s are seen i n thalamic c e l l s , branching off the axon at short distances from the c e l l body, and probably terminating within the same thalamic nucleus. Tombol, and other authors (e.g., McLardy, 1963] have also described interneurones i n the relay n u c l e i . Some workers have disagreed with the model of Andersen and Eccles. Massion [196 8] has argued that neurones of N. r e t i c u l a r i s r a t h e r than VL interneurones may p r o v i d e the i n h i b i t o r y feed-back to the r e l a y c e l l s . C e l l s of N r e t i c u l a r i s appear to send branching axons i n t o the i n t e r i o r of the thalamus, [ S c h e i b e l and S c h e i b e l , 19 66] and have been shown by Massion to have s e v e r a l e l e c t r o p h y s i o l o g i c a l c h a r a c t e r i s t i c s which are a p p r o p r i a t e t o c e l l s having the r o l e of i n h i b i t o r y feed-back to the thalamus. D e s i r a j u and Purpura [1969] have shown t h a t monosynaptic e x c i t a t o r y p o s t -s y n a p t i c p o t e n t i a l s (EPSPs) evoked i n VL c e l l s by EN s t i m u l a t i o n are not, whereas monosynaptic EPSPs evoked i n the same c e l l s by BC s t i m u l a t i o n are succeeded by IPSPs. These o b s e r v a t i o n s are claimed to be i n c o n s i s t e n t with a r e c u r r e n t i n h i b i t i o n h y p o t h e s i s . The p r o p o s a l t h a t t h a l a m i c neurone b u r s t s r e s u l t from PAE i s d i s p u t e d by Maekawa and Purpura [1967] because l a r g e d e p o l a r i z a t i o n s were not observed i n t halamic c e l l s a f t e r h y p e r p o l a r i z a t i o n of the membrane through an i n t r a c e l l u l a r e l e c t r o d e . E l e c t r i c a l d i s c h a r g e p a t t e r n s s u i t a b l e t o the p o s t u l a t e d i n h i b i t o r y i n t e r n e u r o n e s have been r e p o r t e d by one group of workers [Marco and Brown, 19 66]. However, other groups s t u d y i n g t h a l a m i c e l e c t r i c a l a c t i v i t y have not been able t o f i n d such responses [e.g., Massion, 1969]. The r o l e played by the rhythmic a c t i v i t y i s unknown, but one reasonable p r o p o s a l has been o u t l i n e d . Massion [1968, 1969] has suggested t h a t the rhythmic a c t i v i t y may a c t as a g a t i n g mechanism f o r t r a n s m i s s i o n of s i g n a l s through the thalamus to the c o r t e x . In the case of VL, i n f l u e n c e s from the b a s a l g a n g l i a and cerebellum would be 23 f i l t e r e d at the th a l a m i c l e v e l by the rhythmic d i s c h a r g e s so t h a t d u r i n g b u r s t i n g a c t i v i t y the flow of s i g n a l s t o the c o r t e x i s hindered. The g a t i n g system can be v i s u a l i z e d as a d i f f u s e one, a f f e c t i n g the whole nucleus and perhaps r e l a t e d to s l e e p and w a k e f u l l n e s s , or as a l o c a l i z e d one which may a f f e c t s m a l l p a r t s of the nucleus, thus p l a y i n g a r o l e i n s e l e c t i v e movement. C e r e b e l l o - T h a l a m i c The pathway from the deep c e r e b e l l a r n u c l e i has the c a p a c i t y t o i n f l u e n c e s t r o n g l y the a c t i v i t y of VL c e l l s . S e v e r a l workers have used t h i s pathway as p a r t of t h e i r s t u d i e s of p h y s i o l o g i c a l and phar m a c o l o g i c a l f e a t u r e s of the thalamus and the motor system, but only a few have s t u d i e d c h a r a c t e r i s t i c s of the pathway i t s e l f i n any depth. Of the evoked p o t e n t i a l s t u d i e s , probably those of Appelberg [1961] and Angaut and co-workers [Angaut, Guilbaud & Reymond, ' 1969; Conde and Angaut, 1970] are most p e r t i n e n t . By s t i m u l a t i n g the i n d i v i d u a l c e r e b e l l a r n u c l e i of cats these authors have been able t o study t h e i r i n d i v i d u a l p r o j e c t i o n s t o thalamic c e n t e r s . Appelberg found p o t e n t i a l s evoked by s t i m u l a t i o n of N. i n t e r p o s i t u s . These p o t e n t i a l s , judged t o r e p r e s e n t p o s t s y n a p t i c events were found on l y i n the r e g i o n of the c o n t r a l a t e r a l red nucleus, and VL. He found no p o t e n t i a l s i n these areas f o l l o w i n g s t i m u l a t i o n of the dentate or f a s -t i g i a l n u c l e i , t h o u g h he c o n c e d e d t h a t t h e e x i s t e n c e o f s u c h a s c e n d i n g p a t h w a y s w o u l d n o t be r u l e d o u t u s i n g h i s m e t h o d s . E l e c t r i c a l e v e n t s r e p r e s e n t i n g p r e and p o s t s y n a p t i c a c t i v i t y w e r e r e c o r d e d i n VL w i t h 1.0 and 1.5 - 1.8 msec, p o s t s t i m u l u s l a t e n c i e s r e s p e c t i v e l y . The c o n d u c t i o n v e l o c i t y a l o n g c e r e b e l l o - t h a l a m i c f i b r e s was c a l c u l a t e d t o be a b o u t 18 M / s e c . A p p e l b e r g u s e d s m a l l - t i p p e d g l a s s o r t u n g s t e n e l e c t r o d e s i n h i s s t u d i e s , s o t h a t t h e f i e l d s o f e l e c t r i c a l a c t i v i t y r e p r e s e n t e d by h i s r e c o r d i n g s w e r e p r o b a b l y c o m p a r a t i v e l y s m a l l . A n g a u t , G u i l b a u d and Reymond [1969] u s i n g c h l o r a l o s e a n e s t h e s i a and r e c o r d i n g macro e l e c t r o d e s , s t u d i e d t h e p a t h w a y s o r i g i n a t i n g f r o m t h e i n t e r p o s i t u s and f a s t i g i a l n u c l e i . T h e i r f i n d i n g s f o r N i n t e r p o s i t u s w e re s i m i l a r t o t h o s e o f A p p e l b e r g i . e . , t h e p r o j e c t i o n i s l a r g e l y c r o s s e d and a p o s t - s y n a p t i c wave i s s e e n i n and a r o u n d VL. A n g a u t e t a l . a l s o r e c o r d e d p o s t - s y n a p t i c t y p e r e s p o n s e s i n p a r t s o f VA, VPL and N. v e n t r a l i s m e d i a l i s (VM). No n o t a b l e d i f f e r e n c e s w e r e s e e n b e t w e e n t h e r e s p o n s e s o b s e r v e d a f t e r s t i m u l a t i o n o f t h e p o s t e r i o r o r a n t e r i o r p a r t s o f N i n t e r p o s i t u s . T h e s e w o r k e r s f o u n d b i l a t e r a l l y d i s t r i b u t e d t h a l a m i c r e s p o n s e s t o s t i m u l a t i o n o f t h e f a s t i g i a l n u c l e u s , w i t h s y n a p t i c r e s p o n s e s l a r g e l y r e s t r i c t e d t o VL and p a r t s o f VM. I n a s i m i l a r s t u d y o f d e n t a t e n u c l e u s p r o j e c t i o n s , Conde and A n g a u t [1970] f o u n d r e s p o n s e s r e p r e s e n t i n g a w h o l l y c r o s s e d a s c e n d i n g p r o j e c t i o n w i t h p o s t s y n a p t i c r e s p o n s e s a p p a r e n t i n t h e a n t e r i o r p a r t o f t h e r e d n u c l e u s , V L , CM, p a r t s of VA,,and i n F o r e l s ' f i e l d H^. From t h e i r diagrams, p o s t s y n a p t i c responses of the CM - Pf complex were found o n l y i n the medial p a r t , i . e . , Pf as e a r l i e r d e f i n e d . Evidence f o r a r u b r o t h a l a m i c pathway was found i n . t h e form of a p o s t -s y n a p t i c wave f o l l o w i n g the BC p r e - s y n a p t i c f i b r e response and which occ u r r e d o n l y r o s t r a l to the red nucleus. By s t i m u l a t i n g thalamus and r e c o r d i n g a n t i d r o m i c responses i n the dentate nucleus, only a s h o r t l a t e n c y response c o r r e s -ponding to the d i r e c t c e r e b e l l o - t h a l a m i c pathway co u l d be seen, i n d i c a t i n g t h a t the slower wave seen r o s t r a l to NR was not due to a c t i v a t i o n of a slower-conducting group of f i b r e s from the cerebellum. Perhaps the only unexpected data given by these authors i s the s h o r t l a t e n c y o f . t h e c e r e b e l l o - t h a l a m i c responses, i . e . , r e c o r d i n g i n the thalamus, 0.5 - 0.7 msec, l a t e n c y f o r the p r e s y n a p t i c arid 0.8 - 1.3 msec, f o r the p o s t s y n a p t i c waves. Other authors have g e n e r a l l y r e p o r t e d l a t e n c i e s longer than these. However, d i f f e r e n c e s i n the means of measuring l a t e n c y might account f o r some of the d i s c r e p a n c y . Appelberg's f a i l u r e to f i n d VL evoked responses a f t e r f a s t i g i a l and dentate s t i m u l a t i o n i s p u z z l i n g i n view of t h i s work, and t h a t of others who have seen such responses. S i n g l e c e l l s t u d i e s have been conducted on the c e r b e l l o - t h a l a m i c pathways by Sakata, I s h i j i m a and Toyoda [1966] and Uno, Yoshida and H i r o t a [1970] , both groups u s i n g sodium p e n t o b a r b i t a l a n e s t h e s i a . Sakata e t a l . s t u d i e d a l a r g e number of c e l l s i n V L . S t i m u l a t i o n of the BC evoked 26 a p r e s y n a p t i c p o t e n t i a l with l a t e n c y of 0 .7 msec, and p o s t -s y n a p t i c u n i t responses with u s u a l l a t e n c i e s of 1.5 - 2.5 msec. The p o s t s y n a p t i c responses were judged t o r e p r e s e n t monosynaptic e x i t a t i o n of VL c e l l s by BC f i b r e s . Uno e t a l . i n an i n t r a c e l l u l a r study found EPSPs evoked i n VL r e l a y c e l l s w i t h a mean l a t e n c y of 1.7 msec, a f t e r i n t e r p o s i t u s s t i m u l a t i o n and 2.2 msec, a f t e r dentate s t i m u l a t i o n . By s t i m u l a t i n g the BC a t v a r i o u s l e v e l s , they were able t o c a l c u l a t e a mean conduction v e l o c i t y of 18 m/sec f o r the c e r e b e l l a r f i b r e s , thus c o n f i r m i n g Appelberg's d a t a , and a mean s y n a p t i c d elay of 0 .7 m s e c . . i n d i c a t i v e of monosynaptic a c t i o n . Stimulus i n t e n s i t i e s r e q u i r e d f o r t h r e s h o l d a c t i v a t i o n o f i n d i v i d u a l VL c e l l s were compared f o r dentate versus i n t e r p o s i t u s nucleus. The number of c e l l s a c t i v a t e d by s m a l l e r s t i m u l i to N i n t e r p o s i t u s were about equal to those having the a l t e r n a t e p a t t e r n . These authors were a l s o able to demonstrate an i n h i b i t o r y pathway to VL judged t o be d i - s y n a p t i c , a r i s i n g s e l e c t i v e l y from the N i n t e r p o s i t u s . The i n h i b i t o r y , neurone i n the pathway l i e s r o s t r a l to the red nucleus and i s thought to be i n VL i t s e l f . The c e l l s which respond w i t h IPSPs t o i n t e r p o s i t u s s t i m u l a t i o n were a l s o e x c i t e d more r e a d i l y by i n t e r p o s i t u s than by dentate s t i m u l a t i o n , i n d i c a t i n g t h a t N i n t e r p o s i t u s p r o j e c t s both e x i t a t o r y and i n h i b i t o r y i n f l u e n c e s to a s e l e c t e d group of-VL neurones. U n i t a r y EPSPs evoked i n VL c e l l s by a s i n g l e • 27-c e r e b e l l a r v o l l e y were l a r g e (0.8 - 3.2 mV) compared with those seen i n o t h e r c e l l s e.g./ s p i n a l motorneurones, emphas-i z i n g BC's c a p a c i t y f o r s t r o n g l y i n f l u e n c i n g the a c t i v i t y of VL c e l l s . C o r t i c o - VL S e v e r a l groups of workers have r e p o r t e d e l e c t r o p h y s i o -l o g i c a l evidence i n c a t s of n e u r a l connections from the p e r i c r u c i a t e c o r t e x to VL. Sakata, I s h i j i m a and Toyoda [1966] •recorded e x c i t a t o r y , s y n a p t i c responses i n VL w i t h a l a t e n c y range of 2.7 -.13 msec, and a d e p r e s s i o n of BC evoked p o t e n t i a l s f o l l o w i n g p r e c r u c i a t e c o r t i c a l s t i m u l a t i o n . T h i s d e p r e s s i o n had a l a t e n c y of about 20 msec, and a u s u a l d u r a t i o n of 200 msec., and was present even a f t e r c h r o n i c d e c o r t i c a t i o n . The l a t t e r o b s e r v a t i o n i n d i c a t e s t h a t the d e p r e s s i o n i s mediated by the axons of VL c e l l s , as the c o r t i c o - t h a l a m i c f i b r e s should have degenerated i n the three-week s u r v i v a l p e r i o d a f t e r d e c o r t i c a t i o n . The phenomenon would seem analogous, then,.to t h a t observed i n VPL and a t t r i b u t e d t o e x c i t a t i o n of i n h i b i t o r y interneurones by axon c o l l a t e r a l s of t h a l a m i c r e l a y c e l l s (Figure 2). Uno, Yoshida and H i r o t a [1970] showed t h a t the 3-5 msec. EPSPs i n VL c e l l s f o l l o w i n g c o r t i c a l s t i m u l a t i o n were t r a n s m i t t e d m o n o s y n a p t i c a l l y . By s t i m u l a t i n g the i n t e r n a l capsule a t v a r y i n g d i s t a n c e s from VL, they concluded t h a t the conduction v e l o c i t y of the e x i t a t o r y 28 c o r t i c o - t h a l a m i c f i b r e s r a n g e d f r o m 4-10 M / s e c . and by e x t r a p o l a t i o n t h a t t h e t e r m i n a l s o f t h e f i b r e s w o u l d be i n v a d e d o n l y 0.5 - 0 . 7 msec, b e f o r e t h e a p p e a r a n c e o f E P S P s i n t h e VL c e l l s , l e a v i n g t i m e f o r o n l y one s y n a p t i c d e l a y . R i s p a l - P a d e l and M a s s i o n [1970] h a v e e l e c t r o p h y s i o l o g i c a l l y d e m o n s t r a t e d a t o p o g r a p h i c p r o j e c t i o n t o VL f i b r e s o r i g i n a t i n g i n m o t o r c o r t e x . T h e . p a t t e r n o f d i s t r i b u t i o n n o t e d by t h e m, i s t h a t c e l l s i n t h e m e d i a l r e g i o n s o f t h e p r e c r u c i a t e c o r t e x i n c l u d i n g t h e p a r t d e s i g n a t e d a s a r e a 6 p r o j e c t p r e f e r -e n t i a l l y t o m e d i a l a r e a s o f V L , w h e r e a s t h o s e o r i g i n a t i n g i n . more l a t e r a l p a r t s , c o m p r i s i n g a r e a 4 s e n d f i b r e s t o t h e l a t e r a l V L. A r e a 6 o f t h e c o r t e x i s d e p i c t e d as r e p r e s e n t i n g a x i a l r e g i o n s o f b o d y , w h i l e a r e a 4 i s r e l a t e d t o d i s t a l m u s c u l a t u r e > t h e f o r e l i m b s b e i n g p a r t i c u l a r l y w e l l r e p r e s e n t e d . T h i s p a t t e r n l o o k s s i m i l a r t o t h a t d e s c r i b e d a n a t o m i c a l l y f o r f e l i n e c o r t i c o - V L f i b r e s b y Kusama, O t a n i and Kawana [ 1 9 6 6 ] . Dormont and M a s s i o n [1970] a l s o s t u d i e d t h e c o r t i c o - V L p a t h w a y . U s i n g l o w s t i m u l u s i n t e n s i t i e s f o r p r e c r u c i a t e c o r t i c a l a c t i v a t i o n , t h e y f o u n d o n l y 5% o f t h e VL c e l l s w e r e a c t i v a t e d w i t h s h o r t l a t e n c y e i t h e r a n t i d r o m i c a l l y o r s y n a p t i c a l l y . 30% o f t h e s e c e l l s , h o w e v e r , w e r e f o u n d t o e x h i b i t a h y p e r -p o l a r i z a t i o n w i t h 5-10 msec, l a t e n c y f o l l o w e d by a l o n g l a t e n c y d i s c h a r g e . T h e s e a u t h o r s n o t e d t h a t t h e h y p e r -p o l a r i z a t i o n was' s e l d o m a s s o c i a t e d w i t h a n t i d r o m i c a c t i v a t i o n o f t h e c e l l . 29 The s h o r t l a t e n c y e x c i t a t o r y e f f e c t s d e s c r i b e d by a l l the above workers seem t o be b a s i c a l l y s i m i l a r , the only d i f f e r e n c e s b e i n g i n the l a t e n c y ranges r e p o r t e d . The h y p e r p o l a r i z a t i o n s are d i f f e r e n t from those d e s c r i b e d f o r VPL [Andersen and Andersson, 1968] i n t h a t they u s u a l l y are not a s s o c i a t e d w i t h antidromic a c t i v a t i o n of the c e l l s . A few e x p l a n a t i o n s are p o s s i b l e f o r t h i s d i f f e r e n c e . (1) Many of the VL c e l l s depressed by c o r t i c a l s t i m u l a t i o n may not p r o j e c t to the cortex; (2) C o r t i c o - V L i n h i b i t o r y f i b r e s may be p r e s e n t which are absent or l e s s numerous i n the v e n t r o -b a s a l complex; 1 (3) The h y p e r p o l a r i z a t i o n s c o u l d be mediated v i a axon c o l l a t e r a l s and i n h i b i t o r y i n t e r n e u r o n e s , but d i s t r i b u t e d l a r g e l y to surrounding c e l l s , w i t h l i t t l e or no n e g a t i v e feed-back to the c e l l s whose axons were o r i g i n a l l y s t i m u l a t e d . The l a t t e r p o s s i b i l i t y c o u l d p r o v i d e an example of very s i m i l a r mechanisms s e r v i n g two d i f f e r e n t f u n c t i o n s , i . e . , a n e gative feed-back w i t h s u r r o u n d - i n h i b i t i o n i n VPL,. s u i t e d to. g e n e r a t i o n of rhythmic e x i t a b i l i t y a l t e r a t i o n s of a l o c a l i z e d group,of neurones, and in, VL, surround i n h i b i t i o n having l i t t l e or no s e l f - i n h i b i t o r y feed-back, more s u i t e d to f i n e d i s c r i m i n a t i o n of a c t i v i t y between one c e l l and i t s neighbours. " The l a t t e r type of. surround i n h i b i t i o n i s thought to be of importance i n t h e . r e t i n a f o r example. Dormont and Massion have a l s o shown t h a t s t i m u l a t i o n of c o r t i c a l . s e n s o r y areas can i n f l u e n c e VL b i l a t e r a l l y . These i n f l u e n c e s are of longer l a t e n c y than those from p r e c r u c i a t e 30 c o r t e x and t h o u g h t t o be t r a n s m i t t e d b y p a t h w a y s t h r o u g h more t h a n one s y n a p s e . P a l l i d o - t h a l a m i c , A v a r i e t y o f r e s p o n s e s o f VL n e u r o n e s t o s t i m u l a t i o n o f EN h a s b e e n r e p o r t e d . U s i n g e x t r a c e l l u l a r r e c o r d i n g , D o r m o n t , Ohye and A l b e - F e s s a r d [1969] f o u n d a n e g a t i v e f i e l d p o t e n t i a l w i t h a l a t e n c y a s s h o r t as 0 .5 msec, w h i c h a p p e a r e d t o r e p r e s e n t p r e s y n a p t i c a c t i v i t y a n d a l a t e r s m a l l a m p l i t u d e n e g a t i v e wave w i t h l a t e n c y o f 10 -12 msec, and d u r a t i o n o f 40 - 100 msec. The p r e s y n a p t i c r e s p o n s e s w e r e d e t e c t e d i n t h e m e d i a l p a r t s o f VA- VL and i n CM. T h e s e w o r k e r s a l s o f o u n d t h a t r e s p o n s e s t o s o m a t i c s t i m u l a t i o n w e r e d e p r e s s e d f r o m 30 - 200 msec, a f t e r EN s t i m u l a t i o n . T h i r t e e n e x c i t a t o r y c e l l u l a r r e s p o n s e s w e r e s e e n , i n t h e f o r m o f b u r s t s o f a c t i o n p o t e n t i a l s . T h r e e h a d minimum l a t e n c i e s o f 2.5 - 3.0 msec, w h i l e t h e o t h e r s h a d l a t e n c i e s g r e a t e r t h a n 10 msec. R e p o r t s o f i n t r a c e l l u l a r i n v e s t i g a t i o n o f t h i s p a t h w a y h a v e b e e n made by two g r o u p s o f w o r k e r s . On t h e b a s i s o f i n t r a c e l l u l a r r e c o r d i n g s , , F r i g y e s i [1969] r e p o r t e d 8 -10 msec. E P S P s i n VL c e l l s a f t e r EN s t i m u l a t i o n . D e s i r a j u a nd P u r p u r a [1969] f o u n d E P S P s w i t h l a t e n c i e s a s s h o r t a s 0 .7 -0 .9 msec, a f t e r s t i m u l a t i o n o f t h e a n s a l e n t i c u l a r i s (AL) o r EN. T h e s e s h o r t l a t e n c y E P S P s w e r e g e n e r a l l y a s s o c i a t e d w i t h a s i n g l e c e l l u l a r a c t i o n p o t e n t i a l . T h e y n o t e d , h o w e v e r , 3 1 t h a t when a spike was not generated the EPSPs had a d u r a t i o n of 10-20 msec. PSPs of 2-6 msec, l a t e n c y which c o u l d a p p a r e n t l y be e i t h e r e x i t a t o r y or i n h i b i t o r y i n nature were a l s o observed, as were 10-30 msec. EPSP and IPSP sequences. Of p a r t i c u l a r i n t e r e s t was the p r e v i o u s l y mentioned o b s e r v a t i o n t h a t the AL evoked s h o r t l a t e n c y EPSPs were r a r e l y succeeded, by IPSPs, whereas BC evoked EPSPs were f r e q u e n t l y f o l l o w e d by IPSPs. F r i g y e s i and Machek [1970] r e p o r t e d 3-6 msec, l a t e n c y EPSPs a f t e r EN s t i m u l a t i o n i n VA - VL c e l l s evoked by BC. P o l y s y n a p t i c EPSPs succeeded by prolonged IPSPs were superimposed on the shoulders of the s h o r t l a t e n c y e x c i t a t i o n s EPSP l a t e n c i e s t h e r e f o r e of. 0.7 — 0-.9 msec., 2-6 msec. , 8-1.0 msec, and 10-30 msec, have been found. T h i s spread of l a t e n c i e s might be a t t r i b u t a b l e t o the d i f f i c u l t i e s of s e l e c t i v e l y s t i m u l a t i n g EN, to the e x i s t e n c e of groups of p a l l i d o - t h a l a m i c f i b r e s o f d i f f e r e n t s i z e s , or d i r e c t . a n d i n d i r e c t pathways from EN to VL. Probably a combination of these p o s s i b i l i t i e s i s i n v o l v e d . N i g r o - t h a l a m i c F r i g y e s i and Machek [1970] have r e p o r t e d t h a t s t i m u l a t i o n of the s u b s t a n t i a n i g r a e l i c i t e d 2-4 msec. EPSPs which co u l d g i v e r i s e t o s i n g l e s p i l e s i n VL c e l l s . I f the EPSPs d i d not generate a s p i k e , they were succeeded by b r i e f (5-lQ.msec.) h y p e r p o l a r i z i n g p o t e n t i a l s . Such s h o r t l a t e n c y 32 EPSPs were seen most f r e q u e n t l y i n the caudomedial p a r t s of VL i n c e l l s i d e n t i f i e d as r e c e i v i n g a monosynaptic i n p u t from BC. N i g r a l responses - were most commonly found i n c e l l s which c o u l d not be i d e n t i f i e d as r e l a y neurones by i n t e r n a l c apsule s t i m u l a t i o n , and were not observed i n c e l l s which responded to EN s t i m u l a t i o n . I n t r a l a m i n o - VL. S t i m u l a t i o n of m e d i a l - i n t r a l a m i n a r n u c l e i evoked s h o r t l a t e n c y EPSPs succeeded by l a t e r IPSPs which h'ad; a d u r a t i o n of up to 200 msec. [Purpura, S c a r f f and McMurtry, 1965; Purpura, F r i g y e s i , McMurtry and S c a r f f , 1966]. The l a t e n c y o f the EPSPs i s not d e s c r i b e d i n these papers but appears from the f i g u r e s t o be about 10 msec. Because of the prox-i m i t y of the s t i m u l a t i n g and r e c o r d i n g s i t e s , i t would be s u r p r i s i n g i f t h i s l a t e n c y r e p r e s e n t e d t r a n s m i s s i o n along a d i r e c t pathway. In a study of medial thalamic neurones d u r i n g s t i m u l a t i o n of VL, D e s i r a j u , B r o g g i , . P r e l e v i c , S a n t i n i and Purpura, 1969] f a i l e d t o see any a n t i d r o m i c a l l y invaded, c e l l s . I t seems probable, t h e r e f o r e t h a t the e f f e c t s are due t o a c t i v a t i o n of i n d i r e c t pathways. When the medial n u c l e i were s t i m u l a t e d a t a frequency of 7/sec., i t was found t h a t t r a n s m i s s i o n through the c e r e b e l l o - t h a l a m i c pathway was e f f e c t i v e l y b l o c k e d . The e f f e c t s on VL neurones of s t i m u l i t o the b r a i n stem RF have a l s o been s t u d i e d [Purpura, McMurtry and 33 Maekawa, 1966]. RF s t i m u l i . o f t e n evoked EPSPs u s u a l l y w i t h a l a t e n c y s l i g h t l y longer than BC evoked EPSPs. S t i m u l a t i o n at f r e q u e n c i e s of 50-75/sec. g e n e r a l l y caused i n c r e a s e d d i s c h a r g e f r e q u e n c i e s of VL neurones and e l i m i n a t e d IPSPs r e s u l t i n g from BC s t i m u l a t i o n . Purpura and co-workers have proposed t h a t the two systems, the m e d i a l - t h a l a m i c n u c l e i and RF may p l a y r o l e s i n the d e p r e s s i o n and f a c i l i t a t i o n ... r e s p e c t i v e l y of s i g n a l t r a n s m i s s i o n through VL. Other Systems I n f l u e n c i n g VL S t i m u l a t i o n of caudate nucleus [ F r i g y e s i and Purpura, 1964; F r i g y e s i and Machek, 1970] or of d i v e r s e sensory systems [Massion, Angaut and A l b e - F e s s a r d , 1965a, b; Massion, 1968] can a c t i v a t e VL c e l l s . The responses have r e l a t i v e l y long l a t e n c y and are probably t r a n s m i t t e d by mulfeisynaptic pathways. I t i s apparent then t h a t VL a c t i v i t y i s i n f l u e n c e d by a wide v a r i e t y of. i n p u t s o r i g i n a t i n g i n other b r a i n c e n t r e s . In some cases d i f f e r e n t a f f e r e n t pathways do not appear to i n f l u e n c e the same p o p u l a t i o n of neurones e.g., SN and E N , [ F r i g y e s i and Machek, 1970] but some i n d i v i d u a l c e l l s have been found to respond to s e v e r a l a f f e r e n t systems, e.g., EN, BC and c o r t e x . I t i s probable t h a t t h e r e are s u b n u c l e a r . d i v i s i o n s w i t h i n VL, but l i t t l e c o r r e l a t i o n between p h y s i o l o g i c a l and anatomical sub-groups has been found. I t i s a l s o probable t h a t w i t h i n any one area of VL, d i f f e r e n t types of neurones with r e l a t i v e s p e c i f -i c i t y of connections e x i s t . L i t t l e progress has been made i n d e f i n i n g such c e l l types. VL E f f e r e n t The e f f e r e n t pathway of VL t o motor c o r t e x has a l s o been s t u d i e d e l e c t r o p h y s i o l o g i c a l l y . Antidromic i n v a s i o n of VL c e l l s f o l l o w i n g s t i m u l a t i o n , p a r t i c u l a r l y of p r e c r u c i a t e c o r t e x of cats has been r e p o r t e d by Sakata, I s h i j i m a and Toyoda [1966] Uno, Yoshida and H i r o t a [1970] and R i s p a l - P a d e l and Massion [1970] to name a few. Sakata e t a l . u s i n g c o r t i c a l s t i m u l a t i o n were able t o a c t i v a t e a n t i d r o m i c a l l y almost h a l f of BC evoked VL c e l l s . Thus the ascending c e r e b e l l a r pathway appears t o have a d i r e c t l i n e to motor co r t e x , i n v o l v i n g o n l y one synapse i n VL. R i s p a l - P a d e l and Massion showed a topographic p r o j e c t i o n of VL c e l l s t o co r t e x w i t h a p a t t e r n s i m i l a r to t h a t p r e v i o u s l y d e s c r i b e d f o r the cortex-VL p r o j e c t i o n , i . e . c e l l s o f medial p a r t s of VL send f i b r e s to medial p a r t s of p r e c r u c i a t e c o r t e x and l a t e r a l VL c e l l s p r o j e c t to l a t e r a l p r e c r u c i a t e c o r t e x . T h i s p a t t e r n f o r t h a l a m u s - c o r t i c a l f i b r e s appears s i m i l a r t o tha t shown by S t r i c k [1970] . L-Yoshida, Yajima and Uno [1966], Amassian and Weiner [1966] and Leblanc and Cordeau [1969] have shown orga n i z e d s h o r t l a t e n c y e f f e c t s of VL s t i m u l a t i o n on c e l l s of the motor co r t e x which c o n t r i b u t e to the pyramidal t r a c t . Yoshida e t a l . i d e n t i f i e d f a s t . a n d slow pyramidal neurones i n c e r e b r a l c o r t e x by a c t i v a t i n g them antidrom-ically from the medullary pyramids. The f a s t c e l l s were found to be a c t i v a t e d w i t h a l a t e n c y a p p r o p r i a t e f o r a monosynaptic pathway f o l l o w i n g s t i m u l a t i o n of VL. BC s t i m u l a t i o n a c t i v a t e d the same c e l l s , and was judged to occur a f t e r only one synapse i n VL and one a t the c o r t e x . The s l o w l y conduct-i n g pyramidal, c e l l s were a c t i v a t e d l e s s f r e q u e n t l y and w i t h longer and more v a r i a b l e l a t e n c i e s a f t e r V L , s t i m u l a t i o n s , i n d i c a t i v e of t r a n s m i s s i o n over a p o l y s y n a p t i c pathway. The c e r e b e l l o - t h a l a m i c pathway, t h e r e f o r e seems t o have the p r o p e r t i e s r e q u i r e d to e x e r t a d i r e c t and s t r o n g i n f l u e n c e on the a c t i v i t y of f a s t conducting pyramidal c e l l s . T h i s i s s u i t a b l e t o the r o l e some authors have a t t r i b u t e d t o t h i s pathway i n the i n i t i a t i o n of movement [Evarts and Thach, 1969]. Leblanc and Cordeau [1969] have a l s o shown s h o r t l a t e n c y responses of f a s t - c o n d u c t i n g pyramidal neurones to VL s t i m u l a t i o n , but noted f o l l o w i n g prolonged i n h i b i t i o n s succeeded by e x c i t a t i o n s a t about 200-250 and 400-450 msec. By s t i m u l a t i n g VL at. v a r i o u s f r e q u e n c i e s , they concluded t h a t the rhythmic a c t i v i t y i n the l a r g e pyramidal c e l l s responded o p t i m a l l y to VL v o l l e y s a t - r a t e s of 4-5/sec. These authors p o i n t e d out t h a t t h i s frequency i s about the same as that of Parkinsonian tremor and rhythmic e l e c t r i c a l a c t i v i t y noted i n VL of Parkinsonian patients. Monkeys exhibiting tremor after mesencephalic lesions have synchronous e l e c t r i c a l a c t i v i t y i n various brain regions which i s l o s t i n some after immobilization with gallamine t r i e t h i o d i d e , but i s retained i n VL, motor cortex and pyramidal t r a c t [Lamarre and Cordeau, 1964]. These observations allowed the conclusion that the VL-cortex pathway may play a role i n the genesis of tremor of the Parkinsonian type. The conclusion i s p a r t i a l l y supported by the well-known e f f i c a c y of VL lesions i n reducing Parkinsonian tremor, [e.g., Cooper, et al.1968] The responses obtained i n neurones of medial non-s p e c i f i c thalamic nuclei after VL stimulation have been studied by Desiraju et a l . [1969]. They frequently found short latency IPSPs and complex EPSP and IPSP patterns after r e p e t i t i v e stimulation. The authors presented no evidence, however, on the pathways by which these e f f e c t s were trans-mitted, whether they arose s p e c i f i c a l l y from exc i t a t i o n of VL c e l l s or whether activation of f i b r e s passing through VL could have been responsible. 37 ROLES OF VL AND CM The r o l e or r o l e s of the v e n t r o l a t e r a l nucleus are d i f f i c u l t t o p e r c e i v e a t t h i s time ,• and t h i s i s r e f l e c t e d i n the number of" p r o p o s a l s t h a t have been made f o r i t s f u n c t i o n s . That i t a c t s i n p a r t . a s a r e l a y nucleus f o r s i g n a l s p a s s i n g from the c e r e b e l l a r n u c l e i to the motor.cortex i s . u n d e n i a b l e i n l i g h t of the very s t r o n g i n f l u e n c e e x e r t e d on VL c e l l s by BC f i b r e s . The l a r g e s i z e of the u n i t a r y EPSPs r e s u l t i n g from a c t i v a t i o n of BC f i b r e s probably c o n f e r s a l a r g e s a f e t y f a c t o r on c e r e b e l l o - t h a l a m i c t r a n s m i s s i o n , so t h a t though many of these f i b r e s may terminate on a given VL c e l l , simultaneous a c t i v a t i o n of only a few of them i s r e q u i r e d to e f f e c t a post s y n a p t i c d i s c h a r g e . That VL i s much more than a simple r e l a y nucleus i s made evident.by the l a r g e number of n e u r a l c e n t r e s which send f i b r e s t o i t . T h i s p r o f u s i o n of a f f e r e n t s from d i f f e r e n t . b r a i n c e n t r e s leads t o the p o s t u l a t e t h a t VL may f u n c t i o n i n p a r t as an i n t e g r a t i o n c e n t r e f o r i n f o r m a t i o n s u p p l i e d by the b a s a l g a n g l i a . The n o t i o n t h a t the b a s a l g a n g l i a are . r e l a y s t a t i o n s i n a m u l t i s y n a p t i c descending e x t r a p y r a m i d a l system i s r a p i d l y , l o s i n g p o p u l a r i t y . Two reasons e x i s t f o r t h i s . The l a r g e number of c l a s s i c a l anatomical and e l e c t r o -anatomical s t u d i e s c a r r i e d out on the b a s a l g a n g l i a have f a i l e d t o show a descending pathway of any magnitude, i f the r u b r o s p i n a l and r e t i c u l o s p i n a l pathways are excepted.. The 38 two n u c l e i g i v i n g r i s e t o t h e s e p a t h w a y s r e c e i v e r e l a t i v e l y l a r g e numbers o f a f f e r e n t f i b r e s f r o m t h e m o t o r c o r t e x , . a n d a p p a r e n t l y v e r y s m a l l - a m o u n t s , i f a n y , f r o m t h e d i e n c e p h a l i c o r t e l e n c e p h a l i c b a s a l g a n g l i a . The s e c o n d r e a s o n i s t h a t r e c e n t w o r k i n w h i c h t h e i m p o r t a n c e o f , p y r a m i d a l and e x t r a -p y r a m i d a l s y s t e m s h a s b e e n s t u d i e d b y s e l e c t i v e l e s i o n s i n d i c a t e s t h a t d e s c e n d i n g p a t h w a y s o r i g i n a t e i n m e d u l l a r y a n d ^ p o n t i n e r e t i c u l a r , f o r m a t i o n , i n t e r s t i t i a l n u c l e u s o f C a j a l , v e s t i b u l a r n u c l e i and r e d n u c l e u s [ K u y p e r s , 1 9 6 8 ] , Though d i r e c t c o n n e c t i o n s o f o t h e r s u b c o r t i c a l m o t o r c e n t r e s t o t h e s e n u c l e i a r e n o t known, t h e y do r e c e i v e a f f e r e n t s f r o m t h e c e r e b r a l c o r t e x a n d / o r c e r e b e l l u m . T h e r e f o r e t h e m a j o r a v a i l a b l e means f o r t h e . S N and d i e n c e p h a l i c a n d t e l e n c e p h a l i c n u c l e i t o i n f l u e n c e movement w o u l d seem t o be by way o f VL and VA w i t h some r o l e a l s o p o s s i b l e f o r CM. V a r i o u s • e v i d e n c e s h a v e b e e n c i t e d t o s u p p o r t t h e r o l e o f VL i n t h e i n i t i a t i o n o f movement; and t h e c o n n e c t i o n s o f VL w i t h s e n s o r y s y s t e m s and i n c i r c u i t s w i t h o t h e r m o t o r , n u c l e i p r o v i d e s t h e p o t e n t i a l f o r i t t o a c t . a s a c e n t r e f o r r e g u l a t i o n and c o r r e c t i o n o f movement. I t . s e e m s p r o b a b l e t h a t VL p l a y s a r o l e i n e a c h o f t h e s e a s p e c t s o f movement, b u t i t s r e l a t i v e i m p o r t a n c e t o e a c h one i s n o t known. The r o l e p l a y e d b y CM i n movement i s e v e n f a r t h e r f r o m d e f i n i t i o n . The c o n s i d e r a t i o n o f t h e i n t r a l a m i n a r . n u c l e i a s a f u n c t i o n a l g r o u p c o m p r i s i n g a p a r t o f t h e a s c e n d -i n g r e t i c u l a r a c t i v a t i n g s y s t e m may h a v e o b s c u r e d t h e p o t e n t i a l r o l e of c e r t a i n of the component n u c l e i i n motor c o n t r o l . The s p e c i f i c i t y o f , , f o r example, the c o r t i c a l and p a l l i d a l a f f e r e n t i n p u t s t o the p a r v o c e l l u l a r CM and i t s e f f e r e n t c o n n e c t i o n t o l e n t i f o r m nucleus should help t o r e f o c u s a t t e n t i o n on the i n d i v i d u a l n u c l e i of the i n t r a -laminar group. I n t e r p r e t a t i o n of the p h y s i o l o g i c a l l i t e r a t u r e p e r t i n e n t , t o CM i s d i f f i c u l t , b e c a u s e most workers have not attempted to d i f f e r e n t i a t e f i n d i n g s i n v a r i o u s components of the i n t r a l a m i n a r system. S t i m u l a t i o n of v a r i o u s n e u r a l systems have been claimed t o a l t e r a c t i v i t y i n CM,.however,. examination of the data sometimes shows t h a t the responses are l o c a l i z e d to the more medial p a r t s o f . t h e CM-Pf complex. For example the responses .found i n CM f o l l o w i n g dentate N s t i m u l a t i o n [Conde and Angaut, 1970] appear t o be excluded from the l a t e r a l p a r t s of the complex,.and the CM responses t o sensory s t i m u l i d e s c r i b e d by A l b e - F e s s a r d and Fes s a r d , [1963] ' were found,only w i t h i n m e d i a l - l a t e r a l l i m i t s which would exclude most, of the p a r v o c e l l u l a r CM. 40 IONTOPHORESIS AND THE PHARMACOLOGY OF THALAMIC NEURONES The t e c h n i q u e o f i o n t o p h o r e s i s ( m i c r o i o n t o p h o r e s i s , m i c r o e l e c t r o p h o r e s i s ) was f i r s t i n t r o d u c e d by. S u h , Wang and L i m [ 1 9 3 6 ] , and l a t e r u s e d i n s t u d i e s o f t h e n e u r o m u s c u l a r j u n c t i o n [ N a s t u k , 1953] and t h e s p i n a l c o r d [ C u r t i s and E c c l e s , : 1 9 5 8 ] . S i n c e t h a t t i m e , i t h a s b e e n e m p l o y e d i n t h e s t u d y o f a l m o s t e v e r y c e l l u l a r r e g i o n o f t h e mammalian CNS and o f many v e r t e b r a t e and i n v e r t e b r a t e e x c i t a b l e c e l l s . One b a r r e l o f a m u l t i - b a r r e l e d (2-9) g l a s s p i p e t t e a s s e m b l y i s u s e d t o r e c o r d t h e e l e c t r i c a l • , a c t i v i t y o f one o r a s m a l l g r o u p o f c e l l s . I o n s may be e j e c t e d i n t o t h e e x t r a c e l l u l a r e n v i r o n m e n t o f t h e s e c e l l s f r o m t h e n e i g h b o u r i n g b a r r e l s . The m e thod h a s b e e n d e s c r i b e d b y C u r t i s [ 1 9 6 4 ] , K r n j e v i c [ 1 9 6 4 ] , S a l m o i r a g h i and S t e f a n i s [1967] and o t h e r s and b r i e f l y i s a s f o l l o w s . The e l e c t r o d e s may be p r e p a r e d b y f u s i n g s e v e r a l g l a s s c a p i l l a r i e s and t h e n h e a t i n g one c r o s s - s e c t i o n a l r e g i o n i n .a m i c r o f o r g e t o p e r m i t d r a w i n g o f t h e s o f t e n e d g l a s s t o a f i n e d i a m e t e r . I n t h i s way, a f u s e d c l u s t e r o f m i c r o -p i p e t t e s h a v i n g a t o t a l t i p d i a m e t e r o f 1 p o r l e s s , c a n be p r o d u c e d . The t i p i s t h e n b r o k e n b a c k t o a t o t a l c r o s s -s e c t i o n a l d i a m e t e r o f a b o u t 4-6 u, i n o r d e r t o r e d u c e t h e i m p e d a n c e o f t h e - e l e c t r o d e s . The r e c o r d i n g b a r r e l i s f i l l e d w i t h a s t r o n g e l e c t r o l y t e , and t h e . o t h e r s w i t h s o l u t i o n s o f i o n i z e d d r u g s t o be t e s t e d . The a c t i v e i o n o f a d r u g w h i c h i s t o be s t u d i e d c a n be s e l e c t i v e l y d i s c h a r g e d f r o m t h e 41 electrode by passing the appropriate current (-ye to eject anions, +ve to eject cations) through the drug solution. This technique i s of p a r t i c u l a r value i n pharmaco-: l o g i c a l studies of the CNS> not only because small c e l l populations can.be s e l e c t i v e l y exposed to the drug—thus avoiding secondary drug e f f e c t s , but also because many ionized drugs do not gain access to the CNS after systemic adminis-tration:. Iontophoresis has been used mostly i n attempts.to define chemical transmitters at s p e c i f i c s i t e s by f i r s t e stablishing the effects of postulated transmitters on the neuronal a c t i v i t y and then t e s t i n g possible blocking agents on these induced e f f e c t s and on synaptic transmission. Because some chemical agents, p a r t i c u l a r l y some of the d i a c i d i c amino acids, have strong and ubiquitous excitatory actions on neurones, iontophoresis can also be of value i n el e c t r o -ph y s i o l o g i c a l studies. Such agents can be used to find c e l l s which do not discharge spontaneously, and also to induce a steady l e v e l of a c t i v i t y i n such c e l l s so that i n h i b i t o r y or depressant influences can be studied. There are many uncertainties associated.with the iontophoretic method. The concentration of an agent.which i s achieved around a c e l l i s impossible to estimate accurately, due to uncertainties about the amount of drug being extruded by a given current, the d i f f u s i o n c h a r a c t e r i s t i c s of the agent from the electrode t i p and i n the e x t r a c e l l u l a r f l u i d , and the extent of i t s i n t e r a c t i o n with tissue components, e.g., enzymes and membrane b i n d i n g or t r a n s p o r t systems. F u r t h e r , the c o n c e n t r a t i o n s e f f e c t e d are u n l i k e l y t o be uniform, p a r t i c u l a r l y about l a r g e c e l l s , or ones with e x t e n s i v e d e n d r i t i c f i e l d s . Some.of the chemicals used may have more than the one a c t i o n f o r which they are u s u a l l y t e s t e d , p a r t i c u l a r l y a t h i g h l o c a l c o n c e n t r a t i o n s . A t r o p i n e , f o r example seems to have a l o c a l a n e s t h e t i c - t y p e a c t i o n i n a d d i t i o n t o i t s c h o l i n e r g i c b l o c k i n g p r o p e r t i e s [ C u r t i s and P h i l l i s , I960]. Another p o s s i b l e c o m p l i c a t i o n i s t h a t the drug may a f f e c t more than the s i n g l e or few c e l l s being recorded, so t h a t r e s u l t s c o u l d be m o d i f i e d by e f f e c t s of the agent on adjacent i n t e r n e u r o n e s which i n t e r a c t with the c e l l whose a c t i v i t y i s recorded [McLennan, 1971]. The e l e c t r i c a l c u r r e n t used i n p a s s i n g the drugs may i t s e l f modify c e l l a c t i v i t y thus masking the e f f e c t s . o f the chemical agent. F i n a l l y , i n t e r a c t i o n s between systemic drugs (e.g., a n e s t h e t i c s and muscle r e l a x a n t s ) and the substances being a p p l i e d i o n t o p h o r e t i c a l l y may occur, and have been demonstrated i n a number of i n s t a n c e s [e.g., P h i l l i s and T e b e c i s , 1967]. I t i s necessary, t h e r e f o r e to be c a u t i o u s i n the i n t e r p r e t a t i o n of r e s u l t s of i o n t o p h o r e t i c s t u d i e s . The e f f e c t s of i o n t o p h o r e t i c a l l y a p p l i e d amino a c i d s have been t e s t e d i n most c e l l u l a r r e g i o n s of the c e n t r a l nervous system. S e v e r a l of the d i a c i d i c monobasic a c i d s such as g l u t a m i c a c i d , a s p a r t i c a c i d , homocysteic a c i d and many of t h e i r derivatives have strong excitatory actions on. neurpnes,while the monoacidic monobasic ones such as glycine, g'^-alanine and Y - a m i r i o b u t y r i c acid are depressant [Curtis and Watkins, I960]. The excitatory amino acids have been shown to excite neurones i n the spinal cord [Curtis and Watkins, 1960] , l a t e r a l geniculate nucleus [Curtis and Davis, 1962] , cerebral cortex [ P h i l l i s and Krnjevic, 1961] and , apparently every other region tested. Because of thi s property, these amino acids are routinely used i n iontophoretic studies to locate e l e c t r i c a l l y quiescent neurones and as controls for drug e f f e c t s . Their actions are so widespread that i t has been presumed that the exci t a t i o n i s due to a non-specific e f f e c t on neuronal membrane, and not related to activ a t i o n of transmitter receptor s i t e s [Curtis and Watkins, 1963] . The relay nuclei of the thalamus have been studied extensively with the iontophoresis technique i n attempts to i d e n t i f y the transmitter agents released by t h e i r afferent pathways. The f i r s t study of VL was reported by Davis [1966]. His r e s u l t s were very s i m i l a r to those previously reported for the ventrobasal complex [Anderson & Cu r t i s , 1964 a & b]. Iontophoretic application of acetylcholine (ACh) or carbachol caused the VL neurones to discharge at high rates and f a c i l i t a t e d synaptic a c t i v a t i o n after BC stimulation. Muscarone, acetyl-3-methylcholine had s i m i l a r , but less potent excitatory e f f e c t s . The ACh exci t a t i o n was blocked by a t r o p i n e , d i h y d r o - 3 - e r y t h r o i d i n e (DHBE) and t r i h e x y p h e n i d y l b u t t h e s y n a p t i c ^ a c t i v a t i o n o f c e l l s b y BC s t i m u l a t i o n was n o t a f f e c t e d . I f l a r g e r amounts o f a t r o p i n e w e r e e j e c t e d , b o t h t h e ACh e x i t a t i o n a n d . t h e s y n a p t i c a c t i v a t i o n w e r e b l o c k e d , b u t s o was t h e e x c i t a t i o n b y D, L - h o m o c y s t e i c a c i d (DLH) i n d i c a t i n g a n o n - s p e c i f i c b l o c k i n g o f membrane e x c i t a b i l i t y . T h i s l a t t e r o b s e r v a t i o n i s i n a c c o r d w i t h e a r l i e r r e p o r t s t h a t a t r o p i n e c a n b l o c k n e u r o n a l e x c i t a t i o n i n a n o n - s p e c i f i c manner [ C u r t i s a nd P h i l l i s , I 9 6 0 ] . D a v i s ' c o n c l u s i o n s w e r e t h a t t h o u g h i t seemed l i k e l y t h a t VL and t h e o t h e r t h a l a m i c r e l a y n u c l e i may h a v e c h o l i n e r g i c i n p u t s , BC - VL t r a n s m i s s i o n i s n o t c h o l i n e r g i c a l l y m e d i a t e d . I t may a l s o be n o t e d t h a t t h e VL n e u r o n e s w e r e e x c i t e d by b o t h m u s c a r i n i c and n i c o t i n i c c h o l i n e r g i c d r u g s , as w e r e t h e n e u r o n e s o f t h e v e n t r o b a s a l c o m p l e x [ A n d e r s e n & C u r t i s , 1964b] and ACh e x c i t a t i o n c o u l d be p r e v e n t e d b y e i t h e r t h e m u s c a r i n i c b l o c k e r a t r o p i n e o r t h e n i c o t i n i c b l o c k e r d i h y d r o -0 - e r y t h r o i d i n e (DH3E). I n c o n t r a s t t o t h e Renshaw c e l l s , w h i c h a r e s e l e c t i v e l y e x c i t e d by n i c o t i n i c a g e n t s [ C u r t i s and R y a l l , 1966 a,b] and c o r t i c a l _ n e u r o n e s w h i c h a r e a f f e c t e d by m u s c a r i n i c d r u g s [ K r n j i v i c and P h i l l i s , 1963; C r a w f o r d and C u r t i s , 1 9 6 6 ] , t h e t h a l a m i c n e u r o n e s a r e e x c i t e d by b o t h c l a s s e s o f d r u g s . T h i s may be i n t e r p r e t e d a s due t o a p r e s e n c e o f b o t h r e c e p t o r t y p e s o r a h y b r i d r e c e p t o r . F u r t h e r s t u d i e s o n t h a l a m i c r e l a y n u c l e i i n c l u d i n g VL w e r e c a r r i e d o u t by M c C a n c e , P h i l l i s and W e s t e r m a n [1968a] 45 and M c C a n c e , P h i l l i s , T e b e c i s and Westerman [ 1 9 6 8 ] . T h e i r f i n d i n g s w e r e s i m i l a r t o t h o s e o f D a v i s [1966] b u t a l a r g e r number o f s u b s t a n c e s w e r e t e s t e d i o n t o p h o r e t i c a l l y . C a r b a c h o l , ACh and a c e t y l - 3 - m e t h y l c h o l i n e w e r e f o u n d t o be t h e most p o t e n t c h o l i n o m i m e t i c e x c i t a n t s . A t r o p i n e and h y o s c i n e w e r e t h e m o s t e f f e c t i v e b l o c k i n g a g e n t s , a l t h o u g h DH3E, h e x a m e t h o n i u m and m e c a m y l a m i n e a l s o d e p r e s s e d t h e ACh e x c i t a t i o n o f some n e u r o n e s . C h o l i n e s t e r a s e i n h i b i t o r s o f t e n h a d e x c i t a t o r y a c t i o n s o f t h e i r own, p a r t i c u l a r l y t h e q u a t e r n a r y compounds, b u t t h e t e r t i a r y compound e s e r i n e , a n d s o m e t i m e s t h e q u a t e r n a r y i n h i b i t o r s w e r e f o u n d t o e n h a n c e a n d / o r p r o l o n g ACh e x c i t a t i o n w i t h o u t a l t e r i n g t h e e x c i t a t i o n c a u s e d by L-g l u t a m a t e ( L G ) , [ A n d e r s o n and C u r t i s , 1964b; McCance e t a l . , 1 9 6 8 a ] . The l a t t e r a u t h o r s a l s o r e p o r t e d t h a t t h e n u c l e i w i t h t h e g r e a t e s t p r o p o r t i o n s o f ACh s e n s i t i v e c e l l s w e r e N v e n t r a l i s m e d l a l i s VPL and V L . They a l s o made t h e i n t e r e s t i n g o b s e r v a t i o n t h a t a much h i g h e r p e r c e n t a g e (41%) o f s e n s i t i v e n e u r o n e s w e r e f o u n d i n t h e v e n t r a l VL t h a n i n i t s more d o r s a l p a r t s , ( 2 3 % ) . I n t h e i r i n v e s t i g a t i o n o f i n t e r a c t i o n s o f d r u g s w i t h s y n a p t i c a c t i v a t i o n i n t h e t h a l a m u s . M c C a n c e P h i l l i s and Westerman [1968b] f o u n d t h a t t h e a c t i v a t i o n o f v e n t r o b a s a l c o m p l e x c e l l s f r o m t h e m e d i a l l e m n i s c u s was n o t b l o c k e d b y i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e o r DH$E, o r by i n t r a v e n o u s a t r o p i n e i n a d o s e o f 1 mgm/kgm. C o r t i c o - t h a l a m i c r e s p o n s e s 4 6 were t e s t e d w i t h i o n t o p h o r e t i c a p p l i c a t i o n of a t r o p i n e and DH BE w i t h n e g a t i v e r e s u l t s . BC evoked s p i k e responses were not a l t e r e d by i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e , h y oscine or DH(3E, though ACh e x c i t a t i o n was a b o l i s h e d . A t r o p i n e (0.25 - 1.0 mgm/Kg) administered i n t r a v e n o u s l y d i d reduce the.BC evoked f i e l d p o t e n t i a l by 30-40%. C e l l s which were e x c i t e d by s t i m u l a t i o n of mesencephalic r e t i c u l a r , formation (MRF) were s t u d i e d a l s o , and i t was r e p o r t e d t h a t i n some of these the e x c i t a t o r y e f f e c t s of both MRF s t i m u l a t i o n and ACh co u l d be. blocked by i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e or DH3E. In other c e l l s , responses to MRF s t i m u l a t i o n were r e f r a c t o r y t o the c h o l i n e r g i c b l o c k i n g agents. The c o n c l u s i o n s drawn from these s t u d i e s were t h a t the BC - VL pathway i s p a r t i a l l y mediated by ACh and t h a t support was found f o r the i d e a of c h o l i n e r g i c i n p u t s to the thalamus from the MRF. The MRF evoked responses were of long l a t e n c y , i n d i c a t i n g a probable m u l t i s y n a p t i c pathway. That the MRF e f f e c t s were blo c k e d i o n t o p h o r e t i c a l l y i n d i c a t e s t h a t i f a c h o l i n e r g i c synapse had been blocked, i t was a t the l e v e l of VL. The o b s e r v a t i o n t h a t sodium p e n t o b a r b i t a l depressed the ACh s e n s i t i v i t y of thal a m i c neurones without s i g n i f i c a n t l y a l t e r i n g e x c i t a t i o n by amino a c i d s was more f u l l y r e p o r t e d i n another p u b l i c a t i o n [ P h i l l i s & T e b e c i s , 1967]. T h i s r e p o r t showed t h a t very s m a l l doses of I.V. p e n t o b a r b i t a l (2-4 mgm/kgm) markedly reduced NA d e p r e s s i o n of LG f i r i n g i n one c e l l , l e a v i n g the LG e x c i t a b i l i t y a p p a r e n t l y u n a l t e r e d , and completely blocked the ACh s e n s i t i v i t y of another neurone, while LG e x c i t a t i o n was only reduced. The e f f e c t s of ACh and a t r o p i n e on t r a n s m i s s i o n i n the BC - VL pathway had been i n d i c a t e d i n an e a r l i e r r e p o r t by F r i g y e s i and Purpura [1966], who noted t h a t i n j e c t i o n s of ACh i n t o the c a r o t i d a r t e r y t r a n s i e n t l y enhanced the BC evoked f i e l d p o t e n t i a l i n VL, while a t r o p i n e administered by the same route had some b l o c k i n g a c t i o n a t a dose of 250 yg and markedly reduced the response a t 750-1000 yg. The r e p o r t s of F r i g y e s i and Purpura, and McCance e t a l . are s u b j e c t to the same c r i t i c i s m ; t h a t no other pathway to VL was t e s t e d w i t h systemic a t r o p i n e . T h e r e f o r e i f a t r o p i n e had blocked any c h o l i n e r g i c pathway wi t h t o n i c d i s c h a r g e to VL the same e f f e c t might have been expected. I f t h i s were the case, the removal of a t o n i c d e p o l a r i z a t i o n of VL c e l l s c o u l d change the membrane p o t e n t i a l from a t h r e s h o l d to a su b t h r e s h o l d l e v e l f o r the BC evoked EPSPs . • In t h i s s i t u a t i o n , , the responses evoked by any other a f f e r e n t pathway should a l s o have been reduced by a t r o p i n e . P h i l l i s and T e b e c i s [1967] s t u d i e d the e f f e c t s of i o n t o p h o r e t i c a p p l i c a t i o n of monoamines on t h a l a m i c neurones. The e f f e c t s of n o r a d r e n a l i n e (NA), a d r e n a l i n e , i s o p r e n a l i n e and 5-hydroxytryptamine (5HT) were mixed, some c e l l s being e x c i t e d , some depressed and others showing no a p p a r e n t . e f f e c t . C e l l s of the v e n t r o l a t e r a l group of n u c l e i (VBC and VL) were more o f t e n e x c i t e d by these drugs than c e l l s i n other r e g i o n s . Dopamine (DA) d i d not e x c i t e t halamic neurones,, but r a t h e r e i t h e r depressed' them or d i d not a l t e r t h e i r e x c i t a b i l i t y . An i n t e r e s t i n g o b s e r v a t i o n was t h a t a c l e a r s e p a r a t i o n of a and 3 - r e c e p t o r e f f e c t s was not p o s s i b l e from t h i s study. Rather, i t . a p p e a r e d as though the neurones might have both types or mixed r e p o r t s . T h i s i s s i m i l a r i n concept to the o b s e r v a t i o n s r e g a r d i n g the e f f e c t s of m u s c a r i n i c and n i c o t i n i c drugs on thalamic neurones. PARKINSON'S DISEASE That there i s some r e l a t i o n s h i p between the v e n t r o -l a t e r a l nucleus and Parkinson's d i s e a s e i s i n d i c a t e d by the e f f i c a c y o f . l e s i o n s t o VL i n r e l i e v i n g the tremor and r i g i d i t y t y p i c a l of Parkinsonism [Cooper, Samra and Bergmann, 1968]. The reason f o r the e f f e c t i v e n e s s of such l e s i o n s i s not known. I t has been thought important f o r the l e s i o n t o i n t e r r u p t the p a l l i d o - t h a l a m i c pathway, but Copper e t a l . f e e l t h a t the l e s i o n i s most e f f e c t i v e when l o c a l i z e d t o the p o s t e r i o r p a r t of VL, i n a p o s i t i o n i n t e r r u p t i n g both p a l l i d o -t h a l a m i c and c e r e b e l l o - t h a l a m i c pathways. Tremor can be induced i n monkeys by s e l e c t i v e l e s i o n s to the midbrain [ P o i r i e r , I960]. I f t h i s tremor i s accepted as a model of P a r k i n s o n i a n tremor, f u r t h e r evidence f o r an involvement of VL may be found. In such animals u n i t d i s c h a r g e s from s e v e r a l r e g i o n s of the nervous system have been.found t o f i r e i n synchrony with the tremor. A d m i n i s t r a t i o n of a c u r a r i z i n g agent causes t h i s synchronous a c t i v i t y to di s a p p e a r from some l o c a t i o n s , but the synchronous d i s c h a r g e remain i n VL and motor co r t e x . [Lamarre and Cordeau, 1964; Cordeau,,1966]. In addition,..rhythmic a c t i v i t y synchronous with tremor has been recorded i n VL and other t h a l a m i c nucle of P a r k i n s o n i a n p a t i e n t s . [Albe-Fessard, G u i o t , Lamarre and A r f e l , 1966; Jasper and B e r t r a n d , 1966]. I t i s p o s s i b l e t h e r e f o r e t h a t synchronous a c t i v i t y i n VL i s i n v o l v e d i n the g e n e r a t i o n of tremor of the P a r k i n s o n i a n type. A v a r i e t y of ph a r m a c o l o g i c a l agents has been success f u l l y used i n the treatment of Parkinsonism. Belladonna a l k a l o i d s were the f i r s t drugs to be used, and c h o l i n o l y t i c congeners of a t r o p i n e are s t i l l among the most popular t h e r a p e u t i c agents; but the l i s t now a l s o i n c l u d e s a n t i -h i s t a m i n e s , p h e n o t h i a z i n e s and L-dopa. L-dopa i s the only agent f o r which a reasonable p h a r m a c o l o g i c a l r a t i o n a l e can be proposed. Parkinson's d i s e a s e i s o f t e n a s s o c i a t e d w i t h a degeneration or depigmentation of c e l l s of the s u b s t a n t i a n i g r a [ G r e e n f i e l d and Bosanquet, 1953]. A dopaminergic n i g r s t r i a t a l . p a t h w a y has been p o s t u l a t e d on neurochemical [Sourkes & P o i r i e r , 1965] and h i s t o c h e m i c a l [Anden, Dahlstrom Fuxe and Larsson , 1965] grounds.- The corpus s t r i a t u m of P a r k i n s o n i a n p a t i e n t s has been,found t o c o n t a i n l e s s e r 50 amounts o f d o p a m i n e t h a n t h o s e . o f n o n - P a r k i n s o n i a n i n d i v i d u a l s [ E h r i n g e r a n d H o r n y k i e w i c z , I960]. I t h a s b e e n p r o p o s e d , t h e r e f o r e , t h a t L - d o p a c r o s s e s t h e b l o o d b r a i n b a r r i e r a f t e r , s y s t e m i c a d m i n i s t r a t i o n , i s c o n v e r t e d t o d o p a m i n e a nd s u p p l e -m ents t h e s t r i a t a l d o p a m i n e d e f i c i e n c i e s o f P a r k i n s o n i s n p a t i e n t s . The n e u r a l b a s i s f o r e f f e c t i v e n e s s o f t h e o t h e r g r o u p s o f d r u g s i s unknown, a s - i s t h e r e a s o n f o r t h e p o s i t i v e t h e r a p e u t i c r e s u l t s o b t a i n e d w i t h s u c h . w i d e l y d i f f e r e n t compounds. Ahmed and M a r s h a l l [19 62] s t u d i e d t h e l a t t e r q u e s t i o n u s i n g t r e m o r i n e i n d u c e d t r e m o r a s an a s s a y m o d e l f o r CNS a n t i -c h o l i n e r g i c p o t e n c y o f d r u g s . S e v e r a l d r u g s i n c l u d i n g a t r o p i n e and o t h e r a n t i c h o l i n e r g i c a g e n t s , p h e n o t h i a z i n e and a n t i -h i s t a m i n e s w e r e t e s t e d b o t h on t r e m o r i n e t r e m o r and c o m p e t i t i v e i n h i b i t i o n o f ACh a c t i o n o n g u i n e a p i g i l i e u m . T hey f o u n d a d e f i n i t e r e l a t i o n s h i p b e t w e e n t h e p o t e n c y o f t h e v a r i o u s d r u g s i n b l o c k a g e o f t h e e x p e r i m e n t a l t r e m o r , i n h i b i t i o n o f t h e p e r i p h e r a l e f f e c t s o f ACh and t h e a l l e v i a t i o n o f P a r k i n s o n i s m . I t i s p o s s i b l e t h e n t h a t t h e v a r i o u s d r u g s a r e e f f e c t i v e t o a l a r g e d e g r e e b e c a u s e o f t h e i r a n t i c h o l i n e r g i c c h a r a c t e r . . A f u r t h e r i n d i c a t i o n o f some r o l e o f ACh i n P a r k i n s o n i a n t r e m o r i s f o u n d i n t h e w o r k o f D u v o i s i n [1967], who f o u n d t h a t s y s t e m i c a l l y a d m i n i s t e r e d p h y s o s t i g m i n e e x -a c e r b a t e d m a n i f e s t a t i o n s o f P a r k i n s o n i s m . T h e s e e f f e c t s w e r e . p r o m p t l y r e v e r s e d by ' B e n z t r o p i n e o r s c o p o l a m i n e . C o y l e & S n y d e r [1969] on t h e o t h e r h a n d h a v e t r i e d t o r e l a t e , t h e e f f e c t i v e n e s s o f a n t i p a r k i n s o n i a n d r u g s t o t h e i r a b i l i t y t o b l o c k t h e u p t a k e o f d o p a m i n e i n t o s t r i a t a l s y n a p t o s o m a l T h e i r i n v i t r o s t u d i e s showed a v a r i e t y o f a n t i p a r k i n s o n i a n d r u g s t o be i n h i b i t o r s o f t h i s p r o c e s s . F u x e , G o l d s t e i n a n d L j u n g d a h l [1970] t e s t e d t h i s i d e a i n v i v o b y a s s e s s i n g by f l u o r e s c e n c e h i s t o c h e m i s t r y t h e u p t a k e o f . d o p a a nd a - m e t h y l - -NA i n t o t h e n e o s t r i a t u m i n c o n t r o l a n i m a l s and o n e s pre-r i n j e c t e d w i t h v a r i o u s a g e n t s u s e d i n t h e t r e a t m e n t o f . P a r k i n s o n i s m . T hey a l s o f o u n d t h a t a mine u p t a k e seemed p a r t i a l l y . b l o c k e d b y s e v e r a l o f t h e d r u g s , b u t some a n t i h i s t a m i n i c and a n t i c h o l i n e r g i c d r u g s , i n c l u d i n g a t r o p i n e d i d n o t seem e f f e c t i v e . I t seems p r o b a b l e ' t h e r e f o r e t h a t t h o u g h b l o c k i n g o f s t r i a t a l d o p a m i n e u p t a k e may h a v e some r o l e i n a n t i p a r k i n s o n i s m t h e r a p y , a n t i c h o l i n e r g i c a c t i v i t y i s i m p o r t a n t . I f i t i s assumed t h a t a n t i p a r k i n s o n d r u g s a c t a t l e a s t i n p a r t by b l o c k i n g c h o l i n e r g i c f u n c t i o n , t h e p r o b l e m o f t h e i r s i t e o f a c t i o n r e m a i n s . P o s s i b l y b e c a u s e o f t h e r e c e n t i n t e r e s t i n d o p a m i n e a nd t h e n i g r o - s t r i a t a l p a t h w a y and b e c a u s e . o f t h e h i g h c h o l i n e s t e r a s e c o n t e n t , a t t e n t i o n h a s b e e n f o c u s s e d on t h e c o r p u s s t r i a t u m . I n t e r e s t i n g e x p e r i m e n t s i n w h i c h s m a l l amounts o f d r u g s w e r e i n j e c t e d i n t o p a r t s o f t h e c o r p u s s t r i a t u m h a v e b e e n c a r r i e d . o u t b y N a s h o l d [1959] and L a l l e y , R o s s i and B a k e r [1970]. N a s h o l d f o u n d . t h a t ACh i n j e c t e d i n t o g l o b u s p a l l i d u s (GPj. o f P a r k i n s o n p a t i e n t s i n c r e a s e d c o n t r a l a t e r a l 52 t r e m o r , w h i l e t h e a n t i c h o l i n e r g i c a g e n t o x y p h e n o n i u m b r o m i d e r e d u c e d t r e m o r o r r i g i d i t y i n most c a s e s . L a l l e y e t a l . i n j e c t e d s m a l l amounts o f c h o l i n e s t e r a s e i n h i b i t o r s a nd c a r b a c h o l i n t o t h e c a u d a t e n u c l e u s o f a n e s t h e t i z e d c a t s , and f o u n d t h a t r e s t i n g t r e m o r c o u l d be p r o d u c e d . T r e m o r p r o d u c e d b y e i t h e r c h o l i n e s t e r a s e i n h i b i t o r s o r c a r b a c h o l was a n t a g o n i z e d by l o c a l m i c r o i n j e c t i o n s o f s c o p o l a m i n e . T h e r e a l s o ' seems r o o m . t o c o n s i d e r t h e p o s s i b i l i t y t h a t a n t i c h o l i n e r g i c a g e n t s may. e x e r t a p a r t a t l e a s t o f t h e i r a n t i - P a r k i n s o n i a n a c t i o n i n t h e t h a l a m u s . The g r e a t s e n s i -t i v i t y o f VL n e u r o n e s t o a p p l i c a t i o n o f ACh, t h e p r o p o s a l o f two c h o l i n e r g i c p a t h w a y s t o V L , t h e s u g g e s t e d r o l e o f VL as a p a c e m a k e r i n t h e p r o d u c t i o n o f t r e m o r and t h e e f f i c a c y o f VL l e s i o n s i n r e l i e v i n g P a r k i n s o n i a n t r e m o r and r i g i d i t y p r o v i d e p o s s i b l e s u p p o r t f o r t h i s i d e a . CHOLINESTERASE S t u d i e s o f c h o l i n e s t e r a s e d i s t r i b u t i o n i n t h e b r a i n h a v e shown t h a t t h e h i g h e s t c o n c e n t r a t i o n s , o f t h i s enzyme a r e f o u n d i n t h e n e o s t r i a t u m [ e . g . , B u r g e n and C h i p m a n , 1 9 5 1 ] . K o e l l e [1954] r e p o r t e d r e l a t i v e l y i n t e n s e s t a i n i n g i n t h e a n t e r i o r and l a t e r a l t h a l a m i c n u c l e i o f t h e r a t , b u t a r e c e n t s t u d y i n t h e monkey by O l i v i e r , P a r e n t and P o i r i e r [1970] has shown l o w s t a i n i n g d e n s i t y i n VA, V L , VPL and VPM. T h e s e authors r e p o r t e d , however,.that i n an area c o r r e s p o n s i n g to H a s s l e r ' s N. v e n t r o - o r a l i s w i t h i n VL ( i . e . , the v e n t r a l part) " . . . the i n t e n s i t y of the r e a c t i o n i s g e o m e t r i c a l l y accentuated . . ..." T h i s o b s e r v a t i o n i s of i n t e r e s t i n view of McCance e t a l . ' s r e p o r t t h a t a g r e a t e r p r o p o r t i o n of ACh s e n s i t i v e c e l l s was"found i n the v e n t r a l p a r t of VL. The i n t r a l a m i n a r n u c l e i are more densely s t a i n e d than surrounding r e g i o n s [ O l i v i e r , e t a l . ] and the l a t e r a l g e n i c u l a t e nucleus a l s o has enhanced s t a i n i n g p r o p e r t i e s [ P h i l l i s , T e becis and York, 1967; O l i v i e r , e t a l . ] . P h i l l i s [1968] has shown t h a t t h e r e i s a r e l a t i v e l y s m a l l amount of c h o l i n e s t e r a s e w i t h i n the BC, and t h a t the s t a i n i n g i s i n t e n s i f i e d on the c e r e b e l l a r s i d e of a l e s i o n of t h i s t r a c t , i n d i c a t i n g t h a t the s t a i n i n g i s a p r o p e r t y of c e r e b e l l a r e f f e r e n t f i b r e s . Hebb and S i l v e r [1956] found moderate amounts of c h o l i n e a c e t y l a s e i n the p a r t of the thalamus c o n t a i n i n g VL, but t h i s a l s o was very sma l l i n comparison w i t h the s t r i a t a l c ontent. I t would appear, then t h a t by these enzymic c r i t e r i a t here may be some c h o l i n e r g i c a f f e r e n t s to VL, but there i s not l i k e l y t o be a major c h o l i n e r g i c pathway. S i m i l a r l y , the BC may be c h o l i n e r g i c t o some degree. The purpose of the s e r i e s of experiments to be de-. s c r i b e d was to i n v e s t i g a t e the e l e c t r i c a l a c t i v i t y of c e l l s i n the thalamus, p a r t i c u l a r l y VL, to study t h e i r s e n s i t i v i t y to i o n t o p h o r e t i c a l l y a p p l i e d drugs, to d e f i n e the e l e c t r o -p h y s i o l o g i c a l c h a r a c t e r i s t i c s of some of the pathways a f f e r e n t to VL,-and to t e s t whether these pathways are c h o l i n e r g i c . 55 CHAPTER II METHODS PREPARATION OF ANIMALS The animals used i n these experiments were c a t s of e i t h e r sex weighing 2.5 - 3.5 Kgm, although on a few o c c a s i o n s i t was necessary t o use specimens o u t s i d e t h i s weight.range. The c a t s were f a s t e d f o r a minimum of s i x t e e n h o u r s . p r i o r to a n e s t h e s i a . Non-anesthetized d e c e r e b r a t e and a n e s t h e t i z e d animals were used f o r experimentation. A n e s t h e t i c agents used were: D i a l compound ( C i b a - d i a l l y l b a r b i t u r i c a c i d - 100 mgm/ml, ' urethane- 400 mgm/ml, monoethyl urea - 400 mgm/ml) 0.6 - 0.65 ml/kg. i n t r a p e r i t o n e a l l y ; sodium pentobarbital,25-30 mg/kg i n t r a -venously; or a- c h l o r a l o s e 60-70 mg/kg i n t r a v e n o u s l y . A n e s t h e s i a was maintained by p e r i o d i c s m a l l intravenous i n j e c t i o n s of the same a n e s t h e t i c agent. When p e n t o b a r b i t a l or c h l o r a l o s e was used, and i n p r e p a r a t i o n f o r d e c e r e b r a t i o n , a n e s t h e s i a was induced by i n t r o d u c i n g 4-5% halothane i n .oxygen (2-' Lv/min) v i a a Foregger model SA 10 v a p o u r i z e r i n t o a box h o l d i n g the animal. When the animal appeared to be deeply a n e s t h e t i z e d , i t was removed from the box, a n e s t h e s i a being maintained by a nose cone att a c h e d to the v a p o u r i z e r . A tracheotomy was p e r f o r m e d and a g l a s s Y - t u b e i n s e r t e d and s e c u r e d i n t o t h e t r a c h e a . The a n e s t h e s i a was c o n t i n u e d a t 2% , 1},Ly/min. by c o n n e c t i n g t h e v a p o u r i z e r d i r e c t l y t o one arm o f t h e Y - t u b e , t h e s e c o n d arm b e i n g c l a m p e d o f f . An i n d w e l l i n g i n t r a v e n o u s c a n n u l a was t h e n s e c u r e d i n t o a c e p h a l i c v e i n f o r i n t r o d u c t i o n o f o t h e r d r u g s . The h a l o t h a n e was d i s c o n t i n u e d w h i l e t h e s e c o n d a n e s t h e t i c a g e n t was s l o w l y i n t r o d u c e d t h r o u g h t h e I.V. c a n n u l a . A t t h i s s t a g e , t h e e a r b a r s w e re i n s e r t e d i n t o t h e e x t e r n a l a u d i t o r y . m e a t i , and t h e c a t m o u n t e d i n t h e s t e r e o t a x i c frame., When t h e a n i m a l was t o be d e c e r e b r a t e d , i t was m a i n -t a i n e d on 2% h a l o t h a n e a f t e r t r a c h e a l c a n n u l a t i o n , and m o u nted i n t h e s t e r e o t a x i c f r a m e . A f t e r a m i d l i n e s a g i t t a l i n c i s i o n , t h e s k u l l was e x p o s e d by s c r a p i n g b a c k t h e s k i n and t e m p o r a l m u s c u l a t u r e . A r e c t a n g u l a r s l o t was t h e n c u t , a c r o s s t h e m i d l i n e u s i n g a h a n d d r i l l f i t t e d w i t h a s m a l l r o u n d d e n t a l b u r r . A f t e r c u t t i n g t h e d u r a , a row o f s i x o r s e v e n 21 gauge s t a i n l e s s . s t e e l e l e c t r o d e s was l o w e r e d i n t o t h e b r a i n a t r i g h t a n g l e s t o t h e s a g g i t a l p l a n e . T h e s e p a r a l l e l e l e c t r o d e s w e r e s e t 2 mm. a p a r t ( F i g u r e 3 ) , and w e r e i n s u l a t e d ( I n s l - X , e-33 c l e a r ) e x c e p t f o r two mm. a t t h e . t i p o f e a c h e l e c t r o d e . The v e r t i c a l a r r a n g e m e n t o f t h e e l e c t r o d e t i p s c o r r e s p o n d e d a p p r o x i m a t e l y t o t h e s h a p e o f t h e s k u l l a t t h e b a s e o f t h e b r a i n . The e l e c t r o d e s w e r e l o w e r e d t o t h e b a s e o f t h e b r a i n , and a h i g h f r e q u e n c y c u r r e n t o f 20 mA was p a s s e d b e t w e e n t h e t i p s o f e a c h a d j a c e n t p a i r , - u s i n g a W y s s . c o a g u l a t o r , , m o d e l OC-60. The e l e c t r o d e s w e r e 57 t h e n r a i s e d b y 2 mm a n d t h e s e r i e s o f c o a g u l a t i o n s r e p e a t e d . I n t o t a l , c o a g u l a t i o n s w e r e made i n s e v e n v e r t i c a l l e v e l s a t 2 mm i n t e r v a l s . Two t y p e s o f d e c e r e b r a t i o n w e r e u s e d . The one m o s t f r e q u e n t l y u s e d was i n t h e f r o n t a l p l a n e , 1 mm a n t e r i o r t o t h e i n t e r - a u r a l l i n e . T h i s was s i m i l a r t o t h e method r e -c e n t l y p u b l i s h e d b y K o l l e r a n d J e n n y ( 1 9 6 9 ) . I n t h i s t y p e , i t was n e c e s s a r y , when c u t t i n g t h e r e c t a n g u l a r s l o t i n t h e s k u l l , f i r s t t o d r i l l two s m a l l s l o t s , one on«-either s i d e o f t h e m i d l i n e , t o a v o i d d a m a g i n g t h e s a g i t t a l s i n u s . The d u r a a n d s a g i t t a l s i n u s w e r e t h e n t e a s e d away f r o m t h e m i d -l i n e s e c t i o n o f b o n e c o n t a i n i n g t h e s a g i t t a l s u t u r e . T h i s r e m a i n i n g p i e c e o f bone was t h e n c a r e f u l l y r e m o v e d w i t h b one f o r c e p s . W i t h t h e s a g i t t a l s i n u s t h u s e x p o s e d , t h e d u r a was s l i t a t r i g h t a n g l e s t o t h e s i n u s f o r a d i s t a n c e o f a b o u t s i x mm o n e a c h s i d e , t o a v o i d c u t t i n g t h e s i n u s i t s e l f . The b a nk o f s i x e l e c t r o d e s was t h e n l o w e r e d i n t o t h e b r a i n t h r o u g h t h e c u t s i n t h e d u r a , w i t h t h e m i d d l e p a i r p a s s i n g c l o s e l y o n e i t h e r s i d e o f t h e s i n u s . The c o a g u l a t i o n p r o ^ c e d u r e was t h e n f o l l o w e d a s d e s c r i b e d p r e v i o u s l y . The o t h e r t y p e o f d e c e r e b r a t i o n u s e d was s i m i l a r t o t h e Pretrigeminal eat o f Z e r n i c k i ( 1 9 6 8 ) . I n t h i s c a s e a s i n g l e t r a n s v e r s e s l o t was c u t i n t h e b o n e o v e r l y i n g t h e • p o s t e r i o r f o s s a , j u s t p o s t e r i o r t o t h e t e n t o r i u m c e r e b e l l i . A s i n g l e s l i t was made i n t h e d u r a and a row o f s e v e n e l e c t r o d e s 58 l o w e r e d i n t o t h e b r a i n i n a p l a n e i n c l i n e d 30° p o s t e r i o r t o t h e v e r t i c a l . The e l e c t r o d e s p a s s e d b e h i n d t h e t e n t o r i u m and r o u g h l y p a r a l l e l t o i t . W i t h b o t h t y p e s o f d e c e r e b r a t i o n t h e h a l o t h a n e a n e s t h e s i a was m a i n t a i n e d u n t i l t h e r e s t o f t h e s u r g e r y was c o m p l e t e d , t h e n d i s c o n t i n u e d . A p e r i o d o f two h o u r s was r o u t i n e l y a l l o w e d t o e l a p s e b e f o r e e x p e r i m e n t a l r e c o r d s w e r e made. Once t h e d e c e r e b r a t i o n h a d b e e n c o m p l e t e d , o r s u r g i c a l a n e s t h e s i a w i t h t h e m a j o r a n e s t h e s i c h a d b e e n a c h i e v e d , t h e f o l l o w i n g s u r g i c a l p r o c e d u r e s w e r e f o l l o w e d - . By r e m o v i n g t h e bone w i t h d r i l l a n d bone f o r c e p s , t h e c o r t e x on one s i d e o f t h e b r a i n was e x t e n s i v e l y e x p o s e d , f r o m t h e f r o n t a l p o l e t o t h e i n t e r - a u r a l l i n e , a n d f r o m n e a r t h e m i d l i n e l a t e r a l l y f o r a b o u t 15 mm. P a r t i c u l a r l y when t h e more b u l k y g l a s s e l e c t r o d e s w e r e t o be u s e d , t h e bone o v e r l y i n g t h e s a g i t t a l s i n u s a n d a few mm. o f t h e a d j a c e n t c o n t r a l a t e r a l c o r t e x was r e m o v e d , f r o m t h e l e v e l o f t h e i n t e r - a u r a l l i n e t o a b o u t 16 mm. a n t e r i o r t o i t . The d u r a was t h e n s l i t o r r e m o v e d i n r e g i o n s w h e r e r e c o r d i n g and s t i m u l a t i n g e l e c t r o d e s w e r e t o be i n t r o d u c e d . B l e e d i n g f r o m d u r a o r b r a i n was c o n t r o l l e d by a p p l y i n g s m a l l , p l e d g e t s o f G e l f o a m ( U p j o h n ) w h i l e b l e e d i n g f r o m bone was s t o p p e d w i t h bone. wax. The c o r t e x was k e p t m o i s t by c o v e r i n g d u r a and e x p o s e d . c o r t e x w i t h s m a l l c o t t o n b a l l s s o a k e d i n warmed m o d i f i e d L o c k e ' s . s o l u t i o n . When g l a s s e l e c t r o d e s w e r e t o be u s e d , t h e c o r t i c a l g r e y , and w h i t e m a t t e r o v e r l y i n g t h e p a r t o f t h e t h a l a m u s b e i n g s t u d i e d 59 was r e m o v e d b y s u c t i o n , a f t e r c o a g u l a t i o n o f t h e s u r f a c e v e s s e l s w i t h a ^ B i r t c h e r m o d e l 755 c o a g u l a t o r . The c o r t i c a l m a t t e r was r e m o v e d t o a v o i d b r e a k a g e and t i s s u e p l u g g i n g o f e l e c t r o d e t i p s i n c u r r e d by. p u s h i n g t h e e l e c t r o d e t h r o u g h t h e a p p r o x i m a t e l y 10 mm o f c o r t i c a l t i s s u e o v e r l y i n g t h e t h a l a m u s . The r e s u l t i n g c a v i t y was k e p t p a r t l y f i l l e d w i t h warmed L o c k e ' s s o l u t i o n , e x c e p t w h i l e t h e r e c o r d i n g e l e c t r o d e p o s i t i o n was b e i n g c h a n g e d t o a new t r a c k . T h r o u g h o u t t h e e x p e r i m e n t , t h e r e c t a l t e m p e r a t u r e o f t h e a n i m a l was m o n i t o r e d by an EKEG TCU 691 t e m p e r a t u r e c o n t r o l u n i t . The t e m p e r a t u r e was m a i n t a i n e d a t 37-38°C by a 6 V b a t t e r y - s u p p l i e d h e a t i n g p a d p l a c e d u n d e r t h e a n i m a l . L a t e r i n t h e e x p e r i m e n t a l s e r i e s , when a Beckman m o d e l LB-1 M e d i c a l Gas A n a l y z e r became a v a i l a b l e , e n d - t i d a l CC>2 l e v e l s i n t h e t r a c h e a l t u b e w e r e m o n i t o r e d t h r o u g h o u t t h e e x p e r i m e n t . STEREOTAXIC PROCEDURE A s t e r e o t a x i c i n s t r u m e n t ( P r e c i s i o n C i n e m a t o g r a p h i q u e ) was u s e d i n a l l e x p e r i m e n t s . T h i s i n s t r u m e n t u s e s t h e e x t e r n a l a u d i t o r y m e a t i and l o w e r o r b i t f o r h e a d f i x a t i o n r e f e r e n c e p o i n t s . S t e e l e l e c t r o d e s t o be u s e d i n t h e e x p e r i m e n t w e r e c a l i b r a t e d b e f o r e h a n d w i t h r e f e r e n c e t o t h e i n t e r - a u r a l l i n e a t t h e m i d l i n e p o s i t i o n . The c o - o r d i n a t e s o f t h e t a r g e t s t r u c t u r e s w e r e d e t e r m i n e d f r o m t h e s t e r e o -t a x i c a t l a s of Snider and Niemer [1961] but o f t e n w i t h a d d i t i o n a l r e f e r e n c e to the a t l a s e s of Jasper and Ajmone-Marsan [1954] and Reinoso-Suarez [1961]. The proper p o s i t i o n f o r the e l e c t r o d e t i p was then c a l c u l a t e d , a l g e b r a i c a l l y i f the e l e c t r o d e was used i n a v e r t i c a l a x i s , or t r i g o n o m e t r i c a l l y i f used a t some angle to the v e r t i c a l . Because the c a l i b r a t i o n marks on the instrument were found to vary up to 1° from t h e i r d e s i g n a t e d v a l u e s , the e l e c t r o d e s were c a l i b r a t e d w i t h r e f e r e n c e to a plumb l i n e , a f t e r p l a c i n g the frame i n the h o r i z o n t a l plane w i t h the a i d of a s p i r i t . l e v e l . I t was found t h a t t h i s technique allowed a more p r e c i s e placement of e l e c t r o d e s , and p e r m i t t e d a l l e l e c t r o d e s to be c a l i b r a t e d a c c o r d i n g t o the same standard. In s p i t e of t h i s , the s t e r e o t a x i c method was not r e l i a b l e enough t o ensure placement of e l e c t r o d e s i n s m a l l t a r g e t s t r u c t u r e s such as the entopeduncular nucleus and N centrum medianum. Whenever p o s s i b l e t h e r e f o r e , the p o s i t i o n of the e l e c t r o d e was checked by m o n i t o r i n g e l e c t r i c a l a c t i v i t y a t the t i p of the e l e c t r o d e as i t was lowered i n t o p o s i t i o n , , u s i n g the nature of the spontaneous e l e c t r i c a l a c t i v i t y and responses to a c t i v a t i o n of known n e u r a l pathways as a guide. 61 ELECTRODES One or both of two types of s t i m u l a t i n g e l e c t r o d e s were used i n most experiments. A p a r a l l e l p a i r of s t e e l e l e c t r o d e s w i t h 2 mm. s e p a r a t i o n was u s u a l l y used f o r s t i m u l a t i o n of the c e r e b r a l cortex and deep c e r e b e l l a r n u c l e i . These e l e c t r o d e s were c o n s t r u c t e d from 23 gauge needle t u b i n g and i n s u l a t e d w i t h Insl-X,.except f o r 0 . 5 - 1 mm. a t the t i p . Square-wave s t i m u l i of 0.1 msec, d u r a t i o n and 7-10 V amplitude were used. T h i s type of e l e c t r o d e was a l s o used i n i t i a l l y f o r s t i m u l a t i o n of EN and BC, however, i t became apparent t h a t the chances of p l a c i n g the t i p s of both e l e c t r o d e s w i t h i n the d e s i r e d s t r u c t u r e were s l i m . I t a l s o seems probable t h a t c u r r e n t spread from t h i s type of e l e c t r o d e to adjacent n u c l e i and pathways may be s u f f i c i e n t to cause u n i n t e n t i o n a l a c t i v a t i o n of pathways other than the d e s i r e d one. In order t o minimize t h i s problem, s t i m u l a t i o n of EN, CM, BC and VL was e f f e c t e d by c o n c e n t r i c b i p o l a r e l e c t r o d e s . These e l e c t r o d e s were c o n s t r u c t e d by t h r e a d i n g f i n e t e f l o n -coated s t a i n l e s s s t e e l wire through 23 gauge s t a i n l e s s s t e e l t u b i n g u n t i l the wire extended 1/2 - 1 mm beyond the t i p of the t u b i n g . The other end of the wire was l e d through an opening which had been ground i n the s i d e of the t u b i n g . An Amphenol microconnector was then s o l d e r e d t o t h i s end of the w i r e , and the wire f i x e d i n p l a c e by a p p l y i n g epoxy cement a t the opening i n the t u b i n g (Figure 4 ) . The o u t s i d e 6 1 a F i g u r e 4. C o n c e n t r i c b i p o l a r s t i m u l a t i n g e l e c t r o d e . o f t h e e l e c t r o d e b e l o w t h i s j u n c t i o n was c o a t e d w i t h I n s l - X w h i c h , a f t e r d r y i n g was s c r a p e d b a c k 1/2 mm o r l e s s f r o m t h e t i p . S i m i l a r l y , t h e t e f l o n i n s u l a t i o n was remo v e d f r o m t h e t i p o f t h e i n n e r e l e c t r o d e . V o l t a g e s u s e d w i t h t h e s e e l e c t r o d e s v a r i e d w i t h t h e s t r u c t u r e s t i m u l a t e d , b u t were u s u a l l y i n t h e 4-8 V r a n g e . The c u r r e n t i n t e n s i t y o f s t i m u l i t h r o u g h t h e two t y p e s o f e l e c t r o d e w e r e m e a s u r e d on a few o c c a s i o n s u s i n g a T e k t r o n i x Current-Probe.„ The c u r r e n t s c o r r e s p o n d i n g t o t h e u s u a l v o l t a g e r a n g e w e r e 0.05 - 0.1 mA f o r t h e c o n c e n t r i c a n d 1.0 - 1.5 mA f o r t h e p a r a l l e l e l e c -t r o d e s . I n a l l c a s e s s t i m u l i o f 0.1 msec, d u r a t i o n w e r e u s e d . The r e c o r d i n g e l e c t r o d e s w e r e a l s o o f two t y p e s ; s t e e l n e e d l e s a nd e l e c t r o l y t e - f i l l e d g l a s s c a p i l l a r i e s . The s t e e l e l e c t r o d e s w e r e made f r o m s i z e 10 b e a d i n g n e e d l e s b y e r o d i n g t h e s h a f t w i t h p h o s p h o r i c a c i d , i n a p r o c e d u r e : s i m i l a r t o t h a t d e s c r i b e d b y G r e e n [ 1 9 5 8 ] . When t h e n e e d l e h a d b e e n e t c h e d t o t h e d e s i r e d d i a m e t e r and s h a p e , i t was w a s h e d i n d i s t i l l e d w a t e r , d r i e d , and t h e n i n s u l a t e d . The i n s u l a t i o n was e f f e c t e d by l o w e r i n g t h e n e e d l e v e r t i c a l l y i n t o a b e a k e r f i l l e d w i t h I n s l - X , w i t h t h e t i p o f t h e e l e c -t r o d e u p p e r m o s t . U n d e r o b s e r v a t i o n u s i n g a Z e i s s o p e r a t i n g m i c r o s c o p e , t h e e l e c t r o d e was l o w e r e d u n t i l o n l y a s m a l l amount a t t h e t i p was n o t i m m e r s e d . The e l e c t r o d e was t h e n s l o w l y and s t e a d i l y r a i s e d , l e a v i n g a t h i n , e v e n c o a t o f i n s u l a t i o n o v e r a l l b u t t h e t i p o f t h e e l e c t r o d e . The i n s u l a t i o n was d r i e d b y l e a v i n g f o r 24 h o u r s a t room t e m p e r a -t u r e o r by b a k i n g a t 70°C f o r one h o u r . The u n i n s u l a t e d r e g i o n a t t h e t i p v a r i e d i n s i z e , d e p e n d i n g on t h e p u r p o s e o f t h e e l e c t r o d e . When i t was t o be u s e d f o r r e c o r d i n g f i e l d r e s p o n s e s , , t h e t i p was b a r e f o r 200-300 y. F o r r e c o r d i n g o f s i n g l e u n i t a c t i v i t y o n l y 2 0 - 4 0 y was l e f t u n i n s u l a t e d . A s i m i l a r i n s u l a t i n g p r o c e d u r e was f o l l o w e d f o r t h e d e c e r e b r a t i n g and s t i m u l a t i n g e l e c t r o d e s , e x c e p t t h a t t h e s e e l e c t r o d e s w e r e l o w e r e d i n t o t h e I n s l - X w i t h t i p s down, t h e i n s u l a t i o n b e i n g s c r a p e d b a c k f r o m t h e t i p a f t e r d r y i n g . The g l a s s r e c o r d i n g e l e c t r o d e s u s e d w e r e t h e c e n t r e b a r r e l s o f 5 - b a r r e l l e d g l a s s e l e c t r o d e a s s e m b l i e s , t h e o u t e r b a r r e l s b e i n g u s e d f o r i o n t o p h o r e t i c a p p l i c a t i o n o f c h e m i c a l s u b s t a n c e s . The g l a s s e l e c t r o d e b l a n k s w e r e s u p p l i e d b y V a n c o u v e r S c i e n t i f i c i G l a s s b l o w i n g a s an a r r a y o f f i v e f u s e d g l a s s c a p i l l a r i e s , d r a w n o u t t o an o v e r a l l d i a m e t e r o f 2.5-4 mm. ( F i g u r e 5 a ) . T h e s e b l a n k s w e r e h e a t e d and p u l l e d i n a v e r t i c a l C a n b e r r a - t y p e m i c r o e l e c t r o d e p u l l e r . The r e s u l t i n g t i p s o f 0.5 - 1 y d i a m e t e r w e r e b r o k e n b a c k , u n d e r m i c r o s c o p i c o b s e r v a t i o n , t o a t o t a l d i a m e t e r o f 4-8 y. ( F i g u r e 5 b ) . The e l e c t r o d e s w e r e t h e n f i l l e d by b o i l i n g i n f i l t e r e d d i s t i l l e d w a t e r f o r 20 m i n u t e s . A f t e r c o o l i n g , t h e d i s t i l l e d w a t e r was r e m o v e d f r o m t h e w i d e p a r t o f t h e s h a f t w i t h a s y r i n g e and 2 9 gauge n e e d l e > and r e p l a c e d by, an e l e c t r o l y t e (4 M N a C l < f o r t h e r e c o r d i n g b a r r e l ) . The 64 s o l u t i o n s d i f f u s e d t o t h e t i p w i t h i n 24-48 h o u r s . D u r i n g e x p e r i m e n t s , e l e c t r i c a l c o u p l i n g f r o m t h e s o l u t i o n s t o t h e r e c o r d i n g a n d i o n t o p h o r e t i c a p p a r a t u s was a c h i e v e d t h r o u g h a p l a t i n u m a n d s i l v e r w i r e r e s p e c t i v e l y e x t e n d i n g i n t o t h e s o l u t i o n . The w i r e s w e r e s o l d e r e d t o c o p p e r l e a d s w h i c h w e r e c o n n e c t e d t o t h e a m p l i f i e d p r o b e o r i o n t o p h o r e t i c p a n e l . S t e e l r e c o r d i n g e l e c t r o d e s w e r e h e l d i n a s m a l l s t e e l c h u c k f i t t e d t o a N a r a s h i g e m i c r o m a n i p u l a t o r w h i c h was m o d i f i e d f o r u s e w i t h t h e s t e r e o t a x i c f r a m e . A l a r g e r N a r a -s h i g e m i c r o m a n i p u l a t o r m o u n t e d on a b a r a t t a c h e d t o t h e f r a m e was u s e d f o r h o l d i n g a n d m o v i n g t h e g l a s s e l e c t r o d e s . B e c a u s e t h e movement p l a n e s o f t h e l a t t e r e l e c t r o d e h o l d e r w e r e i n d e p e n d e n t o f t h e s t e r e o t a x i c f r a m e , r e s p o n s e s w e r e r o u t i n e l y l o c a t e d f i r s t w i t h s t e e l e l e c t r o d e s . A f t e r f i n d i n g t h e d e s i r e d r e s p o n s e , t h e g l a s s e l e c t r o d e was o r i e n t e d i n t h e same a x i s and l o w e r e d t o t h e d e p t h o f t h e r e s p o n s e s . F u r t h e r e x p l o r a t i o n was c a r r i e d o u t w i t h r e f e r e n c e t o t h i s i n i t i a l p o s i t i o n o f t h e g l a s s e l e c t r o d e . F o r i o n t o p h o r e s i s , d r u g s w e r e d i s s o l v e d , u s u a l l y i n d i s t i l l e d w a t e r , a n d f i l t e r e d t h r o u g h a m i l l i p o r e f i l t e r . The pH o f t h e a m i n o a c i d s o l u t i o n s was a d j u s t e d u p w a r d b y a d d i n g NaOH t o t h e s o l u t i o n w h i l e c h e c k i n g t h e pH w i t h a R a d i o m e t e r pH m e t e r - 25. Some o f t h e p o t e n t b l o c k i n g a g e n t s w e r e u s e d i n v e r y l o w c o n c e n t r a t i o n s , a n d d i s s o l v e d i n 0.15 M N a C l t o a v o i d e x c e s s i v e i m p e d a n c e s f o r t h e s o l u t i o n s a n d t o a l l o w i o n t o p h o r e s i s o f s m a l l d r u g q u a n t i t i e s . T a b l e I l i s t s t h e d r u g s u s e d a n d t h e c h a r a c t e r i s t i c s o f e a c h s o l u t i o n . B e f o r e 65 TABLE I DRUG SOLUTIONS USED FOR IONTOPHORESIS D r u g C o n c e n -t r a t i o n S o l v e n t pH A d j u s t e d To A c t i v e J I o n A c e t y l c h o l i n e b r o m i d e 1.0 M w a t e r c a t i o n A c e t y l c h o l i n e c h l o r i d e 1.0 M w a t e r - c a t i o n A t r o p i n e s u l p h a t e 0.05 M 0.15 M N a C l — • c a t i o n D i h y d r o - 3 - e r y t h r o i d i n e 0.025 M 0.15 M N a C l -c a t i o n S t r y c h n i n e s u l p h a t e 0.025 M 0.15 M N a C l -c a t i o n H y d r o c h l o r i c a c i d 0.1 N w a t e r - c a t i o n L - g l u t a m i c a c i d 0.2 M w a t e r 8-8.5 a n i o n D - g l u t a m i c a c i d 0.2 M w a t e r 8-8.5 a n i o n L - a s p a r t i c a c i d 0; .2 M w a t e r 8-8.5 a n i o n D, L - h o m o c y s t e i c a c i d 0.2 M w a t e r 8-8.5 a n i o n N-methyl-D,'. L - a s p a r t i c a c i d 0.2 M w a t e r 8-8.5 ,, a n i o n a - m e t h y l - D , L - g l u t a m i c a c i d 0.2 M w a t e r 8-8.5 a n i o n N - m e t h y l - D , . L - g l u t a m i c a c i d 0.2 M w a t e r 8-8.5 a n i o n u s e o f g l a s s e l e c t r o d e s , t h e DC r e s i s t a n c e o f e a c h b a r r e l was m e a s u r e d . The u s u a l r a n g e s w e r e 5-12 M Q f o r r e c o r d i n g b a r r e l s a n d 70-90 M Q, f o r i o n t o p h o r e s i s b a r r e l s . D u r i n g u s e o f g l a s s e l e c t r o d e s , r e t a i n i n g c u r r e n t s w e r e a p p l i e d t o e a c h o f t h e d r u g c o n t a i n i n g b a r r e l s t o impede d i f f u s i o n o f t h e a c t i v e i o n f r o m t h e t i p . Thus f o r a s o l u t i o n o f ACh B r . , a n e g a t i v e c u r r e n t was a p p l i e d t o h o l d b a c k t h e p o s i t i v e l y c h a r g e d ACh i o n . C u r r e n t s u s e d t o r e t a i n o r p a s s d r u g s i o n t o p h o r e t i c a l l y w e r e m e a s u r e d on a C a m b r i d g e g a l v a n o m e t e r and w e r e p a s s e d t o t h e b a r r e l s o f t h e e l e c t r o d e t h r o u g h 1000 M Q s e r i e s r e s i s t o r s . R e l a t i v e l y s m a l l v a r i a t i o n s i n r e s i s t a n c e a t t h e e l e c t r o d e t i p s t h e r e f o r e d i d n o t a p p r e c i a b l y a l t e r t h e c u r r e n t f l o w . E x c i t a t o r y e f f e c t s o c c u r r i n g w i t h v e r y s h o r t l a t e n c y a f t e r i o n t o p h o r e t i c a p p l i c a t i o n o f d r u g s and d e p r e s s a n t e f f e c t s w e r e c h e c k e d b y e j e c t i n g a n o n - a c t i v e i o n w i t h t h e same p o l a r i t y f r o m a d i f f e r e n t d r u g b a r r e l . T h i s p r o c e d u r e p e r m i t s s e p a r a t i o n o f d r u g e f f e c t s f r o m t h o s e due t o t h e c u r r e n t u s e d i n p a s s i n g t h e d r u g s . EQUIPMENT The e l e c t r i c a l . s i g n a l s r e c o r d e d f r o m t h e b r a i n w e r e l e d i n t o a G r a s s P6-12 p r e a m p l i f i e r . The p r e a m p l i f i e r o u t p u t s i g n a l was f u r t h e r a m p l i f i e d i n t h e t y p e 2A d i f f e r e n t i a l a m p l i f i e r p l u g - i n u n i t o f a T e k t r o n i x m o d e l 565 d u a l . b e a m o s c i l l o s c o p e . P h o t o g r a p h i c r e c o r d s w e r e made b y a G r a s s C4 c a m e r a f r o m t h e s c r e e n o f t h e T e k t r o n i x 5 6 5. A p a r a l l e l o s c i l l o s c o p e ( T e l e q u i p m e n t t y p e D43R) was a l s o u s e d t o m o n i t o r t h e s i g n a l s b e i n g p h o t o g r a p h e d . The a m p l i f i e d s i g n a l s f r o m t h e T e k t r o n i x s c o p e w e r e a l s o f e d t o a l o u d s p e a k e r and a r a t e m e t e r (EKEG m o d e l RT 6 8 2 - L ) , t h e i n t e g r a t e d r a t e m e t e r o u t p u t b e i n g r e c o r d e d on an E s t e r l i n e - A n g u s R e c t i l i n e a r R e c o r d e r m o d e l AGOIR. The t r i g g e r i n g l e v e l o f t h e r a t e m e t e r was v a r i a b l e s o t h a t i f t h e a c t i o n p o t e n t i a l s o f more t h a n one c e l l w e r e r e c o r d e d by t h e e l e c t r o d e , d i f f e r e n c e s i n t h e s i z e s o f t h e a c t i o n p o t e n t i a l s u s u a l l y p e r m i t t e d s e l e c t i o n o f t h e s i g n a l s . Thus t h e a c t i v i t y o f a s i n g l e c e l l c o u l d be r e p r e s e n t e d i n t h e r a t e m e t e r o u t p u t s i g n a l . F i g u r e 6 i s t h e b l o c k d i a g r a m d e p i c t i n g t h e i o n t o p h o r e s i s and r e c o r d i n g s y s t e m . As h a s b e e n p r e v i o u s l y i n d i c a t e d , " p r o b l e m s w e re e n c o u n t e r e d i n t h e s e l e c t i v e e l e c t r i c a l s t i m u l a t i o n o f some b r a i n s t r u c t u r e s , p a r t i c u l a r l y t h e s m a l l e r o n e s EN and CM. I n t h e c a s e o f CM,.a r e s p o n s e c h a r a c t e r i s t i c f o r t h e l a t e r a l p a r v o c e l l u l a r p a r t was f o u n d t o be e v o k e d b y s t i m u l a t i o n s o f t h e s e n s o r i m o t o r c o r t e x , a s w i l l be d e s c r i b e d i n . t h e s e c t i o n ' R e s u l t s ' . A f t e r t h i s d i s c o v e r y , s t i m u l a t i n g e l e c t r o d e s w e r e p l a c e d a c c o r d i n g t o s t e r e o t a x i c c o o r d i n a t e s and t h i s p o s i t i o n c h e c k e d b y s t i m u l a t i n g t h e c o r t e x ; t o s e e i f t h e a p p r o p r i a t e r e s p o n s e c o u l d be, r e c o r d e d f r o m t h e t i p o f t h e e l e c t r o d e s . The p o s i t i o n o f t h e e l e c t r o d e was t h e n a d j u s t e d t o o b t a i n t h e maximum r e s p o n s e . F o r s t i m u l a t i o n o f CM, a p a r a l l e l p a i r o f c o n c e n t r i c b i p o l a r e l e c t r o d e s was u s e d . One o f t h e s e was p l a c e d i n t h e p a r v o c e l l u l a r CM by t h e a b o v e m e t h o d , w i t h t h e o t h e r l o c a t e d 1.5 mm. m e d i a l t o i t . T h i s a l l o w e d c o m p a r i s o n o f t h e e f f e c t s o f s t i m u l a t i n g t h e l a t e r a l a n d m e d i a l p a r t s o f t h e CM - P f c o m p l e x . T h i s 68 F i g u r e 5. F i v e - b a r r e l e l e c t r o d e u s e d f o r i o n t o p h o r e s i s . (A) e l e c t r o d e b l a n k , (B) e l e c t r o d e d r a w n t o f i n e t i p . F i g u r e 6. B l o c k d i a g r a m f o r i o n t o p h o r e s i s a nd s i g n a l r e c o r d i n g e q u i p m e n t . p r o c e d u r e was f o u n d t o g i v e a l s o an e s t i m a t e o f t h e maximum amount o f e f f e c t i v e c u r r e n t s p r e a d f r o m t h e c o n c e n t r i c b i p o l a r e l e c t r o d e s . The e n t o p e d u n c u l a r n u c l e u s was t h e most d i f f i c u l t s t r u c t u r e t o l o c a t e and t o s t i m u l a t e d i s c r e t e l y . T h i s n u c l e u s l i e s a t t h e v e n t r a l b o r d e r o f p a r t o f t h e i n t e r n a l c a p s u l e , and b u n d l e s o f i n t e r n a l c a p s u l e f i b r e s a p p e a r t o c o u r s e t h r o u g h t h e n u c l e u s . J u s t b e l o w EN l i e s t h e o p t i c t r a c t . T h o u g h no c h a r a c t e r i s t i c e v o k e d r e s p o n s e c o u l d be i d e n t i f i e d i n EN, i t was f o u n d t h a t t h e s t i m u l a t i n g e l e c t r o d e c o u l d be p l a c e d a c c u r a t e l y a f t e r f i n d i n g t h e r e s p o n s e s c h a r a c t e r i s t i c o f t h e i n t e r n a l c a p s u l e and o p t i c t r a c t . The EN r e g i o n showed a v e r y s m a l l e v o k e d r e s p o n s e t o s t i m u l a t i o n o f t h e p r e c r u c i a t e c o r t e x [ s e e a l s o F r i g y e s i and M a c h e k , 1 9 7 0 ] , w h e r e a s t h e i n t e r n a l c a p s u l e a b o v e i t g a v e a l a r g e , s h o r t l a t e n c y r e s p o n s e . V e n t r a l t o EN, s p o n t a n e o u s a c t i v i t y w h i c h r e s p o n d e d t o f l a s h i n g a l i g h t i n t h e a n i m a l s e y e s r e p r e s e n t e d o p t i c t r a c t a c t i v i t y . The e l e c t r o d e t i p was p o s i t i o n e d , t h e r e f o r e , i n t h e e l e c t r i c a l l y q u i e t z o n e b e t w e e n t h e i n t e r n a l c a p s u l e and o p t i c t r a c t r e s p o n s e s . HISTOLOGY 70 A t t h e e n d o f an e x p e r i m e n t , t h e a n i m a l was k i l l e d w i t h a n o v e r d o s e o f p e n t o b a r b i t a l . The h e a d was p e r f u s e d t h r o u g h t h e c a r o t i d a r t e r i e s , . f i r s t w i t h 100 m l 0.15 M Na C I , , t h e n w i t h 200 m l o f 10% f o r m a l i n i n 0.15 M Na C I c o n t a i n i n g 1% p o t a s s i u m f e r r o c y a n i d e . W i t h t h e a n i m a l ; s t i l l m o u nted i n t h e s t e r e o t a x i c h o l d e r , t h e d o r s o l a t e r a l s u r f a c e s o f t h e b r a i n w e r e e x p o s e d and t h e b r a i n was c u t w i t h a s h a r p e n e d p a l l e t - k n i f e b l a d e m ounted on an e l e c t r o d e c a r r i e r a n d a l i g n e d i n t h e f r o n t a l p l a n e . Thus b l o c k s w e r e c u t s o t h a t s e c t i o n s w o u l d be c l o s e l y p a r a l l e l t o e l e c t r o d e t r a c k s w i t h t h e i r a x i s i n t h e f r o n t a l p l a n e . E l e c t r o d e t r a c k s w e r e o f t e n s e e n i n h i s t o l o g i c a l s e c t i o n s , b u t a c c u r a t e l o c a l i z a t i o n o f p r e v i o u s l y r e c o r d e d s i g n a l s r e q u i r e d r e f e r e n c e t o s p o t s m a r k i n g some o f t h e e l e c t r o d e t r a c k s . The s t i m u l a t i n g and s t e e l r e c o r d i n g e l e c t r o d e t r a c k s w e r e m a r k e d b y t h e method d e s c r i b e d by G r e e n [1958], P o s i t i v e c u r r e n t was p a s s e d (20 yA f o r 30 s e c . ) t h r o u g h t h e e l e c t r o d e w h i l e i t was i n t h e p o s i t i o n t o be m a r k e d . I r o n i s e l e c t r o d e p o s i t e d o n l y a t t h e u n i n s u l a t e d e l e c t r o d e t i p , a n d l a t e r c o m p l e x e s w i t h t h e f e r r o c y a n i d e ( P r u s s i a n b l u e r e a c t i o n ) t o l e a v e a d i s t i n c t b l u e - g r e e n s p o t w h i c h i s e a s i l y l o c a t e d i n t h e u n s t a i n e d b r a i n . F i g u r e 7 shows a n e x a m p l e o f s u c h m a r k s . The method o f McCance and P h i l l i s [196 5] was u s e d f o r m a r k i n g t h e g l a s s e l e c t r o d e t r a c k s . F o r t h i s p u r p o s e F i g u r e 8. S p o t m a r k i n g t i p l o c a t i o n o f g l a s s i o n t o p h o r e t i c e l e c t r o d e — H C 1 m e t h o d . 72 0.1 N H C I was u s u a l l y c a r r i e d i n one o f t h e f o u r i o n t o p h o r e s i s b a r r e l s . A t t h e e n d o f . t h e e x p e r i m e n t , o r w h e r e a s p e c i f i c s i t e was t o be m a r k e d , p o s i t i v e c u r r e n t (4 yA f o r 30 s e c . ) was p a s s e d t h r o u g h t h e H C I c o n t a i n i n g b a r r e l . T h e s e m a r k s w e r e more d i f f i c u l t t o l o c a t e b e c a u s e t h e y w e r e n o t u s u a l l y d e t e c t e d i n t h e u n s t a i n e d s e c t i o n s . O f t e n t h e r e f o r e , a f t e r t h e H + h a d b e e n p a s s e d i o n t o p h o r e t i c a l l y , a s t e e l e l e c t r o d e was i n s e r t e d and m a r k e d n e a r b y ( e . g . 1 mm. l a t e r a l t o t h e g l a s s e l e c t r o d e t r a c k . ) t o f a c i l i t a t e l o c a t i o n o f t h e a c i d s p o t s w h i c h w e r e u s u a l l y d e t e c t a b l e i n t h e s t a i n e d s e c t i o n s . F i g u r e 8 shows a n e x a m p l e o f an a c i d mark. Two t y p e s o f h i s t o l o g i c a l p r e p a r a t i o n s , w e r e u s e d . When o n l y g r o s s s t e e l r e c o r d i n g e l e c t r o d e s h a d b e e n u s e d o r when i t was i m p o r t a n t t o o b t a i n t h e r e s u l t s r a p i d l y , a t e c h n i q u e s i m i l a r t o t h a t d e s c r i b e d . b y . Guzman, A l c a r a z and F e r n a n d e z [1958] was u s e d . I n t h i s p r o c e d u r e , t h e b l o c k s o f b r a i n w e r e f i x e d o v e r n i g h t i n 10% f o r m a l i n a nd ' s e c t i o n e d on a f r e e z i n g m i c r o t o m e t h e n e x t d a y . S e c t i o n s o f 50 y - 100 y w e r e c u t , and u s u a l l y showed c l e a r l y t h e e l e c t r o d e t r a c k s and P r u s s i a n b l u e s p o t s . S e c t i o n s w e re p h o t o g r a p h e d f o r p e r m a n e n t r e c o r d s b y p l a c i n g them b e t w e e n two m i c r o s c o p e s l i d e s , m o u n t i n g i n an e n l a r g e r and p r i n t i n g as f r o m a n e g a t i v e . An e x a m p l e o f s u c h a p h o t o g r a p h i s shown i n F i g u r e 9. The s e c o n d h i s t o l o g i c a l m e t hod was u s u a l l y e m p l o y e d when g l a s s e l e c t r o d e s and a c i d m a r k i n g h a d b e e n u s e d i n t h e 7 4 e x p e r i m e n t . The b l o c k s o f b r a i n w e r e f i x e d f o r 2-3 d a y s i n 10% f o r m a l i n , t h e n d e h y d r a t e d , c l e a r e d a n d embedded i n p a r a f f i n a c c o r d i n g t o s t a n d a r d h i s t o l o g i c a l p r o c e d u r e s . S e c t i o n s w e r e c u t a t 25-35 y and t h e d e s i r e d s e c t i o n s s t a i n e d w i t h c r e s y l v i o l e t . CHAPTER I I I RESULTS Experimental animals under a n e s t h e s i a or prepared by midbrain d e c e r e b r a t i o n breathed spontaneously. End t i d a l C 0 2 l e v e l s of about' 4% i n decerebrate and 4 1/2% i n c h l o r a l o s e or b a r b i t u r a t e a n e s t h e t i z e d animals were found c o n s i s t e n t l y . B r a d y c a r d i a and marked c e r e b r a l oedema were frequent c h a r a c t e r i s t i c s of the p r e t r i g e m i n a l decerebrate animals, and t h i s p r e p a r a t i o n was t h e r e f o r e used i n only a small number of experiments. NUCLEUS VENTRALIS LATERALIS E l e c t r o p h y s i o l o g y The a c t i o n p o t e n t i a l s of c e l l s i n VL c h a r a c t e r i s t i c a l l y o c c u r r e d i n spontaneous b u r s t s separated by p e r i o d s of i n a c t i v i t y . The nature of the b u r s t s v a r i e d somewhat from c e l l , t o c e l l , but i n any r e g i o n where more than one c e l l c o u l d be observed s i m u l t a n e o u s l y , the s p i k e s recorded from each f o l l o w e d the same b u r s t i n g p a t t e r n . An example of t h i s i s shown i n F i g u r e 10. The d i s c h a r g e s of s e v e r a l c e l l s (as i n d i c a t e d by the sp i k e amplitude) have been recorded. The c e l l s f i r e i n groups a t about the same time, though each one d o e s n o t n e c e s s a r i l y d i s c h a r g e w i t h e v e r y b u r s t . . The s e r i e s o f b u r s t s o r s p i n d l e s a r e n o r m a l l y a s s o c i a t e d w i t h s l o w wave a c t i v i t y w h i c h i s n o t a p p a r e n t i n F i g u r e 10 b e c a u s e a h i g h p a s s f i l t e r was i n c l u d e d i n t h e r e c o r d i n g s y s t e m . S u c h c o r r e s p o n d e n c e o f s p i k e a n d s l o w wave a c t i v i t y i s a p p a r e n t i n F i g u r e 14. The s p i n d l e s o c c u r r e d w i t h a f r e q u e n c y o f 5 - 1 1 / m i n . , and t h e i n t r a - s p i n d l e b u r s t s had a u s u a l f r e q u e n c y o f 5 - 1 0 / s e c . V e r y s i m i l a r b u r s t i n g a c t i v i t y c o u l d be i n d u c e d i n c e l l s o f VL by s t i m u l a t i o n o f BC o r p r e c r u c i a t e c o r t e x , as w i l l be. d e s c r i b e d s u b s e q u e n t l y . U s i n g a r e l a t i v e l y l a r g e r e c o r d i n g e l e c t r o d e i n V L , t h e u s u a l r e s p o n s e t o s t i m u l a t i o n o f t h e c e r e b e l l a r n u c l e i c o n s i s t e d o f two n e g a t i v e p e a k s ( F i g u r e 11A) w i t h l a t e n c i e s o f 0.8 - 1.2 msec, and 1.5 - 2 msec, r e s p e c t i v e l y . S t i m u -l a t i o n o f BC g a v e a s i m i l a r r e s p o n s e , b u t w i t h s l i g h t l y s h o r t e r l a t e n c y . The e a r l i e r p e a k was u s u a l l y l a r g e , o f r e l a t i v e l y s h o r t d u r a t i o n . , and r e s p o n d e d t o v a r i a t i o n o f t h e s t i m u l u s i n t e n s i t y w i t h a g r a d u a l i n c r e a s e o r d e c r e a s e i n a m p l i t u d e . The s e c o n d p e a k h a d a l o n g e r d u r a t i o n and had a l l - o r - n o n e s p i k e s s u p e r i m p o s e d on i t . The s e c o n d p e a k was n o t s e e n a t t h e v e n t r a l b o r d e r o f VL w h e r e a s t h e f i r s t p e a k w a s , f o u n d t o be p a r t i c u l a r l y l a r g e t h e r e and s m a l l e r i n a m p l i t u d e i n more d o r s a l r e g i o n s . On t h e b a s i s o f t h e s e c h a r a c t e r i s t i c s , , t h e f i r s t a n d s e c o n d p e a k s w e r e c o n s i d e r e d t o c o r r e s p o n d t o BC p r e s y n a p t i c and VL p o s t s y n a p t i c e l e c t r i c a l a c t i v i t y r e s p e c t i v e l y . 77 E l e c t r o d e s w i t h sm a l l t i p s recorded BC responses with a n e g a t i v e peak corresponding to the f i r s t one d e s c r i b e d above o f t e n f o l l o w e d b y . a l l - o r - n o n e s p i k e s evoked w i t h a l a t e n c y of 1.5-2.2 msec. (Figure 11B). For any s i n g l e BC-evoked sp i k e of t h i s k i n d , r e p e t i t i v e s t i m u l a t i o n r e s u l t e d i n l i t t l e v a r i a t i o n i n l a t e n c y . At stimulus f r e q u e n c i e s of 20-40 per second, the s p i k e response began t o f a i l . In some cases, s t i m u l a t i o n of the c e r e b e l l o - t h a l a m i c pathway evoked a b u r s t of 3-5 s p i k e s generated by the same c e l l (Figure 11C). The l a t e n c y of the i n i t i a l s pike of the b u r s t was u s u a l l y 4-6 msec and showed more v a r i a b i l i t y than o c c u r r e d i n c e l l s which responded with a s i n g l e s h o r t - l a t e n c y s p i k e . C e l l s e x h i b i t i n g t h i s " e a r l y b u r s t " response o c c a s i o n a l l y showed a s i n g l e s h o r t - l a t e n c y s p i k e , but the two types of response were mutually e x c l u s i v e , i . e . , the c e l l would respond e i t h e r with a s h o r t l a t e n c y s i n g l e spike or a l a t e r b u r s t , but not both. I t was sometimes-possible to c o n v e r t an e a r l y b u r s t response to a s h o r t l a t e n c y spike by i n c r e a s i n g the stimulus s t r e n g t h . Other means of e f f e c t i n g t h i s type of c o n v e r s i o n w i l l be d e s c r i b e d i n a l a t e r s e c t i o n . The e a r l y b u r s t response to c e r e b e l l a r s t i m u l a t i o n was found most f r e q u e n t l y i n deeply a n e s t h e t i z e d animals, p a r t i c u l a r l y those a n e s t h e t i z e d w i t h c h l o r a l o s e . I t was a l s o seen i n b a r b i t u r a t e a n e s t h e t i z e d animals, but r a r e l y i n d e c e r e b r a t e p r e p a r a t i o n s . A f u r t h e r type of response f o l l o w i n g BC s t i m u l a t i o n was a l o n g - l a t e n c y s e r i e s of b u r s t s which resembled those 78 1 SEC 200 F i g u r e 10. F i g u r e 11. S p o n t a n e o u s b u r s t i n g p a t t e r n o f gc&up o f VL ' n e u r o n e s , r e c o r d e d w i t h h i g h - p a s s f i l t e r . j N e g a t i v e i s up i n t h i s a n d a l l s u b s e q u e n t f i g u r e s . B r C i v \ ' 1 400 200 _ _ V _ 10 MSEC 200 B C - e v o k e d r e s p o n s e s , r e c o r d e d i n VL; (A)' p r e -and p o s t s y n a p t i c f i e I d ' r e s p o n s e s ; (B) p r e s y n a p - ; t i c f i e l d a n d p o s t s y n a p t i c . u n i t ' s p i k e r e s p o n s e s ; ( C ) - p r e - and p o s t s y n a p t i c - f i e l d r e s p o n s e s and e a r l y b u r s t o f s i n g l e n e u r o n e . o c c u r r i n g s p o n t a n e o u s l y ( F i g u r e 12 A ) . T h e r e was u s u a l l y a 100-200 msec, p o s t s t i m u l u s l a t e n c y t o t h e f i r s t o f t h e b u r s t s . The c e l l s e x h i b i t i n g s u c h b u r s t i n g a c t i v i t y o f t e n , b u t n o t n e c e s s a r i l y , a l s o r e s p o n d e d w i t h a s i n g l e , s h o r t -l a t e n c y 'Spike. S u c h s e r i e s o f b u r s t s , w h e t h e r o c c u r r i n g s p o n t a n e o u s l y o r g e n e r a t e d by a s t i m u l u s , w o u l d t e m p o r a r i l y i n t e r r u p t any s p o n t a n e o u s r e g u l a r f i r i n g o f t h e s p i k e s ( F i g u r e 12 B ) . T h e s e s t i m u l u s - i n d u c e d b u r s t s c o u l d be e v o k e d w h e t h e r o r n o t s p o n t a n e o u s b u r s t i n g a c t i v i t y was i n p r o g r e s s , b u t t h e r e s u l t i n g b u r s t i n g p a t t e r n s w e r e o f t e n a b b r e v i a t e d i f a s t i m u l u s was g i v e n d u r i n g s p o n t a n e o u s b u r s t i n g a c t i v i t y . The i n t r a - s p i n d l e f r e q u e n c y o f e v o k e d b u r s t s was a b o u t t h e same as f o r t h e s p o n t a n e o u s o n e s . The e l e c t r i c a l a c t i v i t y e v o k e d i n VL c e l l s b y s t i m u -l a t i o n o f t h e p r e c r u c i a t e c o r t e x h a d t h e same t y p e s o f v a r i a t i o n as t h a t d e s c r i b e d f o r BC s t i m u l a t i o n . The mos t common r e s p o n s e was a s i n g l e s p i k e o f 2-6 msec, l a t e n c y ( F i g u r e 13 A ) , w h i c h was n o t e v o k e d by e v e r y s t i m u l u s a t h i g h f r e q u e n c i e s o f s t i m u l a t i o n . As w i t h BC s t i m u l a t i o n , an e a r l y b u r s t r e s p o n s e was s o m e t i m e s s e e n ( F i g u r e 13 B ) , a g a i n , m o s t f r e q u e n t l y i n a n e s t h e t i z e d , b u t n e v e r i n d e c e r e b r a t e a n i m a l s . The e a r l y b u r s t r e s p o n s e t o c o r t i c a l s t i m u l a t i o n was a l s o c o n v e r t i b l e t o a s i n g l e s h o r t - l a t e n c y s p i k e a s w i l l be d e s c r i b e d . L a t e b u r s t i n g a c t i v i t y s i m i l a r t o t h a t i n d u c e d b y BC s t i m u l a t i o n was o f t e n s e e n a f t e r s t i m u l a t i o n A ' i<fr H # "inii mi • t 500 MSEC B - HI Ml I 1 SEC 200^ F i g u r e 12. B u r s t i n g p a t t e r n s i n VL i n d u c e d by BC s t i m u l u s , - ; ' (A) q u i e s c e n t n e u r o n e s ; (B) s p o n t a n e o u s l y d i s c h a r g i n g n e u r o n e . BC s t i m u l u s a t a r r o w s . 10 MSEC B J, F i g u r e 1 3 . R e s p o n s e s o f s i n g l e VL n e u r o n e s t o s t i m u l a t i o n o f p r e c r u c i a t e c o r t e x ; ^ . (A) s i n g l e s h o r t l a t e n c y s p i k e ; (B) e a r l y b u r s t r e s p o n s e . -81 o f t h e p r e c r u c i a t e c o r t e x . T h i s i s shown, i n t h e r e c o r d o f F i g u r e 14, w h i c h b e c a u s e i t i s u n f i l t e r e d a l s o shows t h e c o r r e s p o n d e n c e o f t h e b u r s t i n g s p i k e s t o s l o w wave a c t i v i t y . I n t h i s r e c o r d t h e b u r s t s o f s p i k e s c a n be s e e n t o o c c u r j u s t b e f o r e a r e l a t i v e l y s l o w n e g a t i v e - p o s i t i v e wave c o m p l e x . The b u r s t i n g s p i k e s i n some c a s e s w e r e , b u t s o m e t i m e s were n o t e v o k e d w i t h s h o r t l a t e n c y b y c o r t i c a l s t i m u l a t i o n . VL c e l l s s o m e t i m e s r e s p o n d e d i n a manner u n i q u e t o s t i m u l a t i o n o f t h e c o r t e x i . e . , a n t i d r o m i c a l l y . The a n t i -d r o m i c n a t u r e o f t h e s p i k e r e s p o n s e was j u d g e d f r o m t h e s h o r t i n v a r i a n t l a t e n c y and t h e a b i l i t y o f t h e c e l l s t o r e s p o n d w i t h an a c t i o n p o t e n t i a l t o e v e r y c o r t i c a l s t i m u l u s , , e v e n a t a f r e q u e n c y o f s e v e r a l h u n d r e d p e r s e c o n d ( F i g u r e 1 5 ) . The r a n g e o f l a t e n c i e s f o r a n t i d r o m i c a l l y e v o k e d c e l l s was 0.7-2.2 msec. C e l l s w e r e o f t e n f o u n d w h i c h w o u l d r e s p o n d t o c o r t i c a l s t i m u l a t i o n w i t h an o r t h o d r o m i c o r a n t i d r o m i c s p i k e , and t o s t i m u l a t i o n o f BC ( s e e e.g. F i g u r e 3 1 ) . I n c h l o r a l o s e p r e p a r a t i o n s , p a r t i c u l a r l y , s i n g l e c e l l s w e r e s o m e t i m e s f o u n d t o r e s p o n d w i t h an e a r l y b u r s t t o b o t h BC and c o r t i c a l s t i m u l i . S t i m u l a t i o n o f t h e e n t o p e d u n c u l a r n u c l e u s a l s o g a v e c h a r a c t e r i s t i c r e s p o n s e s i n VL w h i c h w e r e u s u a l l y f o u n d i n more m e d i a l p a r t s o f t h e n u c l e u s . One t y p e o f r e s p o n s e o f t e n s e e n was a s m a l l , s h o r t l a t e n c y n e g a t i v e p e a k o f 0.8 - 2.0 msec, l a t e n c y . T h i s r e s p o n s e h a d t h e s h o r t e s t l a t e n c y when 500 MSEC J F i g u r e 14. B u r s t i n g p a t t e r n o f VL n e u r o n e s i n d u c e d b y ; p r e c r u c i a t e c o r t i c a l s t i m u l u s — u n f i l t e r e d r e c o r d . 1 B 200 10M F i g u r e 15. U n i t a r y s p i k e p o t e n t i a l i n V L — e v o k e d a n t i d r o m -i c a l l y b y p r e c r u c i a t e c o r t i c a l s t i m u l u s w i t h 1.5 msec l a t e n c y a f t e r s t i m u l u s a r t i f a c t ; (A) s u p e r i m p o s e d : r e c o r d s a t a b o u t 1 0 0 / s e c s t i m u l u s f r e q u e n c y ; l a t e r s m a l l p e a k due t o s t i m u l a t o r s y n c h r o n o u s p u l s e a r t i f a c t ; ( B ) . t r a i n o f f i v e s t i m u l i a t 3 0 0 / s e c e v o k e s s p i k e f o r e v e r y s t i -m u l u s . 83 t h e r e c o r d i n g e l e c t r o d e was n e a r t h e v e n t r a l e x t r e m i t y o f VL (0.8 - 1.0 m s e c . - F i g u r e 16 B) and l o n g e r l a t e n c y i n more d o r s a l p a r t s o f t h e n u c l e u s (1.0 -2.0 msec - F i g u r e 16 A ) . C l e a r l y d i s t i n g u i s h e d u n i t a r y s p i k e s w e r e n e v e r s e e n s u p e r i m -p o s e d on t h i s r e s p o n s e . The o t h e r t y p e o f r e s p o n s e e v o k e d f r o m EN c o n s i s t e d u s u a l l y o f a b u r s t o f s p i k e s s i m i l a r t o t h e e a r l y b u r s t r e s p o n s e t o BC and c o r t i c a l s t i m u l a t i o n , t h o u g h i n a few c e l l s s i n g l e s p i k e r e s p o n s e s w e r e n o r m a l l y e v o k e d . The b u r s t s w e r e u s u a l l y f o r m e d b y 3-6 s p i k e s , w i t h a w i d e l a t e n c y r a n g e ( F i g u r e 1 7 ) . T h e s e r e s p o n s e s s o m e t i m e s w e r e f o u n d a f t e r a s i n g l e s t i m u l u s t o EN, b u t more o f t e n 2-3 s t i m u l i d e l i v e r e d a t a h i g h f r e q u e n c y w e r e r e q u i r e d . Of t h i r t y - t w o c e l l s s t u d i e d , o n l y one r e s p o n d e d w i t h a s i n g l e s h o r t l a t e n c y (1.8 msec) s p i k e ; f o u r o t h e r s r e s p o n d e d w i t h l o n g e r l a t e n c y s i n g l e s p i k e s a nd t h e o t h e r t w e n t y - s e v e n r e s p o n d e d w i t h b u r s t s . The mean r e s p o n s e l a t e n c y f o r e a c h o f t h e s e c e l l s i s shown i n F i g u r e 18. The b u r s t r e s p o n s e s t o EN s t i m u l a t i o n w e r e s e e n w i t h a b o u t e q u a l f r e q u e n c y i n d e c e r e b r a t e and c h l o r a l o s e p r e p a r a t i o n s . No s p i k e r e s p o n s e s t o EN s t i m u l a t i o n w e r e f o u n d u s i n g b a r b i t u r a t e - a n e s t h e t i z e d p r e p a r a t i o n s , t h o u g h s e v e r a l e x p e r i m e n t s w e r e t r i e d u s i n g D i a l o r N e m b u t a l . Two -of t h e EN e v o k e d c e l l s w e r e a n t i d r o m i c a l l y i n v a d e d and t h r e e r e s p o n d e d o r t h o d r o m i c a l l y w i t h s i n g l e s p i k e s t o c o r t i c a l s t i m u l a t i o n . T h o u g h BC s t i m u l a t i o n was n o t 84 F i g u r e 16. EN evoked f i e l d response i n VL—superimposed t r a c e s ; (A) reco r d e d i n mid-region o f V L — 1.5 msec l a t e n c y ; (B) recorded i n v e n t r a l p a r t of VL—1.0 msec l a t e n c y . F i g u r e 17. B u r s t responses of two d i f f e r e n t VL neurones evoked by EN s t i m u l a t i o n ; ( A ) A s i n g l e s t i m u l u s evokes 5 msec l a t e n c y b u r s t ; (B) p a i r o f s t i m u l i evoke b u r s t w i t h 11 msec l a t e n c y a f t e r second s t i m u l u s . a t t e m p t e d i n t h e s e d e c e r e b r a t e e x p e r i m e n t s , o f t h e e l e v e n EN e v o k e d c e l l s f o u n d i n c h l o r a l o s e a n e s t h e t i z e d a n i m a l s two showed an e a r l y b u r s t r e s p o n s e t o BC s t i m u l a t i o n . L o n g l a t e n c y r e c u r r i n g b u r s t s o f s p i k e s s i m i l a r t o t h o s e e v o k e d f r o m BC and c o r t e x w e r e n o t s e e n f o l l o w i n g EN s t i m u l a t i o n . E f f e c t o f D r u g s A c e t y l c h o l i n e A c e t y l c h o l i n e a p p l i e d i o n t o p h o r e t i c a l l y had an e x c i t -a t o r y e f f e c t on most VL c e l l s r e c o r d e d i n t h i s s t u d y . T h o s e c e l l s r e s p o n d i n g w i t h s h o r t l a t e n c y t o BC s t i m u l a t i o n w e r e -9 p a r t i c u l a r l y s e n s i t i v e , and o f t e n o n l y a few nanoamps (10 amperes = nA) o f c u r r e n t p a s s e d t h r o u g h t h e a c e t y l c h o l i n e b a r r e l o f t h e e l e c t r o d e a s s e m b l y I n c r e a s e d -. t h e i r f i r i n g . The e x c i t a t i o n was s e e n as an i n c r e a s e d r a t e o f n o n - e v o k e d d i s c h a r g e o r a g r e a t e r e f f e c t i v e n e s s o f BC o r c o r t i c a l s t i m u l i i n ' e v o k i n g r e s p o n s e s . F i g u r e 19 i s a f i l m e d r e c o r d f r o m a VL c e l l o b t a i n e d d u r i n g t h e i o n t o p h o r e t i c a p p l i c a t i o n o f ACh. The l a t e n c y o f e x c i t a t i o n a f t e r t h e o n s e t o f ACh a p p l i c a t i o n v a r i e d w i d e l y b e t w e e n c e l l s , b u t c o u l d be a s s h o r t , as 1-2 s e c o n d s . The c e l l s o f t e n e x h i b i t e d s u s t a i n e d h i g h l e v e l s o f f i r i n g u n t i l s h o r t l y a f t e r t h e c e s s a t i o n o f t h e ACh c u r r e n t . '"The t e r m i n a t i o n o f . e x c i t a t i o n a l s o v a r i e d c o n s i d e r a b l y , i n some c a s e s b e i n g s h a r p and o f s h o r t l a t e n c y and i n o t h e r s b e i n g g r a d u a l . S p o n t a n e o u s b u r s t i n g a c t i v i t y 86 { i -. , i Latencies of EN evoked Cells in VL 1 Latency (msec) Mean l a t e n c i e s o f t h i r t y - t w o VL n e u r o n e s e v o k e d b y EN s t i m u l a t i o n . L a t e n c y m e a s u r e d f r o m a r t i f a c t o f f i n a l s t i m u l u s o f t r a i n t o f i r s t s p i k e o f b u r s t r e s p o n s e . | 100 500 MSEC F i r i n g o f VL n e u r o n e s i n d u c e d by i o n t o p h o r e t i c | a p p l i c a t i o n o f ACh (10 n A ) . F i r i n g b e g i n s l e s s t h a n 1 s e c a f t e r ACh a d m i n i s t r a t i o n com-menced ( a r t i f a c t ) . 87 was n o t e v i d e n t i n c e l l s e x c i t e d t o a h i g h l e v e l o f d i s c h a r g e e v e n t h o u g h t h e y e x h i b i t e d t h i s b e h a v i o u r b e f o r e t h e ACh was a p p l i e d . A f t e r t h e ACh was t u r n e d o f f and t h e d i s c h a r g e r a t e d i m i n i s h e d , b u r s t i n g a c t i v i t y r e s u m e d . ACh d e p r e s s e d t h e s p o n t a n e o u s o r d r u g i n d u c e d a c t i v i t y o f some c e l l s i n VL a n d ; o t h e r p a r t s o f t h e t h a l a m u s . T h i s was s o m e t i m e s j u d g e d t o be a c u r r e n t e f f e c t a s t h e d e p r e s s i o n o c c u r r e d as s o o n a s t h e c u r r e n t was a p p l i e d , and c o u l d be d u p l i c a t e d b y p a s s i n g p o s i t i v e c u r r e n t t h r o u g h o t h e r b a r r e l s o f t h e e l e c t r o d e . U s u a l l y , h o w e v e r , t h e d e p r e s s i o n was n o t i m m e d i a t e i n o n s e t , a n d c o u l d n o t be r e p r o d u c e d b y e q u i v a l e n t o r h i g h e r c u r r e n t s p a s s e d t h r o u g h ^ o t h e r b a r r e l s ( F i g u r e 2 0 ) . So m e t i m e s a d e p r e s s i o n o f s p o n t a n e o u s b u r s t i n g was s e e n b e t w e e n t h e o n s e t o f ACh a p p l i c a t i o n and t h e e x c i t a t o r y r e s p o n s e . T h i s phenomenon was n o t i n v e s t i g a t e d p h a r m a c o -l o g i c a l l y . C e l l f i r i n g i n d u c e d by ACh c o u l d be b l o c k e d t r a n s i e n t l y b y s t i m u l a t i o n o f BC o r p r e c r u c i a t e c o r t e x . " An a p p a r e n t l y p o w e r f u l and r e l a t i v e l y l o n g d u r a t i o n d e p r e s s i o n o f f i r i n g f o l l o w e d a s i n g l e . s t i m u l u s ( F i g u r e 21 A ) , w h i c h s o m e t i m e s b u t n o t n e c e s s a r i l y e v o k e d t h e s p i k e w i t h s h o r t l a t e n c y . The c e l l s o m e t i m e s d i s p l a y e d l a t e b u r s t i n g a c t i v i t y d u r i n g t h e p e r i o d o f d e p r e s s i o n o f t h e s t e a d y f i r i n g l e v e l ( F i g u r e 2 1 B ) . S t i m u l a t i o n o f EN s o m e t i m e s d e p r e s s e d ACh i n d u c e d c e l l a c t i v i t y , b u t when i t d i d . o c c u r t h e d u r a t i o n o f t h e d e p r e s s i o n was s h o r t e r t h a n a f t e r BC o r c o r t i c a l s t i m u l a t i o n ( F i g u r e 2 2 ) . F i g u r e 20 200 yV 1 s e c . ACh d e p r e s s i o n o f s p o n t a n e o u s b u r s t i n g o f t h a l a -m i c n e u r o n e ; A. s p o n t a n e o u s b u r s t i n g p a t t e r n ; B. 40 n A ACh d e p r e s s e s s p o n t a n e o u s b u r s t i n g a b o u t 1-2 s e c a f t e r c u r r e n t t u r n e d o n ( a r t i f a c t ) C. s p o n t a n e o u s b u r s t i n g r e s u m e s a b o u t 2 s e c a f t e r c u r r e n t t u r n e d o f f ; D. N a + 40 n A d o e s n o t n o t i c e a b l y a l t e r s p o n t a n e o u s b u r s t i n g . 89 A « t t B 1 "mirn'ij mm#m i n « n i i n n i a f i i i n — i f i f I I 1SEC | t'lfll'fl'iWl ^ • f1 • I* i ' I I I l!h!f'!ff!'ll!!'! 500 M t 100 wV 200 . >i t t > \ i t t.t t> F i g u r e 2 1 . BC s t i m u l u s d e p r e s s e s n e u r o n a l f i r i n g i n d u c e d by ACh; A. c o m p l e t e s u p p r e s s i o n o f f i r i n g f o r a l m o s t 0.5 s e c - - s t i m u l i a t a r r o w s ; B. s t e a d y . f i r i n g i s r e p l a c e d b y r h y t h m i c b u r s t s " a f t e r . BC s t i m u l a t i o n . L o w e r t r a c e o f e a c h p a d x i n d i c a t e s , o u t p u t o f p u l s e g e n e r a t o r t o r a t e n i e t e l : . ' N o t e d i f f e r e n t t i m e a n d v o l t a g e c a l i b r a t i o n s . 100 i F i g u r e 22. D e p r e s s i o n o f ACh i n d u c e d a c t i v i t y o f VL n e u r o n e s by EN s t i m u l i . S t i m u l i a t a r r o w s . 90 S y n a p t i c a c t i v a t i o n o f VL c e l l s c o u l d a l s o be a l t e r e d b y ACh. C e l l s n o t e x c i t e d b y a s t i m u l u s t o BC o r c o r t e x c o u l d s o m e t i m e s be e v o k e d by t h e same s t i m u l u s d u r i n g ACh a p p l i c a t i o n . ( F i g u r e 23) . A n o t h e r f o r m o f s u c h a l t e r a t i o n o f a c t i v i t y , was s e e n i n VL c e l l s i n w h i c h a n e a r l y b u r s t was e v o k e d by BC o r c o r t i c a l s t i m u l a t i o n . ACh a p p l i e d t o s u c h c e l l s c o n v e r t e d t h e e a r l y b u r s t s t o s i n g l e s h o r t l a t e n c y s p i k e r e s p o n s e s ( F i g u r e 2 4 ) . VL c e l l r e s p o n s e s e v o k e d by E N . s t i m u l a t i o n showed a v a r i e t y o f e f f e c t s w i t h i o n t o p h o r e t i c a p p l i c a t i o n o f ACh. F i v e o f t h e t w e n t y - t w o c e l l s t e s t e d w e r e u n a f f e c t e d b y ACh. S e v e n c e l l s showed some s i g n o f e x c i t a t i o n , t a k i n g t h e f o r m e i t h e r o f an i n d u c e d f i r i n g a n d / o r a r e d u c t i o n i n t h e l a t e n c y o f t h e e v o k e d r e s p o n s e . Where t h e l a t e n c y o f t h e r e s p o n s e was s h o r t e n e d , t h e b u r s t was u s u a l l y r e d u c e d t o a s i n g l e s p i k e ( F i g u r e 2 5 ) . The s e v e n c e l l s s h o w i n g e x c i t a t i o n w i t h ACh w e r e f o u n d i n d e c e r e b r a t e a n i m a l s and none w e r e s e e n i n c h l o r a l o s e p r e p a r a t i o n s . Two o t h e r c e l l s r e s p o n d e d t o ACh w i t h a c o n v e r s i o n o f t h e b u r s t t o a s i n g l e s p i k e , b u t w i t h o u t a c h a n g e i n l a t e n c y . I t i s n o t c l e a r w h e t h e r t h i s t y p e o f r e s p o n s e b e l o n g s w i t h t h e e x c i t e d o r d e p r e s s e d g r o u p . The t h i r d e f f e c t o f ACh, a d e p r e s s i o n o f t h e r e s p o n s e t o EN s t i m u l a t i o n was f o u n d i n e i g h t c e l l s . I n t h e s e c e l l s t h e r e was e i t h e r a c o m p l e t e s u p p r e s s i o n o f r e s p o n s e ( F i g u r e 2 6 ) , o r . a m a r k e d r e d u c t i o n i n t h e f r e q u e n c y o f s u c c e s s w i t h w h i c h 91 B 10MSEC F i g u r e 23, F a c i l i t a t i o n o f s p i k e d i s c h a r g e e v o k e d b y BC ' s t i m u l u s ; A. f i e l d r e s p o n s e t o BC s t i m u l u s w i t h o u t ACh; B. same w i t h 20 n A ACh a p p l i e d i o n t o p h o r e t i c a l l y . B N 25MSEC F i g u r e 24. C o n v e r s i o n o f c o r t i c a l l y e v o k e d e a r l y b u r s t o f -•. VL. n e u r o n e t o . s h o r t - , l a t e n c y s p i k e ; A. e a r l y ^  4 . b u r s t e v o k e d w i t h no .drugs:;.-' B. same c e l l ['•>: e v o k e d d u r i n g a p p l i c a t i o n o f 30, n ,A, ACh.. .; 92 . a s t i m u l u s evoked a response. The d e p r e s s i o n was not immediate i n onset and was never d u p l i c a t e d by p o s i t i v e c u r r e n t s passed through other b a r r e l s of the e l e c t r o d e . D epression of responses was seen i n both c h l o r a l o s e (3 c e l l s ) and decerebrate (5 c e l l s ) p r e p a r a t i o n s . N e i t h e r BC nor c o r t i c a l evoked responses were ever found t o be depressed by ACh. ACh d e p r e s s i o n of EN evoked responses c o u l d not be c o r r e l a t e d w i t h the l a t e n c y o f response, as they were found i n c e l l s showing almost the complete range of l a t e n c i e s (6-15 m s e c . ) v By c o n t r a s t , s i x of the•seven cases i n which <•; ' ACh caused' an e x c i t a t i o n were found with c e l l s responding i' w i t h mean l a t e n c i e s between 2 and 8 msec. There was a s l i g h t d i f f e r e n c e i n the mean l a t e n c i e s of EN responses i n c h l o r a l o s e (12.6 msec.) and decerebrate (10.1 msec.) prepar-a t i o n s , but the sample s i z e does not j u s t i f y s t a t i s t i c a l a n a l y s i s . Amino A c i d s The v a r i o u s d i a c i d i c amino a c i d s used i n t h i s study appeared to have q u a l i t a t i v e l y s i m i l a r e x c i t a t o r y e f f e c t s : on c e l l s of the thalamus. These e f f e c t s were very s i m i l a r t o those d e s c r i b e d . above-for--ACh. Most t h a l a m i c c e l l s i n and o u t s i d e of VL co u l d be induced to d i s c h a r g e r a p i d l y over r e l a t i v e l y long p e r i o d s of time by the amino a c i d s . Spontaneous B 93 200 '•1 Figure 25. VL neurone evoked by EN stimulus pa i r ; A. r e s p o n s e w i t h no d r u g s ; B. s e v e r a l t r a c e s 1 s u p e r i m p o s e d d u r i n g a p p l i c a t i o n o f 30 n A ACh. i N o t e t h a t e a c h s t i m u l u s now e v o k e s s i n g l e s h o r t j. l a t e n c y s p i k e r e s p o n s e . B 400 25MSEC ' F i g u r e 26, VL n e u r o n e e v o k e d by s i n g l e EN s t i m u l u s ; A. l o n g l a t e n c y b u r s ^ t r e s p o n s e w i t h n o ' d r u g s ; B. no r e s p o n s e e v o k e d d u r i n g a p p l i c a t i o n o f 35 n A ACh; C. s h o r t l a t e n c y s i n g l e s p i k e e v o k e d d u r i n g a p p l i c a t i o n o£ 40 n A DLH. b u r s t i n g a c t i v i t y was n o t a p p a r e n t d u r i n g h i g h l e v e l s o f i n d u c e d a c t i v i t y , b u t i f p r e s e n t i n t h e r e s t i n g s t a t e , w o u l d r e t u r n a f t e r t h e d r u g a p p l i c a t i o n was d i s c o n t i n u e d . S u c h i n d u c e d f i r i n g o f V L c e l l s was u s u a l l y d e p r e s s e d m a r k e d l y b y BC o r p r e c r u c i a t e c o r t e x s t i m u l a t i o n , b u t l e s s , i f a t a l l b y EN s t i m u l a t i o n . V L c e l l s n o t e v o k e d b y BC o r c o r t i c a l s t i m u l a t i o n w i t h o u t d r u g s c o u l d s o m e t i m e s be e v o k e d w i t h s h o r t l a t e n c y when one o f t h e e x c i t a t o r y a m i n o a c i d s was a p p l i e d . As f o r ACh, BC o r c o r t i c a l e v o k e d e a r l y b u r s t r e s p o n s e s c o u l d u s u a l l y be c o n v e r t e d t o s i n g l e s h o r t - l a t e n c y s p i k e s w i t h t h e a i d o f an amino a c i d . T h e s e e f f e c t s w e r e i n d i s t i n g u i s h a b l e f r o m t h o s e r e s u l t i n g f r o m ACh a p p l i c a t i o n . F o u r t e e n c e l l s e v o k e d b y EN w e r e t e s t e d w i t h D L H and o f t h e s e 9 w e r e e x c i t e d . F i v e o f t h e s e c l e a r l y showed a c o n v e r s i o n f r o m a b u r s t t o a s i n g l e s p i k e r e s p o n s e w i t h a s h o r t e r l a t e n c y when D L H was a p p l i e d . The l a t e n c i e s o f t h e s e f i v e r e s p o n s e s h a d a r a n g e o f 7-15 msec, w i t h o u t d r u g s and 2-7 msec. when DLH was a p p l i e d . An e x a m p l e i s shown i n F i g u r e 26 C. The r e s p o n s e o f t h i s c e l l , a s p r e v i o u s l y n o t e d , was d e p r e s s e d b y ACh. The a p p l i c a t i o n o f D L H , h o w e v e r , c o n v e r t e d t h e b u r s t t o a s i n g l e s p i k e a n d r e d u c e d t h e mean l a t e n c y f r o m 15 msec, t o 5 msec. T h r e e o f t h e f i v e " n o n - e x c i t e d " c e l l s showed no e f f e c t o f D L H , one. showed a r e d u c t i o n f r o m a b u r s t t o a s i n g l e s p i k e w i t h o u t a r e d u c t i o n i n l a t e n c y , a n d one showed a d e p r e s s i o n o f t h e r e s p o n s e n o t d u p l i c a t e d by n e g a t i v e 95 c u r r e n t a p p l i e d f r o m o t h e r b a r r e l s . E x c e p t f o r t h i s one c a s e , no. i n d i c a t i o n o f d e p r e s s i o n o f any e v o k e d r e s p o n s e o r r e s t i n g d i s c h a r g e was s e e n when LG o r DLH was a p p l i e d . , - A l t h o u g h t h e v a r i o u s e x c i t a t o r y a m i n o a c i d s t e s t e d d e m o n s t r a t e d q u a l i t a t i v e l y s i m i l a r e f f e c t s , i t became a p p a r e n t d u r i n g t h e s e s t u d i e s t h a t t h e r e w e r e q u a n t i t a t i v e d i f f e r e n c e s b e t w e e n some o f t h e i r a c t i o n s . F o r e x a m p l e , i t was r o u t i n e l y n e c e s s a r y t o p a s s a g r e a t e r amount o f c u r r e n t t o c a u s e t h e same e x t e n t o f c e l l f i r i n g f o r D g l u t a m a t e (DG) t h a n f o r L g l u t a m a t e ( L G ) . Of t w e n t y - e i g h t c e l l s upon w h i c h t h e s e two d r u g s w e r e c o m p a r e d o n l y t h r e e w e r e a c t i v a t e d b y l e s s DG t h a n LG, w h e r e a s t w e n t y a p p e a r e d more s e n s i t i v e t o LG. DLH and n - m e t h y l - a s p a r t i c a c i d (NMA) h a d a p p r o x i m a t e l y e q u a l p o t e n c i e s as m e a s u r e d by c o m p a r i n g t h e i r e f f e c t s on c e l l s i n and o u t -s i d e o f VL. T h e s e two a m ino a c i d s w e r e g e n e r a l l y more e f f e c t i v e i n e x c i t i n g c e l l s t o a c t i v i t y t h a n e i t h e r L o r D, g l u t a m a t e . I t was, h o w e v e r , n o t i c e d t h a t t h e r a t i o s o f t h e p o t e n c i e s o f LG/DLH f o r e x a m p l e , v a r i e d c o n s i s t e n t l y b e t w e e n c e l l s o f VL and t h o s e o f more d o r s a l p a r t s o f t h e t h a l a m u s . T h a t i s , LG h a d a g r e a t e r e f f e c t on VL c e l l s t h a n on o t h e r s t e s t e d , w h e r e a s DLH was r o u g h l y e q u i p o t e n t i n t h e s e two g r o u p s . T h i s phenomenon i s i l l u s t r a t e d b y F i g u r e 2 7 , w h i c h shows t h e r e l a t i v e e f f i c a c y o f ACh, DLH and NMA a n d LG i n i n d u c i n g e x c i t a t i o n o f c e l l s e n c o u n t e r e d i n a t r a c k t h r o u g h t h e t h a l a m u s . C e l l s w i t h i n t h e r e g i o n i n w h i c h s h o r t l a t e n c y BC r e s p o n s e s a r e f o u n d a r e shown to. be more s e n s i t i v e t o ACh and LG. t h a n t h o s e o u t s i d e o f i t . DLH and NMA m a i n t a i n e d a b o u t t h e same o r h a d a d e c r e a s e d p o t e n c y on VL c e l l s . T h e s e d a t a w e r e t a k e n f r o m an e x p e r i m e n t on a D i a l a n e s t h e t i z e d a n i m a l . S i m i l a r d i f f e r e n c e s w e r e s e e n i n d e c e r e b r a t e p r e p - , a r a t i o n s . H i s t o l o g i c a l e x a m i n a t i o n o f t h e b r a i n f o r F i g u r e 27 showed a c l o s e c o r r e l a t i o n b e t w e e n VL and t h e p a r t o f t h e t r a c k i n w h i c h BC r e s p o n s e s w e r e r e c o r d e d . A d i f f e r e n t a n a l y s i s o f d a t a f r o m t h e s e and o t h e r e x p e r i m e n t s i s p r e s e n t e d i n F i g u r e 28. C e l l s s t u d i e d i n s e v e r a l e x p e r i m e n t s w e r e s e p a r a t e d i n t o two c a t e g o r i e s , t h o s e e v o k e d w i t h s h o r t l a t e n c y b y BC w i t h o u t t h e a i d o f e x c i t a t o r y d r u g s , a nd t h o s e n o t e v o k e d , i n c l u d i n g n o n - e v o k e d c e l l s w i t h i n V L . The amount o f c u r r e n t r e q u i r e d t o e j e c t e n o u g h o f e a c h o f t h e s u b s t a n c e s t o e x c i t e t h e c e l l was t a b u l a t e d , and t h e means o f t h e s e numbers w e r e c a l c u l a t e d f o r e a c h s u b s t a n c e i n t h e two c e l l c a t e g o r i e s . O n l y c e l l s on w h i c h a l l f o u r a g e n t s w e r e t e s t e d w e r e u s e d f o r t h i s a n a l y s i s , s o t h a t t h e f i g u r e s w i t h i n e a c h o f t h e f o u r . g r o u p s a r e d e r i v e d f r o m t h e same p o p u l a t i o n o f c e l l s . The d a t a w e r e c o l l e c t e d f o r a t o t a l o f t h i r t y - s e v e n BC e v o k e d and f o r t y - n i n e n o n -e v o k e d c e l l s . ACh and LG h a v e a g r e a t e r a b i l i t y t o e x c i t e e v o k e d r a t h e r t h a n n o n - e v o k e d c e l l s w h e r e a s DLH and NMA h a v e a b o u t t h e same p o t e n c y on t h e two g r o u p s . The two c e l l g r o u p s w e r e c o m p a r e d s t a t i s t i c a l l y f o r e a c h d r u g - u s i n g t h e 97 ACh *• c or T 6 I I Depth below fornix , mm. F i g u r e 27. S e n s i t i v i t y o f t w e n t y - t h r e e t h a l a m i c n e u r o n e s a t d i f f e r e n t l e v e l s i n a s i n g l e e l e c t r o d e t r a c k t o i o n t o p h o r e t i c a l l y a p p l i e d d r u g s . C e l l s b e t w e e n d o t t e d l i n e s w e r e e v o k e d w i t h s h o r t l a t e n c y by BC s t i m u l a t i o n . N o t e e n h a n c e d s e n s i t i v i t y o f e v o k e d c e l l s t o ACh a n d LG. 9.8 M a n n - W h i t n e y U t e s t [ S i e g e l , 1956]. They w e r e s i g n i f i c a n t l y d i f f e r e n t a t t h e 0.01 p r o b a b i l i t y l e v e l f o r ACh a n d LG, b u t n o t f o r DLH o r NMA. S i m i l a r d a t a f o r a s m a l l e r g r o u p o f c e l l s i s p r e s e n t e d f o r DG and LG i n F i g u r e 29. The r e s u l t s f o r s e v e n e v o k e d and t h i r t e e n n o n - e v o k e d c e l l s i n d i c a t e t h a t DG i s s i m i l a r i n p a t t e r n . t o LG, r a t h e r t h a n t o DLH and NMA. The d i f f e r e n c e s . b e t w e e n t h e two c e l l g r o u p s a r e s i g n i f i c a n t a t p = 0.01 f o r e a c h d r u g . . [ M a n n ^ W h i t n e y U. t e s t ] . I n t h e e x p e r i m e n t s on w h i c h F i g u r e s 23 and 24 a r e b a s e d , c u r r e n t s up t o 110 nA we r e t e s t e d f o r e a c h s u b s t a n c e . . I f no e x c i t a t i o n was o b t a i n e d a t t h i s l e v e l , 110 was assumed as t h e minimum e f f e c t i v e c u r r e n t f o r c a l c u l a t i o n s . B l o c k i n g A g e n t s ( i ) v C h o l i n e r g i c b l o c k e r s - The c h o l i n e r g i c b l o c k i n g a g e n t s a t r o p i n e and DHBE w e r e t e s t e d i o n t o p h o r e t i c a l l y a n d i n some c a s e s i n t r a v e n o u s l y t o d e t e r m i n e t h e i r a b i l i t i e s t o b l o c k s y n a p t i c a c t i v a t i o n o f VL c e l l s and t h e e f f e c t s o f i o n t o p h o r e t i c a l l y a p p l i e d ACh. B o t h o f . t h e s e a g e n t s w ere a b l e t o b l o c k t h e e x i t a t i o n by ACh o f BC e v o k e d and n o n - e v o k e d c e l l s . The b l o c k i n g e f f e c t e d i o n t o p h o r e t i c a l l y by t h e s e d r u g s v a r i e d i n t h e amount a nd d u r a t i o n o f c u r r e n t a p p l i c a t i o n n e c e s s a r y .and i n t h e t i m e r e q u i r e d f o r r e c o v e r y . B l o c k i n g by DH3E was u s u a l l y r e v e r s i b l e w i t h i n few m i n u t e s a f t e r t h e a p p l i c a t i o n was- s t o p p e d ( F i g u r e 30). S u c h q u i c k l y r e v e r s i b l e Mean Current required to excite Cell (nA) H-0 H fD M 00 i H i 0 Pi > o g k 0 3 fD O . H - f D tf H 3 - rt & • rt rt H- fD 3 < 0> 1 H - fD 3 rt O 13 H Pi Pi 3- C H- rt pj hi 3 •< H i DJ M I"! a> i M 3 (u (D cn O H- 3 3 • 3 fD 3 3 H- rt J O < . 0 O ' 3 fD 0 H t 1 3 tr 0) 3 CD ' . fD cn n fD iQ ' < CD fD H 1 C C 0 <! H Pi l-j H-W O < cn o H fD X pj . 3 fD & fD rt fD Pi 1 P. H- cn > O O S rt f fD PJ 3 fD c: 0 ' M 3 cn p; cn 1—' P i 3 H- fD cn cn 3 x i iQ 1 [ * 1 fD | to ID rt Hi O o S 3- hi O H- fD H- Q rt &» l~S 3 0) CD 3 rt 3 fD cn Pi rt 0 fD CD 5* C 3 < (T1 PJ H fD O M H 3 3 • P> fD 0 3 3 3 fD fi-1 < S O fD 0 CD ts < ft) 3 CD 0 CD 3 CD *P X P i CO d CD H-Pi 0> Pi 0 H 3 fD 3 CD O Pi H fD Pi (D W- cn < rt H 1 C O cn Pi cn • H-3 ex-66 o L_ ro o O 0> o 00 o _ l C/5 CD 3 (/> —••' >< O -*> W o CD < O 7T o CD a. > O Q 3 " a. O 3 3 Q. on > i CD > OA 3T CD Q. H 3 " a o 3 o O cp_ 5T Mean Current required to excite Cell (nA) TO o i 4k O cn o CD o o o _ l 3 o 3 I co s a. CD < O a. 5 cn o 3 < o $ g? o TO _ 03 O O O CD O O 3 X-a. CD _ a. I a _ 3 O a. S. 3 Q O 3 = o i a. dHSE 50 ACh32 DLH40 ACh 32 DLH40 ACh 32 DLH40 ACh 32 ACh DLH 4 0 32 ACh DLH 32 4 0 F i g u r e ,30. R a t e m e t e r r e c o r d o f p r e f e r e n t i a l b l o c k i n g o f ACh ' e x c i t a t i o n o f s i n g l e VL n e u r o n e by DH3E and a t r o p i n e . , T r a c e s t o p p e d , f o r 8 r a i n a t (a) and . 4. m i n a t (b);-Vv% ', •'. 101 b l o c k i n g was s o m e t i m e s s e e n w i t h a t r o p i n e as i n F i g u r e 3 0, b u t f r e q u e n t l y a f t e r i o n t o p h o r e t i c a t r o p i n e o n l y p a r t i a l r e c o v e r y . o f ACh e x c i t a t i o n o c c u r r e d e v e n 2.0-30 m i n u t e s a f t e r a t r o p i n e d e l i v e r y was d i s c o n t i n u e d . I n a few i n s t a n c e s i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e a p p e a r e d t o b l o c k t h e e x c i t a t i o n o f c e l l s by amino a c i d s as w e l l a s ACh, s o t h a t a n o n - s p e c i f i c d e s e n s i t i z a t i o n t o i o n t o p h o r e t i c a l l y a p p l i e d e x c i t a n t s seemed t o h a v e o c c u r r e d . N o n - s p e c i f i c b l o c k i n g was n o t s e e n w i t h DH3.E. A t r o p i n e was u s u a l l y s u c c e s s f u l i n b l o c k i n g ACh e x c i t a t i o n s , w h e r e a s DH3E h a d e i t h e r no o r o n l y a s m a l l b l o c k i n g e f f e c t on a b o u t h a l f t h e n e u r o n e s t e s t e d . The e f f e c t s o f a t r o p i n e a n d DH3E on s y n a p t i c a c t i v a t i o n o f VL c e l l s b y BC, c o r t i c a l and EN s t i m u l a t i o n w e r e t e s t e d . D u r i n g t h e s e t e s t s , a t t e m p t s w e r e u s u a l l y made t o u s e two t y p e s o f c o n t r o l . To e n s u r e t h a t a n o n - s p e c i f i c b l o c k o f n e u r o n e e x c i t a b i l i t y h a d n o t o c c u r r e d , t h e s e n s i t i v i t y o f t h e n e u r o n e s t o i o n t o p h o r e t i c a l l y a p p l i e d DLH was t e s t e d p e r i o d i c a l l y . The e f f e c t , o f ACh was a l s o u s u a l l y t e s t e d t o o b s e r v e any. c o r r e l a t i o n o f a r e d u c t i o n o f ACh s e n s i t i v i t y a n d o f s y n a p t i c a c t i v a t i o n . T h e s e c o n t r o l s w e r e n o t a l w a y s p o s s i b l e i n c a s e s w h e r e c e l l s d i d n o t seem s e n s i t i v e t o ACh. The e f f e c t o f i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e was e x a m i n e d i n n i n e c e l l s e v o k e d b y BC, s i x w i t h a s h o r t l a t e n c y s p i k e a n d t h r e e w i t h an e a r l y b u r s t r e s p o n s e . No 102 c h a n g e i n t h e e v o k e d r e s p o n s e s was s e e n a l t h o u g h t h e ACh s e n s i t i v i t y o f t h e c e l l s was b l o c k e d . F i g u r e 31 shows a t y p i c a l r e s u l t i n a c e l l . w h i c h was e v o k e d o r t h o d r o m i c a l l y b y b o t h BC and c o r t i c a l s t i m u l a t i o n . The s p i k e i n t h i s c a s e was s l i g h t l y s m a l l e r a f t e r ACh s e n s i t i v i t y h a d b e e n a b o l i s h e d ( F i g u r e 31 - b u t t h i s was p r o b a b l y due t o a s l i g h t , d i s p l a c e m e n t o f t h e e l e c t r o d e t i p w i t h r e s p e c t t o t h e c e l l i n t h e t i m e b e t w e e n t h e two p h o t o g r a p h s . The l a t e n c i e s o f t h e e v o k e d s p i k e a n d t h e r e g u l a r i t y o f s u c c e s s f u l r e s p o n s e s was n o t a l t e r e d f o r e i t h e r s t i m u l u s b y t h e ACh b l o c k . DH/3E was a p p l i e d i o n t o p h o r e t i c a l l y t o n i n e BC e v o k e d c e l l s , i n c l u d i n g two e v o k e d w i t h e a r l y b u r s t s . E i g h t o f t h e s e r e s p o n s e s showed no c h a n g e d u r i n g o r a f t e r t h e DH3E a p p l i c a t i o n a l t h o u g h t h e ACh s e n s i t i v i t y o f t h e c e l l s was m a r k e d l y r e d u c e d o r a b o l i s h e d . The a c t i v a t i o n o f one c e l l was r e v e r s i b l y b l o c k e d b y DH3E. T h i s c e l l r e s p o n d e d w i t h a s h o r t l a t e n c y s p i k e , a n d t h e s y n a p t i c b l o c k i n g c o r r e l a t e d c l o s e l y w i t h d e p r e s s i o n o f t h e c e l l ' s s e n s i t i v i t y t o ACh ( F i g u r e 3 2 ) . . The c e l l was t e s t e d t w i c e i n s u c c e s s i o n , a n d i n e a c h c a s e b o t h t h e ACh s e n s i t i v i t y a n d BC a c t i v a t i o n w e r e b l o c k e d w i t h i n 1-2 m i n u t e s . a f t e r t h e DH3E was a p p l i e d and r e c o v e r e d a f ew m i n u t e s a f t e r i t s c e s s a t i o n . A t r o p i n e s u l p h a t e w a s . i n j e c t e d i n t r a v e n o u s l y i n t h r e e c a s e s w h i l e BC e v o k e d c e l l s w e r e r e c o r d e d . Two o f t h e c e l l s t e s t e d r e s p o n d e d t o BC s t i m u l a t i o n w i t h an e a r l y b u r s t 103 F i g u r e 31. I . S i n g l e VL n e u r o n e e v o k e d o r t h o d r o m i c a l l y by'.: • b o t h c o r t i c a l ( f i r s t r e s p o n s e ) and BC ( s e c o n d r e s p o n s e ) . A. w i t h no d r u g s ; B. a f t e r ACh e x c i t a t i o n b l o c k e d by i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e ( s e e f i g u r e 3 1 - 1 1 ) . DLH 20 ACh60 DLH 20 ACH60 DLH 20 ACh 60 DLH 20 ACh 60 DLH 20 AGn 60 F i g u r e 3 1 . I I . , - R e c o r d . o f s p e c i f I c b l o c k i n g o f ACh e x c i t a -: t i o r i . o f n e u r o n e s h o w r i ' i n ; '31-1. T r a c e s o f 31-1 o b t a i n e d , a t a a n d b. , 104 r e s p o n s e , a n d a p p e a r e d t o be u n a f f e c t e d by a t r o p i n e i n j e c t e d i n d o s e s o f 1.0 and 1.5 mgm/Kg r e s p e c t i v e l y . I n t h e t h i r d c a s e , a s m a l l g r o u p o f n e u r o n e s was e v o k e d w i t h s h o r t l a t e n c y , and c e r e b e l l a r a c t i v a t i o n o f t h e s e c e l l s was d e p r e s s e d b y 0. 5 mgm/Kg o f a t r o p i n e ( F i g u r e 33). The c e l l s w e r e e v o k e d r e l i a b l y b e f o r e t h e i n j e c t i o n , b u t o n l y r a r e l y a f t e r w a r d . T h a t t h e c e l l s h a d n o t b e e n l o s t , and w e r e s t i l l s e n s i t i v e t o i o n t o p h o r e t i c a l l y a p p l i e d DLH i s shown i n F i g u r e 33 C, w h i c h was r e c o r d e d d u r i n g a DLH a p p l i c a t i o n . E v e n d u r i n g t he'DLH e x c i t a t i o n , t h e c e l l s w e r e n o t e v o k e d b y c e r e b e l l a r s t i m u l a t i o n . The r e d u c t i o n i n DLH s e n s i t i v i t y s o m e t i m e s f o u n d a f t e r i o n t o p h o r e t i c a t r o p i n e was n e v e r s e e n a f t e r 1. V. a t r o p i n e , e v e n a t d o s e s o f 2 mgm/Kg. I n two o t h e r e x p e r i m e n t s , f i e l d r e s p o n s e s i n VL t o s t i m u l a t i o n o f BC and c o r t e x w e r e r e c o r d e d by a s i n g l e e l e c - . t r o d e w h i l e 1 mg/Kg o f a t r o p i n e was i n j e c t e d i n t r a v e n o u s l y . I n e a c h c a s e t h e r e s p o n s e t o BC s t i m u l a t i o n was m a r k e d l y r e d u c e d w h i l e t h a t f o l l o w i n g s t i m u l a t i o n o f p r e c r u c i a t e c o r t e x was n o t n o t i c e a b l y a l t e r e d ( F i g u r e 3 4 ) . S y n a p t i c a c t i v a t i o n o f s i n g l e VL c e l l s b y p r e c r u c i a t e . c o r t e x s t i m u l a t i o n was t e s t e d f o r b l o c k i n g i n . f o u r c a s e s w i t h i o n t o p h o r e t i c a t r o p i n e and i n t h r e e w i t h DHpE. No ch a n g e i n t h e r e s p o n s e was s e e n i n any o f t h e s e t e s t s ( e . g . , F i g u r e 31). A t o t a l o f t w e n t y - t w o t e s t s w e r e c o n d u c t e d on c e l l s e v o k e d ; by EN s t i m u l a t i o n , u s i n g a t r o p i n e o r DH3E. F o r t w e l v e F i g u r e 32, i F i g u r e 33. mil ilnirtilij 105 10 MSEC 1 SEC B l o c k i n g o f BC e v o k e d s p i k e (on l e f t ) a n d i t s ACh e x c i t a t i o n (on r i g h t ) b y i o n t o p h o r e t i c DH3E; A. o n l e f t , BC e v o k e d s p i k e w i t h no d r u g s ; on r i g h t 10 n A ACh i n d u c e d c e l l d i s -c h a r g e s ; B.' a f t e r DH$E 65;';n A f o r 1% m i n , BC d o e s n ' t e v o k e s p i k e , ACh no . l o n g e r c a u s e s c e l l f i r i n g ; C. 2% m i n a f t e r D H 3 E - a p p l i c a t i o n s t o p p e d , BC s t i m u l u s 'evoke's'..:spike a n d ACh e x c i t a t i o n h a s r e t u r n e d . B r~(^ 100 10 MSEC 100 MSEC VL c e l l s e v o k e d w i t h s h o r t l a t e n c y b y BC s t i -m u l u s . E a c h p a i r o f t r a c e s b e g i n s a t same t i m e , b u t sweep t i m e d i f f e r e n t ; A. w i t h no d r u g s ; B. 4 m i n a f t e r .0.5img/kg a t r o p i n e s u l p h a t e . LV,-C. d u r i n g i o n t o p h o r e t i c a p p l i c a t i o n - - o f " 40 nA DLH. 105a F i g u r e 34. BC and c o r t i c a l l y evoked f i e l d responses i n VL ; V'^ '- b e f o r e and a f t e r 1.0 mg/kg a t r o p i n e sulphate ;j ."':.'• I.V. Each column d e p i c t s f i v e c o n s e c u t i v e ",b r e c o r d s o b t a i n e d a t 1 sec i n t e r v a l s . ' ; A".;-BC evoked responses b e f o r e a t r o p i n e ; B.'.same, 4 min, | a f t e r i n j e c t i o n ; C. c o r t i c a l l y evoked responses • b e f o r e a t r o p i n e ; D. same, 5 min a f t e r i n j e c t i o n . • 106 o f t h e t h i r t e e n i o n t o p h o r e t i c a t r o p i n e t e s t s , no c h a n g e i n t h e e v o k e d r e s p o n s e was o b s e r v e d . I n t h e o t h e r c a s e , t h e r e s p o n s e was p a r t i a l l y d e p r e s s e d i n t h a t t h e c e l l was n o t e v o k e d as f r e q u e n t l y a f t e r t h e a t r o p i n e a p p l i c a t i o n . ACh was more e f f e c t i v e t h a n a t r o p i n e i n d e p r e s s i n g t h e r e s p o n s e o f t h i s c e l l . T h r e e o f t h e s e t h i r t e e n c e l l s w e r e e x c i t e d b y A C h , and i n e a c h c a s e t h e e x c i t a t o r y r e s p o n s e was b l o c k e d by a t r o p i n e . The r e s p o n s e s o f s i x o f t h e c e l l s w e r e d e p r e s s e d by ACh. T h i s d e p r e s s i o n was e l i m i n a t e d b y i o n t o p h o r e t i c a t r o p i n e i n t h r e e c a s e s , p a r t i a l l y b l o c k e d i n o n e , a n d u n a f f e c t e d i n t h e r e m a i n i n g t w o . F i v e E N - e v o k e d c e l l s w e r e s t u d i e d w i t h i n t r a v e n o u s a t r o p i n e i n minimum d o s e s o f 1 mg/Kg and i n no c a s e was t h e e v o k e d r e s p o n s e a l t e r e d . . One o f t h e s e c e l l s was i n d u c e d t o r e s p o n d t o s t i m u l a t i o n w i t h a s i n g l e 2 msec, l a t e n c y s p i k e d u r i n g ACh a p p l i c a t i o n , and t h i s e x c i t a t i o n was b l o c k e d by t h e a t r o p i n e ( F i g u r e 35B) i 'The same c e l l and a n o t h e r one w e r e a l s o i n d u c e d t o r e s p o n d w i t h s h o r t l a t e n c y by.DLH, b u t t h e DLH e x c i t a t i o n was not a f f e c t e d by I.V. a t r o p i n e ( F i g u r e 35C) . The EN r e s p o n s e s o f t h r e e o f t h i s g r o u p o f c e l l s w e r e d e p r e s s e d by ACh. A t r o p i n e I.V. p a r t i a l l y b l o c k e d t h e d e p r e s s i o n i n two and d i d n o t a f f e c t t h e d e p r e s s i o n i n t h e t h i r d . DHgE was t e s t e d i n t r a v e n o u s l y (0.5 mg/Kg) on two c e l l s t h e EN r e s p o n s e s o f w h i c h w e r e b o t h d e p r e s s e d by i o n t o p h o r e t i c ACh. N e i t h e r t h e ACh d e p r e s s i o n n o r t h e e v o k e d r e s p o n s e s w e r e a l t e r e d . I o n t o p h o r e t i c DH3E d i d n o t 107 A B C i 25 MSEC F i g u r e 35. E f f e c t o f 1 mg/kg a t r o p i n e s u l p h a t e I . V . on EN e v o k e d VL n e u r o n e , a n d i t s e x c i t a t i o n by" ACh a n d DLH; A. t o p t r a c e — E N e v o k e d b u r s t w i t h no d r u g s ; b o t t o m - - 8 m i n a f t e r a t r o p i n e i n j e c t i o n ; . B. t o p — A C h 30 nA c o n v e r t s ' l o n g ' l a t e n c y b u r s t t o s h o r t l a t e n c y s i n g l e • s p i k e ; b o t t o m - - 5 m i n a f t e r a t r o p i n e i n j e c t i o n , e x c i t a t i o n d i s a p p e a r s and ACh d e p r e s s i o n ' o f b u r s t . i s e v i d e n t ; C. t o p — D L H 40 nA c o n v e r t s b u r s t ' r e s p o n s e t o s h o r t l a t e n c y s i n g l e s p i k e ; b o t t o m — 7 m i n a f t e r a t r o p i n e i n j e c t i o n , DLH s t i l l c a u s e s s h o r t l a t e n c y r e s p o n s e s . Top p a r t s o f B and C ar.e;: composed o f s e v e r a l s u p e r i m p o s e d t r a c e s . ^ 5 108 B 200 25MSEC ; F i g u r e 36. I . VL n e u r o n e e v o k e d i n b u r s t by E N . s t i m u l u s ; A. w i t h no d r u g ; B. a f t e r ACh e x c i t a t i o n r e -d u c e d b y i o n t o p h o r e t i c DH3E; C. after\<gGhy:A. " e x c i t a t i o n , b l o c k e d • b y i o n t o p h o r e t i c ''atropine,'... 40 a 20 L0 (B) i r DLH 40 ACh 40 (D) dHpE 50 i — i i — i DLH 40 ACh 40 Atr. 60 • 1 DLH 40 ACh 40 ACh 40 DLH 40 ACh 40 F i g u r e ' 36. I I . Ratemeter.- r e c o r d o f ACh and DLH e x c i t a t i o n | o f n e u r o n e d e p i c t e d i n 3 6 - 1 . T r a c e s f o r 3 6 - 1 - j ;>.;',.'•''.'• A/ B and C o b t a i n e d ' a t ' a , b , and c r e s p e c t i v e l y . j J 109 a f f e c t t h e EN r e s p o n s e o f two o t h e r c e l l s t e s t e d . The ACh e x c i t a t i o n w h i c h h a d b e e n s e e n i n one o f t h e s e was/ h o w e v e r , b l o c k e d . An e x a m p l e o f one o f t h e " c e l l s t e s t e d i o n t o p h o r - _ e t i c a l l y w i t h b o t h a t r o p i n e a n d DHBE i s shown i n F i g u r e 36. The r e s u l t s o f t h e c h o l i n e r g i c b l o c k i n g e x p e r i m e n t s on E N - e v o k e d c e l l s a r e s u m m a r i z e d i n T a b l e I I . TABLE I I EFFECTS OF CHOLINERGIC BLOCKING AGENTS ON EN-EVOKED CELLS RESPONSE ATROPINE DHBE TOTAL i o n t o -p h o r e t i c I.V. i o n t o -p h o r e t i c I.V. E N - e v o k e d 1/13 0/5 0/2 0/2 1/22 ACh e x c i t a t i o n 3/3 1/1 1/1 5/5 ACh d e p r e s s i o n 4/6 2/3 - 0/2 6/11 * r a t i o i n d i c a t e s s u c c e s s f u l b l o c k i n g a t t e m p t s / t o t a l t r i a l s ( i i ) A m i no A c i d D e r i v a t i v e s - S e v e r a l e x p e r i m e n t s w e r e c a r r i e d o u t t o t e s t t h e a b i l i t y o f two g l u t a m i c a c i d d e r i v a t i v e s a - m e t h y l - D , L - g l u t a m i c a c i d (aMG) and N - m e t h y l - D , - L - g l u t a m i c a c i d (NMG) , t o a l t e r e x c i t a t i o n o f t h a l a m i c c e l l s e x c i t e d , , by LG. The N - m e t h y l d e r i v a t i v e was n o t f o u n d t o e x e r t any c o n s i s t e n t b l o c k i n g a c t i o n . , I n a few c a s e s i t seemed s l i g h t l y : t o d e p r e s s o r p o t e n t i a t e t h e g l u t a m a t e e x c i t a t i o n ( F i g u r e 3 7 ) . 110 M o r e i n t e r e s t i n g r e s u l t s w e r e o b t a i n e d w i t h a - m e t h y l g l u t a m a t e . I n n i n e o f f i f t y - f o u r c e l l s t e s t e d a d e f i n i t e a n d a t l e a s t p a r t i a l l y s p e c i f i c b l o c k o f LG a c t i v a t i o n was n o t e d . I n t w e n t y o t h e r s , LG e x c i t a t i o n was b l o c k e d o n l y p a r t i a l l y , o r i n a d e q u a t e c o n t r o l s w e r e a v a i l a b l e t o d i s t i n g u i s h t h e s p e c i f i c i t y o f t h e e f f e c t s . The b l o c k i n g by aMG was f o u n d i n two f o r m s . . I n a few c e l l s , b l o c k a d e o c c u r r e d d u r i n g aMG a p p l i c a t i o n and f o r a s h o r t t i m e a f t e r w a r d ( F i g u r e 3 7 ) . I n m o s t c a s e s , h o w e v e r , o n l y a p a r t i a l r e c o v e r y o f LG a c t i v a t i o n was s e e n a f t e r t h e e n d o f a p p l i c a t i o n and i n d e e d t h e e x t e n t o f d e p r e s s i o n o f t h e g l u t a m a t e r e s p d n s e was f r e q u e n t l y e v e n g r e a t e r a f t e r aMG a p p l i c a t i o n e n d e d ( F i g u r e 3 8 ) . The a n t a g o n i s t i c a c t i o n o f aMG t o w a r d s g l u t a m a t e d i d n o t a p p e a r t o be r e l a t e d t o w h e t h e r t h e c e l l was e v o k e d by BC s t i m u l a t i o n , as b l o c k i n g was s e e n i n b o t h BC e v o k e d VL c e l l s and i n n o n - e v o k e d VL c e l l s a n d i n n o n - e v o k e d c e l l s l y i n g d o r s a l t o VL. I n f o u r i n s t a n c e s t h e r e s p o n s e t o BC s t i m u -l a t i o n was n o t a f f e c t e d by aMG, a l t h o u g h a d e f i n i t e b l o c k o f LG a c t i v a t i o n o c c u r r e d . S e v e r a l o f t h e c e l l s whose LG s e n s i -t i v i t y was d e p r e s s e d by aMG a l s o became l e s s s e n s i t i v e t o DLH o r a s p a r t a t e a f t e r aMG a p p l i c a t i o n s , h o w e v e r , t h e e f f e c t on LG a c t i o n s was a l m o s t a l w a y s g r e a t e r . I n t h o s e c a s e s w h e r e ACh was u s e d as a c o n t r o l , t h e ACh s e n s i t i v i t y was u n a l t e r e d , e v e n when LG a c t i v a t i o n was c o m p l e t e l y d e p r e s s e d . .1.1.1. NMG 90 aMG 9 0 100 r 50 50 <o X 2 50 50 5 0 5 0 50 5 0 5 0 50 6 0 s . : F i g u r e '37. R a t e m e t e r . r e c o r d o f e x c i t a t i o n o f VL n e u r o n e b y 1 ': . : • 50 nA LG d u r i n g i o n t o p h o r e t i c a p p l i c a t i o n s o f NMG 90 nA and aMG 9.0 nA.-x 2 5 GUJT DLH 70 IS F i g u r e 38. 30 s. R a t e m e t e r r e c o r d s . o b t a i n e d f r o m two d i f f e r e n t c e l l s . . • A-D. e x c i t e d by'.*LG.-:75 nA and ACh 3 5 " nA;JyE "and'.ivE" e x c i t e d b y -jLG".!70 nA arid'' DLH 15 nA; A\0^' e:'* c o n t r o l s ;^B:;.'arid';"F;; .3./min a f t e r .the s%3$& • o f a M G a p p l i c a t i o n . ; ' . ; ( 9 0 . n A ) ; C. 1 m i n and. , D . ^ l l m i n a f t e r ,theV-aMG; c u r r e n t was t e r m i n a t e d , The m i n i m a ; minate'd-iT j - j - unn ar.rer ..tnev-aMG1 current was terminated. 5 • ^S&esponse •in ' . ' - t h^cell of Ev--E'-'showed only tirnaRecovery i5.,!M-n>;aftef aMG'.current ter-112 ( i i i ) S t r y c h n i n e - The e f f e c t s o f s t r y c h n i n e w e r e s t u d i e d on t h e s p o n t a n e o u s and e v o k e d b u r s t i n g a c t i v i t y o f VL c e l l s , a n d / o r t h e d e p r e s s i o n o f d r u g i n d u c e d f i r i n g by BC s t i m u l a t i o n . S t r y c h n i n e was a p p l i e d i o n t o p h o r e t i c a l l y t o n i n e c e l l s a n d i n t r a v e n o u s l y d u r i n g t h e s t u d y o f two o t h e r s . No c h a n g e i n t h e b u r s t i n g a c t i v i t y o r BC d e p r e s s i o n o f f i r i n g was s e e n i n any o f t h e s e c a s e s ( F i g u r e 39) . ( i v ) A n e s t h e t i c s - O b s e r v a t i o n s i n - m a ny e x p e r i m e n t s on c h l o r a l o s e a n d b a r b i t u r a t e a n e s t h e t i z e d and d e c e r e b r a t e a n i m a l s s u g g e s t e d t h a t d i f f e r e n t r e s u l t s c o u l d be o b t a i n e d u n d e r t h e s e v a r i o u s c o n d i t i o n s . I n c h l o r a l o s e a n e s t h e t i z e d a n i m a l s , f o r e x a m p l e , c e l l s i n VL e x h i b i t e d l e s s s p o n t a n e o u s a c t i v i t y and w e r e g e n e r a l l y much l e s s s e n s i t i v e o r i n s e n s i t i v e t o t h e i o n t o p h o r e t i c a p p l i c a t i o n o f ACh. T h i s e f f e c t was d i r e c t l y e x a m i n e d i n one d e c e r e b r a t e p r e p a r a t i o n t o w h i c h c h l o r a l o s e was a d m i n i s t e r e d i n t r a v e n o u s l y w h i l e t h e d r u g s e n s i t i v i t y o f a VL c e l l was t e s t e d ( F i g u r e 4 0 ) . T h i s c e l l was e x c i t e d b y i o n t o p h o r e t i c a l l y a p p l i e d AChi.and DLH b e f o r e t h e c h l o r a l o s e was g i v e n . W i t h i n 2 m i n u t e s o f t h e i n j e c t i o n o f 30 mg/Kg t h e c e l l h a d become i n s e n s i t i v e t o ACh, and was l e s s s e n s i t i v e t o DLH. W h i l e t h e DLH s e n s i t i v i t y r e c o v e r e d i n . a s h o r t t i m e , ACh r e m a i n e d u n a b l e t o e x c i t e t h e c e l l , e v e n when t h e e j e c t i n g c u r r e n t was c o n s i d e r a b l y i n c r e a s e d . VL c e l l s w e r e f r e q u e n t l y v e r y s e n s i t i v e t o ACh i n D i a l a n e s t h e t i z e d a n i m a l s , b u t t h e same d e g r e e o f s e n s i t i v i t y I i I A m MMliMM mmmtm 113 B i^ w#»f >< Wvii^j^ M" > > j mm t 1 SEC 200 JLiV F i g u r e 39. E f f e c t o f . i o n t o p h o r e t i c a l l y a p p l i e d s t r y c h n i n e •''"')) o n BC e v o k e d r h y t h m i c b u r s t i n g a n d d e p r e s s i o n o f . f i r i n g . A. f i r i n g i n d u c e d i n VL n e u r o n e b y 6'riA DLH ' i s d e p r e s s e d b y BC s t i m u l a t i o n ; B. ' same a f t e r 90 nA s t r y c h n i n e f o r 4 m i n ; C. f i r i n g !' i n d u c e d i n VL n e u r o n e s b y 12 nA ACh i s c o n v e r t e d , t o b u r s t i n g b y BC s t i m u l a t i o n ; D. same a f t e r p 90 nA s t r y c h n i n e , f o r 2 m i n . ( A ) ( C ) i A C h D L H A C h D L H A C h D L H 10 5 5 10 5 5 10 5 5 fU A C h 10 D L H 5 5 D L H 5 5 A C h 5 0 A C h 8 0 D L H 5 5 D L H 5 5 F i g u r e 40.' R a t e m e t e r r e c o r d o f e f f e c t o f a - c h l o r a l o s e o n f DLH •and: ACh i n d u c e d ' f i r i n g o f a VL n e u r o n e i n |o d e c e r e b r a t e a n i m a l ; A..' c o n t r o l ; B. 2 min_ j; a f t e r 30 n g / k g a - c h l o r a l o s e I.V.; C. 28 m i n : a f t e r c h l o r a l o s e . i n j e c t i o n . ' • 114 1 i 1 i 1 i 1 DLH 40 ACh 15 DLH 40 ACh 15 I : 1 1-DLH40 ACh 15 -I I 1 I DLH 4 0 ACh 15 40 6 0 sec. I 1 ; 2 0 -DLH 40 ACh 15 DLH 40 ACh 15 ! co Figure- 41. -Ratemeter r e c o r d of e f f e c t o f sodium-pentobar-' b i t a l on DLH and ACh induced f i r i n g . ';6f/ a VL . neurone i n de c e r e b r a t e animal. 2 mg'/kg' pento-v , b a r b i t a l i n j e c t e d a t ( . a ) 1 0 min, l a t e r a d d i -t i o n a l 2 m'g/kg i h j e c t e d ^ a t (B;)'. . 10 min l a t e r a d d i t i o n a l 4 mg'/kg i n j e c t e d - a t (c) . 115 was not u s u a l l y seen i n p e n t o b a r b i t a l p r e p a r a t i o n s . T h i s agent was, t h e r e f o r e , t e s t e d i n the same way as c h l o r a l o s e was above. The p e n t o b a r b i t a l was g i v e n as a s e r i e s of s m a l l i n j e c t i o n s . F i g u r e 41 shows t h a t the ACh s e n s i t i v i t y of a VL c e l l d isappeared when the cumulative dose reached 8 mgm/Kg . DLH e x c i t a b i l i t y was at the same time reduced but i t was s t i l l e v i d e n t i n s p i t e of the g r e a t e r e f f e c t of ACh i n the c o n t r o l p e r i o d . There appears, t h e r e f o r e , to have been a p r e f e r e n t i a l b l o c k of ACh s e n s i t i v i t y . I t should be noted t h a t the l a t e n c y of the ACh e x c i t a t i o n i n c r e a s e d as a d d i t i o n a l s m a l l i n j e c t i o n s were g i v e n . Sodium t h i o p e n t a l was s i m i l a r l y t e s t e d i n another decer e b r a t e p r e p a r a t i o n . At r e l a t i v e l y h i g h doses, the ACh s e n s i t i v i t y seemed depressed, w h i l e DLH s e n s i t i v i t y remained a t about the c o n t r o l l e v e l s (Figure 42). A more marked e f f e c t was the i n c r e a s e i n l a t e n c y to e x c i t a t i o n by ACh. The l a t e n c y f o r e x c i t a t i o n by DLH was a l s o i n c r e a s e d , but t o a much l e s s e r extent as shown i n F i g u r e 43. An e f f e c t noted e a r l i e r , i . e . a d e p r e s s i o n of spontaneous f i r i n g between the onsets of ACh a p p l i c a t i o n and the e x c i t a t i o n caused by ACh i s apparent i n F i g u r e s 4 i & 42. In the experiment re p r e s e n t e d i n F i g u r e 41, the s p i k e r e c o r d e d on paper was evoked w i t h s h o r t l a t e n c y by BC stimu-l a t i o n , as were a few s m a l l e r s p i k e s . A f t e r 8 mg/Kg of p e n t o b a r b i t a l , h o w e v e r t h e c e l l s o f t e n responded to BC s t i m u l a t i o n w i t h an e a r l y b u r s t type response (Figure 44).. 116 40 r ACh 10 DLH20 ACh 10 DLH 20 ACh 10 60 sees. 20 • ACh 10 DLH 20 ACh IQ- • DLH 20 ACh 10 ACh 10 DLH 20 ACh 10 DLH 20 ACh 10 . DLH 20 ACh 10 F i g u r e 42. Ratemeter r e c o r d of e f f e c t . o f sodium t h i o p e n t a l on DLH and ACh induced f i r i n g of a VL neurone ; • i n a dec e r e b r a t e animal; t o p • r e c o r d - - c o n t r o l ; second r e c o r d — a f t e r 2.5 .mg/kg 'sodium ..thiopental I.V. ; t h i r d r e c o r d— 2 0 m i t i ^ later"''Rafter/7.-. 5 mg/kg t h i p p e n t a l t o t a l ; • • bp^om^re.cofd'--'20'- m'in l a t e r a f t e r 20 mg/kg • t h i o p e n t a l ^ t p t a l . I:'y *•' Effect of Thiopental on Latency of A C h and DLH Excitation of a V L Neurone ! 2 01 " — i i-x 1 1 T — 0 10 20 30 40 50 Latency (sees.) F i g u r e 43. G r a p h o f l a t e n c y t o c e l l f i r i n g a f t e r a p p l i c a -t i o n o f DLH o r ACh f o l l o w i n g I.V. i n j e c t i o n s of,; s o d i u m t h i o p e n t a l . I n j e c t i o n s . a d m i M l s t e r e d . ' . at 10 m i n i n t e r v a l s . '.'"1 J' • A flW1"" ••' - 4oo I J [ F i g u r e 4 4 . BC e v o k e d .VL c e l l s . A. . c o n t r o l ; -B. a f t e r ' 8 mg/. ' kg. s o d i u m p e n t o b a r b i t a l . ; , ; 118 NUCLEUS CENTRUM MEDIANUM' Many e x p e r i m e n t s were, c o n d u c t e d t o s t u d y t h e n e u r a l p a t h w a y f r o m t h e e n t o p e d u n c u l a r n u c l e u s and f r o m p r e c r u c i a t e c o r t e x t o t h e n u c l e u s c e n t r u m medianum.. S t i m u l a t i o n i n t h e r e g i o n o f t h e e n t o p e d u n c u l a r n u c l e u s was f o u n d t o e v o k e s h o r t l a t e n c y (2-10 msec.) a c t i v i t y i n c e l l s o f t h e c e n t r u m medianum c o m p l e x . M o s t c e l l s r e s p o n d e d w i t h s i n g l e s p i k e s , t h o u g h some showed b u r s t r e s p o n s e s . C e l l s w e r e e v o k e d i n t h e more m e d i a l as w e l l as t h e l a t e r a l p o r t i o n s o f t h e CM -P f c o m p l e x . Some e x p e r i m e n t s i n w h i c h h i s t o l o g i c a l , s t u d y showed t h e EN s t i m u l a t i n g e l e c t r o d e l y i n g i n t h e i n t e r n a l c a p s u l e a b o v e EN g a v e a s i m i l a r v a r i e t y o f r e s p o n s e s . I t was n o t p o s s i b l e t h e r e f o r e t o a t t r i b u t e any p a r t i c u l a r t y p e o r l a t e n c y o f r e s p o n s e s p e c i f i c a l l y t o a c t i v a t i o n o f t h e EN-CM p a t h w a y . S t i m u l a t i o n o f t h e p r e c r u c i a t e c o r t e x g a v e more d e f i n i t i v e r e s p o n s e s i n t h e CM.- P f c o m p l e x . A c o m p l e x n e g a t i v e - p o s i t i v e - n e g a t i v e wave r e s p o n s e was o f t e n s e e n . C h e c k i n g o f t h e a n a t o m i c a l l i m i t s o f t h i s r e s p o n s e i n s e v e r a l e x p e r i m e n t s i n d i c a t e d t h a t i t c o u l d be e l i c i t e d o n l y i n t h e l a t e r a l p a r t o f t h e n u c l e a r c o m p l e x and t h r e e e x p e r i r -m e n t s u s i n g l a r g e - ^ t i p p e d e l e c t r o d e s w e r e t h e r e f o r e p e r f o r m e d t o p r o v i d e a p r e c i s e map o f t h e l o c a l i z a t i o n o f t h e r e s p o n s e . I n t h e s e e x p e r i m e n t s , a f t e r t h e r e s p o n s e was l o c a t e d , t h e 119 r e c o r d i n g e l e c t r o d e was moved .down i n 1 mm i n t e r v a l s t o f i n d t h e v e r t i c a l l i m i t s ' o f t h e r e s p o n s e . The e l e c t r o d e was t h e n moved l a t e r a l l y b y 0.5 - 0.6 mm t o a new t r a c k and t h e v e r t i c a l m a p p i n g was r e p e a t e d . . F u r t h e r t r a c k s w e r e made i n t h e l a t e r a l d i r e c t i o n u n t i l one was f o u n d w h i c h g a v e no s i g n o f t h e r e s p o n s e a t any v e r t i c a l ' l e v e l . The same p r o c e d u r e was t h e n r e p e a t e d i n t r a c k s m e d i a l t o t h e o r i g i n a l o n e , a g a i n u n t i l t h e r e s p o n s e c o u l d no l o n g e r be f o u n d . S e v e r a l e x a m p l e s o f t h e r e s p o n s e w e r e r e c o r d e d a t e a c h o f t h e 1 mm v e r t i c a l , s t e p s . The m a p p i n g p r o c e s s was c a r r i e d o u t i n two o r t h r e e d i f f e r e n t f r o n t a l p l a n e s i n e a c h o f t h e s e e x p e r i m e n t s . A l l t h e s e - e x p e r i m e n t s c o n f i r m e d t h e i m p r e s s i o n g a i n e d f r o m t h e p r e l i m i n a r y w o r k t h a t t h e r e s p o n s e was l a r g e l y c o n f i n e d t o CM a n d was a b s e n t o r m a r k e d l y r e d u c e d i n m e d i a l p a r t s o f t h e CM-Pf c o m p l e x and i n o t h e r a d j a c e n t n u c l e i . The r e s u l t s f r o m one o f . t h e m a p p i n g e x p e r i m e n t s a r e shown and r e l a t e d t o M e h l e r ' s [1966] d e f i n i t i o n o f t h e b o u n d a r i e s o f CM i n F i g u r e 45. I t c a n be s e e n f r o m t h i s f i g u r e t h a t t h e e a r l i e r n e g a t i v e wave was r e c o r d e d o v e r a l a r g e a r e a , b u t t h e p o s i t i v e a n d l a t e r n e g a t i v e waves w e r e l a r g e o n l y i n CM. The-c o r r e l a t i o n w i t h M e h l e r ' s l i m i t s i s v e r y c l o s e a t f r o n t a l p l a n e s A 7.5 and A 7.0, and o n l y s l i g h t l y d i v e r g e n t a t A 6;5. V e r y s i m i l a r r e s p o n s e s , w i t h a b o u t . t h e same d i s t r i b u t i o n i n CM-Pf w e r e r e c o r d e d a f t e r s t i m u l a t i o n o f . o t h e r p a r t s o f t h e s e n s o r i - m o t o r , b u t n o t o f p a r i e t a l , o c c i p i t a l 120 F i g u r e 45. On l e f t — f i g u r e t a k e n f r o m M e h l e r ( 1 9 6 6 ) — \ h o r i z o n t a l b a r s i n d i c a t e p a r v o c e l l u l a r p a r t o f CM i n c a t b r a i n a s o u t l i n e d b y M e h l e r on a n a - • j tqmical;.'.bases a t f r o n t a l p l a n e s A 6.5, A 7.0 ,• .: A"-7v5; c o m p a r e d w i t h CM" as d e s i g n a t e d b y J a s p e r a n d A j m o n e - M a r s a n (19 54) on r i g h t si'de£\. R i g h t s i d e o f s e c t i o n s h a v e b e e n m o d i f i e d ( c o u r t e s y C o l u m b i a U n i v . P r e s s , a n d W.R. Mehler.)•;£•-. by s u p e r i m p o s i n g M e h l e r " s CM l i m i t s ( d o t t e d o u t l i n e s ) and e l e c t r o d e t r a c k ^ p s i t i o n s . ; R e c o r d i n g on r i g h t c o r r e s p o n d d , t ' d ' t r a c i n g s o b - . ~\ t a i n e d a f t e r p r e c r u c i a t e c o r t e x s t i m u l a t i o n a t - • p o s i t i o n s i n d i c a t e d b y b l a c k s w e l l i n g s on t r a c k s . ; - L e f t - m o s t c o l u m n o f t r a c i n g s c o r r e s p o n d s t o l e f t r m o s t r e c o r d i n g t r a c k , t o p row,.,of t r a c i n g s c o r r e s p o n d s t o t o p row o f r e c o r d i n g p o s i t i o n s , . | e t c . B l a c k d o t s o v e r t r a c i n g s i n d i c a t e , t h o s e ,: j t r a c i n g s o b t a i n e d i n p o s i t i o n s , f a l l i n g w i t h i n j M e h l e r • s l i m i t s o f CM. -'I 1 2 1 o r t e m p o r a l c o r t e x . The r e s p o n s e s w e r e l a r g e s t , and m o s t r e l i a b l y e v o k e d , h o w e v e r , a f t e r s t i m u l a t i o n o f p r e c r u c i a t e c o r t e x . S t i m u l a t i o n o f p r e c r u c i a t e c o r t e x . w h i l e r e c o r d i n g f r o m t h e l a t e r a l p a r t o f CM w i t h a f i n e - t i p p e d e l e c t r o d e i n v a r i a b l y e v o k e d t h e c o m p l e x wave r e s p o n s e n o t e d a b o v e , w i t h e v o k e d s p i k e s s u p e r i m p o s e d . The c e l l s e v o k e d u s u a l l y r e -sponded: w i t h a b u r s t o f 2-3 s p i k e s w h i c h o c c u r r e d d u r i n g t h e p o s i t i v e wave, u s u a l l y on i t s p o s i t i v e - g o i n g s l o p e . The l a t e n c i e s o f t h e t r o u g h o f t h e p o s i t i v e wave v a r i e d f r o m 9 - 1 5 msec, w h i l e t h e l a t e n c i e s t o t h e i n i t i a l s p i k e o f t h e b u r s t s w e r e 5 - 1 2 msec. A t y p i c a l r e s p o n s e w i t h s p i k e s s u p e r -i m p o s e d i s shown i n F i g u r e '46 . The c e l l s i n CM w e r e o f t e n f o u n d t o b u r s t s p o n t a n e o u s l y , i n a f a s h i o n s i m i l a r t o t h o s e : i n V L , t h o u g h w i t h l e s s r e g u l a r i t y . S t i m u l i o f t h e - p r e c r u c i a t e c o r t e x w e r e s o m e t i m e s f o l l o w e d by s e r i e s o f l a t e b u r s t s o f c e l l s , a g a i n s i m i l a r t o t h o s e s e e n i n V L . The c e l l s o f CM seemed r e l a t i v e l y i n s e n s i t i v e t o ACh a p p l i c a t i o n , e v e n i n d e c e r e b r a t e a n i m a l s . C h o l i n e r g i c i n v o l v e m e n t i n t h e c o r t e x - C M r e s p o n s e was t e s t e d i n t h e s e e x p e r i m e n t s by c o m p a r i n g t h e r e s p o n s e s b e f o r e and a f t e r i n t r a v e n o u s i n j e c t i o n s o f a t r o p i n e i n -doses o f 1-2 mgm/Kgm. No c h a n g e i n t h e e v o k e d s p i k e s o r c o m p l e x wave w e r e f o u n d . 122 ENTOPEDUNCULAR NUCLEUS Some e x p e r i m e n t s w e r e d e s i g n e d t o t e s t w h e t h e r t h e p a l l i d o - t h a l a m i c p a t h w a y a c t u a l l y o r i g i n a t e s f r o m c e l l s o f t h e e n t o p e d u n c u l a r n u c l e u s . I t was f o u n d t h a t s t i m u l a t i o n o f e i t h e r CM o r VL c a u s e d a n t i d r o m i c a c t i v a t i o n o f c e l l s i n EN. I n e x p e r i m e n t s i n w h i c h b o t h VL and CM s t i m u l i w e r e t e s t e d a t o t a l ' o f t h i r t y c e l l s w e r e f o u n d w h i c h w e r e a n t i d r o m i c a l l y e v o k e d by CM and e l e v e n by VL s t i m u l a t i o n . T h e s e numbers i n c l u d e s i x w h i c h w e r e e v o k e d by s t i m u l a t i o n o f b o t h n u c l e i . I n , t h e l a t t e r c e l l s , i t was f o u n d t h a t a c t i o n p o t e n t i a l s f o l l o w e d e v e r y s t i m u l u s t o e i t h e r CM o r VL a t f r e q u e n c i e s up t o s e v e r a l h u n d r e d p e r s e c o n d . H o w e v e r , a c o n d i t i o n i n g s t i m u l u s t o one o f t h e s e s i t e s , f o l l o w e d a f t e r a 1-2 msec, d e l a y by s t i m u l a t i o n o f t h e o t h e r , showed ;an o c c l u s i o n o f t h e s e c o n d r e s p o n s e . When b o t h n u c l e i w e r e s t i m u l a t e d , c o n s i d e r a b l y l o n g e r i n t e r s t i m u l u s i n t e r v a l was r e q u i r e d b e f o r e t h e c e l l r e s p o n d e d t o b o t h . An e x a m p l e o f . t h e s e e x p e r i m e n t s i s r e p r o d u c e d i n F i g u r e 47. I n two e x p e r i m e n t s t h e r e l a t i v e e f f i c a c y o f a p a i r o f b i p o l a r c o n c e n t r i c s t i m u l a t i n g e l e c t r o d e i n t h e CM-Pf c o m p l e x was t e s t e d . One e l e c t r o d e t i p was s i t u a t e d i n t h e l a t e r a l p a r t o f t h e CM-Pf c o m p l e x , a n d t h e o t h e r 1.5 mm m e d i a l t o i t , a l s o i n t h e CM-Pf c o m p l e x . Of t h e t w e n t y - f i v e EN c e l l s w h i c h w e r e a n t i d r o m i c a l l y a c t i v a t e d by a t l e a s t one o f t h e s e WW** i 123 50 MSEC F i g u r e 46. M i c r o e l e c t r o d e r e c o r d i n g i n l a t e r a l p a r t o f .CM*-Pf''' c o m p l e x a f t e r '-. s i f t g l ^ , ^ r ^ q r v | a ' c i a t e c o r t e x s t i m u l u s a t - a r r o w . '• A m B * C S _ l 2 : 10 MSEC 200 V D * F i g u r e 47. S u p e r i m p o s e d t r a c e s s h o w i n g a n t i d r o m i c a c t i v a -t i o n , o f EN n e u r o n e b y - CM and. VL s t i m u l a t i o n . A • each':.of two s t i m u l i w i t h 1.5 msec i n t e r s t i m u l u s i n t e r v a l e v o k e s a n t i d r o m i c a c t i o n p o t e n t i a l . . r e s p o n s e ; B. same f o r two VL s t i m u l i ; C. CM, "then VL' s t i m u l a t i o n on l e f t — 1 . 5 ' msec i n t e r v a l ; o n ' . r i g h t — 2 . 4 msec i n t e r s t i m u l u s i n t e r v a l i s r e q u i r e d b e f o r e - s p i k e . p o t e n t i a l i s r e l i a b l y e v o k e d b y VL stimulus;,. ' D. V L , t h e n CM s t i m u l u s ..--on l e f t — 1 . 5 m s e c i n t e r v a l ; o n r i g h t — 1 . 9 msec i n t e r s t i m u l u s i n t e r v a l i s r e q u i r e d b e f o r e s p i k e p o t e n t i a l i s r e l i a b l y e v o k e d b y CM s t i m u -l a t i o n . •.;••'•• 124 e l e c t r o d e s , t w e n t y - t w o w e r e a c t i v a t e d b y t h e more l a t e r a l e l e c t r o d e and s i x by t h e more m e d i a l o n e , t h r e e c e l l s b e i n g e v o k e d b y b o t h . ' The a v e r a g e l a t e n c y o f r e s p o n s e i n EN c e l l s was 1.6 msec - f o l l o w i n g CM s t i m u l a t i o n and 1.7 msec a f t e r s t i m u l a t i o n o f VL. CHAPTER I V DISCUSSION The s p o n t a n e o u s and s t i m u l u s - i n d u c e d b u r s t i n g a c t i v i t y i n VL s e e n i n t h i s s t u d y i s v e r y s i m i l a r t o t h a t d e s c r i b e d by A n d e r s e n and A n d e r s s o n [1968] f o r t h e VBC, and more r e -c e n t l y by A n d e r s e n e t a l . [19 69] f o r V L . S t r y c h n i n e a d m i n i s t e r e d i o n t o p h o r e t i c a l l y o r s y s t e m a t i c a l l y h a d no a p p a r e n t e f f e c t on t h e b u r s t i n g a c t i v i t y o r p o s t - s t i m u l u s d e p r e s s i o n . I f t h e c u r r e n t c o n c e p t t h a t a p r i n c i p a l a c t i o n o f s t r y c h n i n e as a g l y c i n e b l o c k i n g a g e n t [ C u r t i s , Duggan a n d J o h n s t o n , 1969] i s a c c e p t e d , t h i s w o u l d i n d i c a t e t h a t g l y c i n e i s n o t t h e i n h i b i t o r y , t r a n s m i t t e r w h i c h g e n e r a t e s t h e I P S P s o f t h e b u r s t i n g p a t t e r n . Duggan and M c L e n n a n , [1971] h a v e , h o w e v e r , f o u n d t h a t b i c u c u l l i n e d o e s b l o c k a t l e a s t some i n h i b i t o r y phenomena i n t h a l a m i c r e l a y n u c l e i . B i c u c u l l i n e h a s b e e n p r o p o s e d as a r e l a t i v e l y s p e c i f i c a n t a g o n i s t o f t h e a c t i o n o f y - a m i n o b u t y r i c a c i d (GABA) [ C u r t i s , Duggan, F e l i x and J o h n s t o n , 1970], I t i s p o s s i b l e t h e r e f o r e t h a t GABA may f u n c t i o n as an i n h i b i t o r y t r a n s m i t t e r i n t h e t h a l a m u s and c o u l d be t h e a g e n t w h i c h g e n e r a t e s t h e r h y t h m i c h y p e r p o l a r -i z a t i o n s a s s o c i a t e d w i t h t h e s p o n t a n e o u s b u r s t i n g a c t i v i t y . The o b s e r v a t i o n s t h a t s t i m u l a t i o n o f EN d i d n o t g e n e r a t e p o s t - s t i m u l u s b u r s t i n g a c t i v i t y i n VL c e l l s and 126 t h a t t h e p o s t s t i m u l u s d e p r e s s i o n was e i t h e r a b s e n t o r much w e a k e r t h a n t h a t p r o d u c e d by, c o r t i c a l o r BC s t i m u l a t i o n ( F i g u r e s 2 1 , 22) i s i n a g r e e m e n t w i t h t h e f i n d i n g s o f D e s i r a j u a n d P u r p u r a [1969] t h a t some E P S P s o f VL c e l l s f o l l o w i n g EN s t i m u l a t i o n w e r e n o t s u c c e e d e d b y I P S P s . T h e s e d a t a a p p e a r t o c o n t r a d i c t t h e c o l l a t e r a l - i n h i b i t o r y f e e d b a c k m o d e l o f A n d e r s e n a nd E c c l e s [ 1 9 6 2 ] . I t i s p o s s i b l e h o w e v e r , t h a t t h e EN p a t h w a y t e r m i n a t e s i n l a r g e p a r t on a t y p e o f c e l l w h i c h i s n o t i n f l u e n c e d b y t h e i n h i b i t o r y f e e d -b a c k m e c h a n i s m . T h a t t h i s c o u l d be t h e c a s e m i g h t be i n d i c a t e d by t h e r e s u l t s o f t h i s s t u d y i n w h i c h r e l a t i v e l y s m a l l p r o p o r t i o n s o f t h e EN e v o k e d c e l l s w e r e a l s o a c t i v a t e d w i t h l e s s t h a n 10 msec l a t e n c y b y c e r e b e l l a r ( 3 / 1 1 c e l l s ) -or , c o r t i c a l (3/28 c e l l s ) s t i m u l a t i o n . I n a d d i t i o n , i f EN f i b r e s a l s o i n f l u e n c e t h e p r o p o s e d i n h i b i t o r y i n t e r n e u r o n e s , . e . g . , by p r e s y n a p t i c i n h i b i t i o n , s t i m u l a t i o n o f t h i s n u c l e u s c o u l d p r o d u c e d i f f e r e n t e f f e c t s t h a n s t i m u l a t i o n o f o t h e r a f f e r e n t s y s t e m s . A t h i r d e x p l a n a t i o n w o u l d be t h a t b e c a u s e EN s t i m u l a t i o n a p p e a r s t o h a v e a l e s s e r e x c i t a t o r y i n f l u e n c e on VL c e l l s t h a n c o r t i c a l o r BC s t i m u -l a t i o n , t h e r e s u l t i n g i n h i b i t i o n , w h i c h b y t h e m o d e l i s d e p e n d e n t on p r i o r e x c i t a t i o n o f n e u r o n e s , s h o u l d a l s o be e x p e c t e d t o be w e a k e r . I t i s d i f f i c u l t t o a r g u e a g a i n s t t h e a x o n c o l l a t e r a l p a r t o f t h e m o d e l i n f a c e o f t h e e v i d e n c e o f S a k a t a e t a l . [1966] t h a t t h e p o s t s t i m u l u s d e p r e s s i o n i s s t i l l p r e s e n t w i t h c o r t i c a l s t i m u l a t i o n a f t e r d e g e n e r a t i o n ' o f t h a l a m o -127 c o r t i c a l f i b r e s . H o w e v e r , b e c a u s e o f t h e o t h e r o b j e c t i o n s t o t h e m o d e l ( p . 22) and b e c a u s e an i n h i b i t o r y . i n f l u e n c e o n VL f r o m i n t e r p o s i t u s n u c l e u s w h i c h c o u l d a l s o c a u s e a p o s t -s t i m u l u s d e p r e s s i o n h a s b e e n f o u n d [Uno, e t a l . , , 1970] more p o s i t i v e e v i d e n c e w i l l be n e c e s s a r y t o d e t e r m i n e i t s v a l i d i t y . I t s h o u l d be p o i n t e d o u t t h a t t h e e v i d e n c e on w h i c h t h e A n d e r s e n - E c c l e s m o d e l was b a s e d was; g a t h e r e d f r o m e x p e r i m e n t s on t h e VBC., The o b j e c t i o n s t o t h e m o d e l , . h o w e v e r , h a v e a l m o s t e x c l u s i v e l y b e e n b a s e d on f i n d i n g s i n VL. I t i s p o s s i b l e t h e r e f o r e t h a t t y p e s o f e l e c t r i c a l a c t i v i t y i n t h e s e n s o r y r e l a y m u c l e i and VL w h i c h a r e s i m i l a r i n a p p e a r a n c e ( e . g . , p o s t - s t i m u l u s d e p r e s s i o n ) c o u l d be c a u s e d b y d i f f e r e n t m e c h a n i s m s . The e x c i t a t o r y e f f e c t s o f c e r e b e l l a r n u c l e i a nd BC s t i m u l a t i o n s e e n on VL c e l l s i n t h i s s t u d y a r e v e r y s i m i l a r t o t h o s e r e p o r t e d b y o t h e r i n v e s t i g a t o r s , w i t h t h e e x c e p t i o n t h a t t h e e a r l y b u r s t r e s p o n s e h a s n o t b e e n p r e v i o u s l y r e p o r t e d i n VL e x c e p t b y M c L e n n a n , H u f f m a n and M a r s h a l l [19 6 8a]. A s i m i l a r phenomenon was o b s e r v e d b y C u r t i s a nd D a v i s [1963] i n t h e l a t e r a l g e n i c u l a t e n u c l e u s (LGN). The b u r s t r e s p o n s e s t o o p t i c t r a c t s t i m u l a t i o n s e e n • b y t h e s e w o r k e r s c o u l d a l s o be c o n v e r t e d t o s i n g l e , s h o r t -l a t e n c y s p i k e s by i n c r e a s e d s t i m u l u s i n t e n s i t y o r by i o n t o -p h o r e t i c a p p l i c a t i o n o f ACh. They p r o p o s e d t h a t t h e b u r s t s w e r e due t o - s y n a p t i c a c t i v a t i o n a t a b o u t t h e t h r e s h o l d l e v e l . T h e i r s u g g e s t i o n i s a p l a u s i b l e o n e , b u t i f t h i s i s t h e s o l e 128 e x p l a n a t i o n , t h e r e p l a c e m e n t o f t h e b u r s t w i t h o n l y a s i n g l e s p i k e i s d i f f i c u l t t o u n d e r s t a n d . B e c a u s e t h i s t y p e o f r e s p o n s e was f o u n d f r e q u e n t l y i n a n e s t h e t i z e d , b u t o n l y o n c e i n a l a r g e s e r i e s o f d e c e r e b r a t e a n i m a l s , i t s a p p e a r a n c e i n p l a c e o f a s h o r t l a t e n c y s i n g l e s p i k e may. be r e l a t e d t o t h e e f f e c t s o f a n e s t h e t i c a g e n t s . A s • t h e e a r l y b u r s t h a s b e e n s e e n f r e q u e n t l y i n b o t h b a r b i t u r a t e and c h l o r a l o s e p r e p a r a t i o n s , i t may be r e l a t e d t o a n o n - s p e c i f i c d e p r e s s i o n o f t h e p o s t - s y n a p t i c membrane o r t r a n s m i t t e r r e l e a s e m e c h a n i s m s . The i d e a t h a t t h e phenomenon may be r e l a t e d t o a n e s t h e s i a i s s t r e n g t h e n e d b y t h e o b s e r v a t i o n t h a t i n one c a s e s i n g l e s h o r t l a t e n c y BC e v o k e d s p i k e s w e r e c o n v e r t e d t o e a r l y b u r s t s a f t e r I.V. p e n t o b a r b i t a l . The a b i l i t y o f d e p o l a r i z i n g d r u g s (ACh and t h e amino a c i d s ) r e v e r s i b l y t o c h a n g e t h e e a r l y b u r s t s t o a s h o r t l a t e n c y s p i k e s u g g e s t s t h a t s h o r t l a t e n c y E P S P s a r e s t i l l p r o d u c e d b y a c t i v a t i o n o f t h e a f f e r e n t s y s t e m s , b u t t h a t t h e y a r e s u b t h r e s h o l d f o r s p i k e g e n e r a t i o n . I n t r a c e l l u l a r r e c o r d i n g m i g h t i n d i c a t e w h e t h e r t h e e a r l y b u r s t i s due t o r e d u c e d , t e m p o r a l l y d i s p e r s e d E P S P s o r t o some o t h e r m e c h a n i s m . The p o s s i b i l i t y t h a t E P S P s w h i c h n o r m a l l y o c c u r s y n c h r o n o u s l y a f t e r an a f f e r e n t s t i m u l u s c o u l d u n d e r c e r t a i n a n e s t h e t i c c o n d i t i o n s be s p r e a d o u t o v e r a l o n g t i m e c o u l d e x p l a i n t h e e a r l y b u r s t r e s p o n s e . S u c h t e m p o r a l l y d i s p e r s e d E P S P s m i g h t r e a c h t h e a c t i o n p o t e n t i a l t h r e s h o l d w i t h a l o n g e r l a t e n c y a n d m a i n t a i n t h e membrane a b o u t t h r e s h o l d l e v e l f o r a l o n g e r t i m e t h a n w o u l d 129 a s y n c h r o n o u s b a r r a g e o f E P S P s . I t i s c o n c e i v a b l e t h a t c e r t a i n a n e s t h e t i c s c o u l d r e d u c e • t h e a c t i o n p o t e n t i a l c o n -d u c t i o n r a t e i n a f f e r e n t t e r m i n a l s , g i v i n g t h i s r e s u l t . The r e s p o n s e s o f VL c e l l s t o c o r t i c a l s t i m u l a t i o n w e r e s i m i l a r t o t h o s e d e s c r i b e d by s e v e r a l o t h e r a u t h o r s [ e . g . , S a k a t a e t a l . , 1966; Uno, e t a l . , 1970], a g a i n w i t h t h e e x c e p t i o n o f t h e e a r l y b u r s t s . EN s t i m u l a t i o n g a v e a v a r i e t y o f r e s p o n s e s i n VL c e l l s . The o n l y e x t r a c e l l u l a r s t u d y o f . t h e p a l l i d o - t h a l a m i c p a t h w a y t o w h i c h t h e s e r e s u l t s may be c o m p a r e d i s t h a t o f D o r m o n t , Ohye and A l b e - F e s s a r d [1969]. T h e s e w o r k e r s a l s o f o u n d a s h o r t l a t e n c y n e g a t i v e f i e l d r e s p o n s e and j u d g e d i t t o be p r e s y n a p t i c i n n a t u r e . T h e i r s a m p l e o f t h i r t e e n p o s t - s y n a p t i c s p i k e r e s p o n s e s was d i v i s i b l e i n t o two g r o u p s — t h r e e c e l l s g i v i n g b u r s t r e s p o n s e s w i t h l a t e n c i e s as s h o r t as 2.5-3 m s e c , and t e n c e l l s w i t h l a t e n c i e s t o b u r s t r e -s p o n s e s g r e a t e r t h a n 10 msec. The a n a l y s i s i n t h i s s t u d y was done on t h e b a s i s o f mean l a t e n c i e s , s o t h a t c o m p a r i s o n i s d i f f i c u l t h o w e v e r , s e v e r a l o f t h e c e l l s w i t h mean l a t e n c i e s 5-9 msec, o c c a s i o n a l l y w e r e e v o k e d w i t h 3-4 msec, l a t e n c y . The r e d u c t i o n i n l a t e n c y and number o f s p i k e s o f t h e r e s p o n s e d u r i n g i o n t o p h o r e t i c a p p l i c a t i o n o f ACh o r DLH i s s i m i l a r t o t h a t o b s e r v e d on e a r l y b u r s t i n g c e l l s e v o k e d by c o r t i c a l o r BC s t i m u l a t i o n . The b u r s t r e s p o n s e s t o EN s t i m u l a t i o n a p p e a r e d t o be u n a f f e c t e d b y c h l o r a l o s e - a n e s t h e s i a h o w e v e r , . s i n c e t h e y w e r e e n c o u n t e r e d w i t h a b o u t e q u a l f r e -130 q u e n c y i n c h l o r a l o s e and d e c e r e b r a t e p r e p a r a t i o n s . A s i m i l a r i t y o f u n d e r l y i n g m e c h a n i s m i s p o s s i b l e i f t h e E P S P s e v o k e d by. EN s t i m u l a t i o n a c h i e v e o n l y p e r i - t h r e s h o l d l e v e l s , o r i f t h e y a r e t e m p o r a l l y d i s p e r s e d . The b u r s t r e s p o n s e c o u l d a l s o be r e l a t e d t o t h e a b s e n c e o f I P S P s f o l l o w i n g a c t i v a t i o n b y p a l l i d o - t h a l a m i c p a t h w a y s t i m u l a t i o n w h i c h h a s b e e n n o t e d b y D e s i r a j u and P u r p u r a [1969] a f t e r p a l l i d o -t h a l a m i c a c t i v a t i o n . I t w o u l d be n e c e s s a r y t h e n t o c o n s i d e r w h e t h e r t h e e a r l y b u r s t s e v o k e d by. BC o r c o r t i c a l s t i m u l a t i o n a r e r e l a t e d t o a l t e r a t i o n s i n t h e i n h i b i t o r y f e e d b a c k mechan-i s m s b y a n e s t h e t i c a g e n t s . D e s i r a j u a nd P u r p u r a ' s o b s e r v a t i o n t h a t t h e s h o r t - l a t e n c y E P S P s w h i c h do n o t g e n e r a t e an a c t i o n p o t e n t i a l h a v e a p r o l o n g e d d u r a t i o n (10-20 msec.) c o u l d f o r m t h e b a s i s f o r f u r t h e r s p e c u l a t i o n on t h e m e c h a n i s m o f t h e b u r s t r e s p o n s e . L i t t l e c o m p a r i s o n o f t h e r e s u l t s r e p o r t e d h e r e w i t h t h e i n t r a c e l l u l a r s t u d i e s o f t h e EN - VL p a t h w a y [ F r i g y e s i , 1969; D e s i r a j u and P u r p u r a , 1969; F r i g y e s i and M a c h e k , 1 9 7 0 ] ; i s p o s s i b l e as t h e s e r e p o r t s h a v e n o t m e n t i o n e d b u r s t r e s p o n s e s . However t h e . r a n g e o f EPSP l a t e n c i e s o b s e r v e d i n t h o s e s t u d i e s i s c o n s i s t e n t w i t h t h e l a t e n c i e s o f e x t r a c e l l u l a r r e s p o n s e s o b s e r v e d . B o t h F r i g y e s i a n d Machek, and D e s i r a j u and P u r p u r a h a v e r e p o r t e d E P S P s w i t h l a t e n c i e s o f a b o u t 2-3 msec. T h i s c o r r e s p o n d s w i t h t h e p r e s e n t o b s e r v a t i o n s t h a t s e v e r a l c e l l s c o u l d be i n d u c e d t o f i r e w i t h 2-3 msec, l a t e n c y d u r i n g i o n t o p h o r e t i c a p p l i c a t i o n o f an e x c i t a t o r y a g e n t . The 131 p r e s e n c e o f t h e p r e s y n a p t i c t y p e a c t i v i t y o f 0.8-2 msec, l a t e n c y and t h e c o n f i r m a t o r y mean l a t e n c y o f 1.6 msec, f o r a c t i v a t i o n o f EN c e l l s a f t e r t h a l a m i c s t i m u l a t i o n s u g g e s t s t h a t t h e d i r e c t - E N - VL pathway- i s r e p r e s e n t e d by t e r m i n a l a c t i v a t i o n a b o u t 1.5-2 msec a f t e r EN s t i m u l a t i o n . The t i m e r e q u i r e d f o r m o n o s y n a p t i c t r a n s m i s s i o n and s p i k e g e n e r a t i o n w o u l d r e s u l t i n a t o t a l l a t e n c y o f 2-3 msec. The r e l a t i v e l y l a r g e d i f f e r e n c e i n l a t e n c y o f t h e p r e s y n a p t i c - t y p e EN r e s p o n s e b e t w e e n t h e v e n t r a l p a r t o f EN ( 0 . 8-1.0 msec) and more d o r s a l p a r t s (1.5-2.0 msec, F i g u r e 16) i s p u z z l i n g , as t h e l a t e n c y a p p e a r s t o i n c r e a s e by a b o u t a f a c t o r o f two i n l o c a t i o n s s e p a r a t e d b y o n l y a few mm. I f t h e a b s e n c e o f u n i t a r y p o t e n t i a l s s u p e r i m p o s e d on. t h e wave i s a c c e p t e d a s e v i d e n c e t h a t t h e s i g n a l r e c o r d e d i n more d o r s a l r e g i o n s s t i l l r e p r e s e n t s p r e s y n a p t i c a c t i v i t y , t h e n t h e most, l i k e l y e x p l a n a t i o n i s t h a t EN a x o n s b r a n c h e x t e n s i v e l y u p o n e n t e r i n g V L , t h u s g i v i n g r i s e t o v e r y f i n e c a l i b r e c o l l a t e r a l s h a v i n g a r e l a t i v e l y s l o w c o n d u c t i o n v e l o c i t y . A d i f f i c u l t y o f t h i s p r o p o s a l i s D e s i r a j u and P u r p u r a " s o b s e r v a t i o n o f 0.7-0.9 msec, l a t e n c i e s a f t e r p a l l i d a l , s t i m u l a t i o n . O n l y o n c e was e l e c t r i c a l a c t i v i t y s e e n i n t h i s s t u d y w h i c h c o u l d be. r e l a t e d t o s u c h a c t i v i t y — a s i n g l e 1.8 msec l a t e n c y s p i k e . The minimum l a t e n c y o b s e r v e d f o r a n t i d r o m i c a c t i v a t i o n o f EN c e l l s f r o m VL was 1.1 msec, w h i c h i s l o n g e r t h a n t h e v e r y s h o r t l a t e n c y E P S P s o f D e s i r a j u and P u r p u r a . I t m u s t be q u e s t i o n e d w h e t h e r t h e i r f i n d i n g s w e r e 132 d u e , e.g. t o a c t i v a t i o n o f c o r t i c o - t h a l a m i c p a t h w a y s c l o s e t o EN. The l a t e r b u r s t r e s p o n s e s n o r m a l l y s e e n c o u l d be e x p l a i n e d b y some o f t h e s u g g e s t i o n s a l r e a d y made f o r e a r l y b u r s t s e v o k e d by BC o r c o r t i c a l s t i m u l a t i o n , o r c o u l d be due t o a d d i t i o n a l a c t i v a t i o n a t l o n g e r l a t e n c i e s v i a i n d i r e c t p a t h w a y s . The a n a t o m i c a l s u b s t r a t e s f o r s u c h p a t h w a y s a r e t h o u g h t t o e x i s t i n t h e p a l l i d o - C M - V L o r p a l l i d o - n i g r o - V L s y s t e m s . E v e n more i n d i r e c t r o u t e s h a v e b e e n p r o p o s e d by F r i g y e s i , [19 6 9 ] . The a n t i d r o m i c a c t i v a t i o n o f EN n e u r o n e s by s t i m u l a t i o n o f VL and o f CM, and t h e c o l l i s i o n d e m o n s t r a t e d h a v e a l s o b e e n o b s e r v e d by Ohye, Dormont and R i s p a l [196 8 ] . T h e s e w o r k e r s e s t i m a t e d t h a t 40% o f t h e p a l l i d o - t h a l a m i c f i b r e s d i c h o t o m i z e d a t t h e t h a l a m i c l e v e l . T h a t 20% o f t h e a n t i d r o m i c a l l y i d e n t i f i e d EN c e l l s i n t h i s s t u d y w e re i n v a d e d by b o t h CM and VL s t i m u l a t i o n s u g g e s t s t h a t t h e p r o p o r t i o n may be c o n s i d e r a b l y h i g h e r , a s t h e s t i m u l i a r e t h o u g h t t o h a v e a c t i v a t e d n e u r o n e s o n l y w i t h i n a s m a l l v o l u m e . The o b s e r v a t i o n s o f n o n - o v e r l a p p i n g e x c i t a t i o n by, p a i r s o f c o n c e n t r i c e l e c t r o d e s w i t h 1.5 mm s e p a r a t i o n i n d i c a t e t h a t t h e s p h e r e o f i n f l u e n c e o f e a c h e l e c t r o d e was n o t more t h a n a b o u t , 1.5 mm.•in d i a m e t e r . I t m i g h t be e x p e c t e d t h a t s u c h an e l e c t r o d e l o c a t e d i n CM w o u l d a c t i v a t e a r e l a t i v e l y h i g h p r o p o r t i o n o f t h e p a l l i d o - t h a l a m i c f i b r e s c o u r s i n g t o i t , b e c a u s e o f t h e more c o m p a c t s i z e o f t h i s n u c l e u s . S t i m u l a t i o n 133 o f VL on t h e o t h e r h a n d may e x c i t e o n l y a s m a l l p r o p o r t i o n o f p a l l i d o - V L f i b r e s b e c a u s e t h e y p r o b a b l y p r o j e c t - t o a l a r g e r a r e a . E s t i m a t e s o f t h e p e r c e n t a g e o f a x o n s w h i c h s e n d c o l l a t e r a l s t o b o t h VL and CM s h o u l d t h e r e f o r e be b a s e d on t h e p r o p o r t i o n o f VL a n t i d r o m i c a l l y e v o k e d EN c e l l s w h i c h a r e a l s o a n t i d r o m i c a l l y e v o k e d b y CM s t i m u l a t i o n . On t h i s b a s i s t h e a d m i t t e d l y s m a l l s a m p l e o f e l e v e n VL e v o k e d c e l l s , s i x o f w h i c h w e r e a l s o a c t i v a t e d b y CM s t i m u l a t i o n , s u g g e s t s t h a t a minimum-^of 50% o f t h e EN c e l l s s e n d i n g f i b r e s t o VL h a v e c o l l a t e r a l s t o CM. B e c a u s e t h i s i s a minimum f i g u r e , i t - i s t h o u g h t t h a t most o r a l l o f t h e E N - t h a l a m i c a x o n s s e n d c o l l a t e r a l s t o t h e s e n u c l e i . The s i g n i f i c a n c e o f t h e c o l l a t e r a l p a t h w a y s t o VL and CM i s n o t p r e s e n t l y u n d e r s t o o d . The CM-VL p a t h w a y s u g g e s t e d b o t h by a n a t o m i s t s [ e . g . , N a u t a a nd W h i t l o c k , 1954] and e l e c t r o p h y s i o l o g i s t s [ P u r p u r a , S c a r f f a n d M c M u r t r y , 1965] w o u l d p r o v i d e a b a s i s f o r a d o u b l e a c t i o n on VL by t h e same g r o u p o f EN n e u r o n e s . I t i s p o s s i b l e o f c o u r s e t h a t t h e EN-CM i n f l u e n c e s a r e n o t t r a n s m i t t e d t o VL and t h a t t h e y h a v e a r o l e i n d e p e n d e n t o f VL. I f t h e i d e a p r e s e n t e d i n t h e I n t r o d u c t i o n , i . e . , . / . t h a t t h e b a s a l g a n g l i a i n f l u e n c e m o t o r f u n c t i o n t h r o u g h VL i s a c c e p t e d i t i s s u r p r i s i n g t h a t t h e i r m a j o r o u t p u t p a t h w a y t o VL d o e s n o t seem t o e x e r t a s t r o n g i n f l u e n c e t h e r e . L a r g e f i e l d p o t e n t i a l s s i m i l a r t o t h o s e s e e n a f t e r BC o r 134 c o r t i c a l s t i m u l a t i o n a r e n o t f o u n d w i t h EN s t i m u l a t i o n [ D o r m o n t , Ohye and A l b e - F e s s a r d , 1969]. T h i s c o u l d be a t t r i b u t e d i n p a r t t o t h e d i s p e r s e d n a t u r e o f EN, s o t h a t s t i m u l i i n any one r e g i o n do n o t a c t i v a t e a l a r g e p a r t o f t h e p a l l i d o - t h a l a m i c p a t h w a y . H o w e v e r , . a n a t o m i c a l e v i d e n c e [ S z a b o , 1965] a l s o s u g g e s t s t h a t t h i s p a t h w a y i s l e s s p o w e r f u l t h a n t h e c e r e b e l l o - t h a l a m i c o n e . I n t e r f e r e n c e b y c h o l i n e r g i c b l o c k i n g a g e n t s w i t h s y n a p t i c t r a n s m i s s i o n i n VL was s e e n i n f r e q u e n t l y i n t h i s s t u d y . The c o m p l e t e l a c k o f b l o c k i n g b y a t r o p i n e o r " DH3E o f t h e n e u r o n a l f i r i n g and g r o s s r e s p o n s e e v o k e d . b y c o r t i c o -t h a l a m i c a c t i v a t i o n i n d i c a t e s t h a t t h i s p a t h w a y h a s no s i g n i f i c a n t c h o l i n e r g i c c o m ponent. T h i s i s t o be c o n t r a s t e d w i t h t h e c o r t i c o - m e d i a l g e n i c u l a t e p a t h w a y w h i c h T e b e c i s [1970] h a s s u g g e s t e d may be p a r t l y c h o l i n e r g i c . As no s y n a p t i c f i e l d p o t e n t i a l s w e r e c l e a r l y d i s t i n g u i s h e d f o l l o w i n g EN s t i m u l a t i o n , o n l y s i n g l e c e l l r e s p o n s e s c o u l d be t e s t e d . The s i n g l e EN e v o k e d r e s p o n s e w h i c h was p a r t i a l l y d e p r e s s e d b y i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e was a l s o d e p r e s s e d b y ACh. I t i s p o s s i b l e t h a t t h i s c e l l was d e p r e s s e d b y a t r o p i n e i n a n o n - s p e c i f i c manner a s no DLH c o n t r o l was o b t a i n e d i n t h i s c a s e . I n no t e s t was t h e r e any c h a n g e o f t h e EN r e s p o n s e s a f t e r a d m i n i s t r a t i o n o f DH3E o r I.V. a t r o p i n e . I f t h e c e l l s i n d u c e d t o f i r e w i t h a b o u t 3 msec l a t e n c y a r e t a k e n as r e p r e s e n t i n g t h e d i r e c t p a t h w a y , n e g a t i v e r e s u l t s w i t h I.V. a t r o p i n e w e r e f o u n d i n a l l c a s e s . 1 3 5 The n e g a t i v e e f f e c t s o f a t r o p i n e and DH3E on o t h e r r e s p o n s e s i n d i c a t e t h a t p r o b a b l y no - c o m p o n e n t s o f t h e d i r e c t o r p o s t u l a t e d i n d i r e c t p a t h w a y s a r e c h o l i n e r g i c a l l y m e d i a t e d . T h i s c o n c l u s i o n i s s u p p o r t e d b y t h e o b s e r v a t i o n o f O l i v i e r , P a r e n t , S i m a r d and P o i r i e r [ 1 9 6 8 ] t h a t t h e p a l l i d o - t h a l a m i c p a t h w a y i s d e v o i d : o f c h o l i n e s t e r a s e . The e f f e c t s o f c h o l i n e r g i c b l o c k e r s o n t h e p a t h w a y f r o m t h e c e r e b e l l a r n u c l e i t o VL a r e p a r a d o x i c a l . - W i t h t h e o n e . e x c e p t i o n f o r DH3E ( F i g u r e 3 2 ) , no b l o c k i n g o f BC e v o k e d a c t i o n p o t e n t i a l s was s e e n w i t h i o n t o p h o r e t i c a t r o p i n e o r DH3E. The two BC e v o k e d e a r l y b u r s t r e s p o n s e s t e s t e d w e r e n o t b l o c k e d b y I . V . - a t r o p i n e . I n e a c h c a s e , h o w e v e r , B C - e v o k e d f i e l d r e s p o n s e s w e r e m a r k e d l y r e d u c e d by I.V. / a t r o p i n e . T h a t t h e a t r o p i n e b l o c k i n g o f t h e f i e l d r e s p o n s e s was s p e c i f i c was i n d i c a t e d i n two c a s e s by t h e l a c k o f e f f e c t on c o r t i c a l e v o k e d f i e l d r e s p o n s e s a t t h e same l o c a t i o n ( F i g u r e 3 4 ) and i n t h e t h i r d by t h e r e t a i n e d DLH e x c i t a b i l i t y o f t h e c e l l s ( F i g u r e 3 3 ) . T h a t t h e i o n t o p h o r e t i c a p p l i c a t i o n o f b l o c k i n g a g e n t s d i d n o t a f f e c t t h e BC-VL r e s p o n s e s s h o u l d n o t , p e r h a p s be s u r p r i s i n g . B e c a u s e o f t h e l a r g e s i z e o f t h e u n i t a r y E P S P s o b s e r v e d a f t e r a c t i v a t i o n o f t h i s p a t h w a y , [Uno, e t a l . , 1 9 7 0 ] , " i t h a s b e e n e s t i m a t e d [ E c c l e s , I t o and S z e n t a g o t h a i , 1 9 6 7 ] t h a t a c t i v a t i o n o f f e w e r t h a n t e n s y n a p t i c t e r m i n a l s on a VL c e l l may be s u f f i c i e n t t o g e n e r a t e an a c t i o n p o t e n -t i a l . A s t h i s f i g u r e may r e p r e s e n t a v e r y s m a l l p r o p o r t i o n 136 o f t h e t o t a l number o f e n d i n g s , , i t i s p o s s i b l e t h a t many o f t h e BC s y n a p s e s on a VL c e l l c o u l d be b l o c k e d w i t h o u t a f f e c t i n g p o s t s y n a p t i c s p i k e g e n e r a t i o n . The s m a l l s i z e o f t h e m i c r o e l e c t r o d e a s s e m b l y c o m p a r e d w i t h t h a t o f t h e r e l a y n e u r o n e s (4-6 y v s . 25-30 y) w i l l l e a d t o a v e r y n o n -u n i f o r m c o n c e n t r a t i o n o f e j e c t e d d r u g i n t h e p e r i n e u r o n a l f l u i d , w i t h t h e r e s u l t t h a t t h e c o n c e n t r a t i o n s o f a t r o p i n e a t t h e membrane c l o s e , t o t h e e l e c t r o d e t i p c o u l d be s u f f i c i e n t t o b l o c k t h e ACh and a m ino a c i d s e n s i t i v i t y b e f o r e c o n c e n -t r a t i o n s a d e q u a t e t o b l o c k s y n a p t i c t r a n s m i s s i o n a r e a c h i e v e d a t more d i s t a n t s i t e s . B l o c k a d e o f t h e BC e v o k e d s p i k e c o i n c i d e n t w i t h d e p r e s s i o n o f a m i n o a c i d e x c i t a b i l i t y h a s b e e n s e e n i n o t h e r s t u d i e s [ e . g . , D a v i s , 1 9 6 6 ] , b u t h a s b e e n r e g a r d e d as n o n - s p e c i f i c d e p r e s s i o n o f membrane e x c i t a b i l i t y . I n t r a v e n o u s a d m i n i s t r a t i o n o f a t r o p i n e p r e s u m a b l y a c h i e v e s o v e r a l l c o n c e n t r a t i o n s s u f f i c i e n t t o b l o c k a l m o s t a l l c h o l i n e r g i c s y n a p s e s w i t h o u t d e p r e s s i n g membrane e x c i t a b i l i t y . The a b s e n c e o f b l o c k i n g by I.V. a t r o p i n e on two BC e v o k e d e a r l y b u r s t r e s p o n s e s may be a t t r i b u t a b l e t o o n l y a p a r t i a l c h o l i n e r g i c m e d i a t i o n o f t h e c e r e b e l l o - t h a l a m i c p a t h w a y , as p r e v i o u s l y s u g g e s t e d by M c C a n c e , P h i l l i s and W e s t e r m a n [196 8 b ] . B e c a u s e o f t h e i n d i c a t i o n t h a t t h e BC - VL pathway-i s a t l e a s t p a r t l y c h o l i n e r g i c , t h e BC - VL c h o l i n e r g i c s y n a p s e s c o u l d be p r o p o s e d as t h e b a s i s f o r a c t i o n o f a n t i -137 c h o l i n e r g i c d r u g s i n P a r k i n s o n ' s d i s e a s e . I t o [1969] h a s r e p o r t e d t h a t n e u r o n e s o f N.. i n t e r p o s i t u s h a v e a r e s t i n g d i s c h a r g e r a t e o f 50-100/sec. B e c a u s e o f t h e m a g n i t u d e o f t h e u n i t a r y • E P S P s e v o k e d i n VL c e l l s by c e r e b e l l a r s t i m u -l a t i o n [Uno, e t a l . , 1970], s u c h a h i g h r e s t i n g d i s c h a r g e s h o u l d h a v e a s t r o n g t o n i c a l l y d e p o l a r i z i n g i n f l u e n c e on VL n e u r o n e s . A p a r t i a l r e m o v a l o f c h o l i n e r g i c a l l y m e d i a t e d d e p o l a r i z a t i o n m i g h t l o w e r t h e membrane p o t e n t i a l e nough so t h a t c e l l s d o n o t r e s p o n d t o t r e m o r g e n e r a t i n g m e c h a n i s m s . The e f f e c t s o f ACh on EN e v o k e d r e s p o n s e s i n VL c e l l s a r e a l s o d i f f i c u l t t o e x p l a i n . T h e r e a p p e a r s t o be a d u a l e f f e c t ; an e x c i t a t i o n w h i c h was n o t s e e n i n c h l o r a l o s e a n e s t h e t i z e d a n i m a l s b e c a u s e o f t h e i n d i c a t e d i n t e r f e r e n c e w i t h ACh e x c i t a t i o n by t h i s d r u g , a nd a d e p r e s s i o n w h i c h i s i n d e p e n d e n t o f t h e e x c i t a t i o n o r a t l e a s t more r e f r a c t o r y t o b l o c k b y c h l o r a l o s e a n d a t r o p i n e . The d e p r e s s i o n d i d n o t a p p e a r t o be b l o c k e d by DH3E and was l e s s s e n s i t i v e t o a t r o p i n e t h a n was t h e e x c i t a t i o n . The d e p r e s s i o n o b s e r v e d h e r e c o u l d be r e l a t e d t o t h a t s e e n by C u r t i s a n d R y a l l [1966] i n Renshaw c e l l s . T h e s e a u t h o r s p r o p o s e d a t y p e o f c h o l i n e r g i c r e c e p t o r w h i c h p r o d u c e s d e p r e s s i o n a nd w h i c h m i g h t be common t o m a n y . t y p e s o f n e u r o n e . I n t h i s c a s e , t h e a p p a r e n t s p e c i f i c i t y o f EN-VL b l o c k i n g m i g h t be e x p l a i n e d s i m p l y b y s u g g e s t i n g t h a t b e c a u s e t h e EN s t i m u l a t i o n e v o k e s a l e s s p o w e r f u l s y n a p t i c e x c i t a t i o n t h a n BC o r c o r t i c a l s t i m u l a t i o n , t h e EN-VL r e s p o n s e i s d e p r e s s e d more e a s i l y t h a n t h e o t h e r s . B e c a u s e s y n a p t i c a c t i v a t i o n by BC and c o r t i c a l s t i m u l a t i o n w e r e n o t d e p r e s s e d b y ACh, t h e d e p r e s s i o n o f EN e v o k e d r e s p o n s e s seems t o h a v e some s p e c i f i c i t y . The s p e c i f i c i t y could be a p r o p e r t y o f t h e t y p e o f n e u r o n e , r e c e i v i n g EN a f f e r e n t s , b u t some o f t h e d e p r e s s e d EN r e s p o n s e s w e r e s e e n i n c e l l s w h i c h w e r e a c t i v a t e d by. o t h e r a f f e r e n t s y s t e m s w i t h o u t d e p r e s s i o n - b y ACh. The s p e c i f i t y c o u l d a l s o l i e a t t h e s y n a p s e , b u t no m e c h a n i s m c a n be p r o p o s e d f o r t h i s . As a t h i r d p o s s i b i l i t y , ACh m i g h t s e l e c t i v e l y a f f e c t n e r v e t e r m i n a l s o f t h e p a l l i d o - t h a l a m i c p a t h w a y . D e p o l a r i z -a t i o n o f t h e s e n e r v e t e r m i n a l s c o u l d r e d u c e t h e amount o f t r a n s m i t t e r r e l e a s e d t h u s r e d u c i n g t h e s i z e o f t h e E P S P s t o a s u b - t h r e s h o l d l e v e l . H o w e v e r , t h e d e p r e s s a n t e f f e c t s on EN r e s p o n s e s i n a l m o s t t h e t o t a l l a t e n c y r a n g e w o u l d i n d i c a t e e i t h e r t h a t t h e d i f f e r e n t l a t e n c y r e s p o n s e s do n o t r e p r e s e n t d i f f e r e n t p a t h w a y s o r t h a t t h e ACh d e p r e s s i o n i s n o t s p e c i f i c t o EN-VL f i b r e s . I n t r a c e l l u l a r r e c o r d i n g m i g h t a l s o p r o v i d e t h e a n s w e r t o t h e s e p r o b l e m s . The a mino a c i d s h a v e b e e n s u g g e s t e d t o h a v e n o n -s p e c i f i c e x c i t a t o r y a c t i o n s on n e u r o n e s [ C u r t i s a nd W a t k i n s , 1 9 6 3 ] . N i s h i , S o e d a and K o k e t s u [ 1 9 6 5 ] , h o w e v e r , . f o u n d t h a t p e r f u s i o n s o f b a r i u m g l u t a m a t e d i d n o t d e p o l a r i z e s p i n a l g a n g l i o n c e l l s o f t h e f r o g • I t i s i n t e r e s t i n g t h a t t h e s e -c e l l s a r e t h o u g h t n o t t o h a v e s y n a p t i c e n d i n g s on them. 139 F i n d i n g s o f d i f f e r e n t i a l s e n s i t i v i t y t o amino a c i d s i n d i f f e r e n t c e l l p o p u l a t i o n s p r e s e n t e d i n t h i s s t u d y t e n d t o c o n f i r m t h e i d e a t h a t s p e c i f i c i t i e s e x i s t a t t h e n e u r o n a l l e v e l f o r d i f f e r e n t g r o u p s o f e x c i t a t o r y , a m i n o a c i d s [ M c L e n n a n , . H u f f m a n and M a r s h a l l , 1968 b] . S u p p o r t i n g d a t a h a v e b e e n p r e s e n t e d i n r e c e n t r e p o r t s . B o a k e s , B r a d l e y , B r i g g s a n d D r a y [1969] h a v e r e p o r t e d t h a t i n b r a i n s t e m RF n e u r o n e s w h i c h a r e e x c i t e d b y i o n t o p h o r e t i c a l l y a p p l i e d 5-HT, l y s e r g i c a c i d d i e t h y l a m i d e (LSD) b l o c k s e x c i t a t i o n b y LG b u t n o t b y DLH. M c L e n n a n [1970] h a s i n d i c a t e d d i f f e r e n c e s b e t w e e n t h e s e same amino a c i d s i n t h e c e r e b r a l c o r t e x . E x c i t a t i o n s o f c o r t i c a l n e u r o n e s i n d u c e d b y i o n t o p h o r e t i c LG w e r e d e p r e s s e d f o r a l o n g e r t i m e b y s t i m u l i t o n e i g h b o u r i n g c o r t e x t h a n w e r e c o m p a r a b l e l e v e l s o f DLH e x c i t a t i o n . The d i f f e r e n c e s s e e n w e r e n o t t h e same a t a l l c o r t i c a l d e p t h s , b u t w e r e more p r o n o u n c e d a t d e p t h s c o r r e s p o n d i n g t o l a y e r s I I a n d V. I t a p p e a r s t h e r e f o r e t h a t t h e r e a r e d i f f e r e n c e s i n t h e e x c i t a t i o n s i n d u c e d by, t h e s e two amino a c i d s . The s t u d i e s w i t h NMA and DG i n d i c a t e t h a t i f t w o , r a t h e r t h a n m u l t i p l e a m i n o a c i d e x c i t a t i o n m e c h a n i s m s a r e p o s t u l a t e d , DG a f f e c t s t h a l a m i c n e u r o n e s i n t h e same way as LG, t h o u g h w i t h l e s s p o t e n c y , w h i l e t h e e f f e c t s o f NMA r e s e m b l e c l o s e l y t h o s e o f DLH. Tho u g h t h e s e n s i t i v i t y d i f f e r e n c e b e t w e e n t h e two c e l l g r o u p s s t u d i e d d i d n o t seem as g r e a t f o r DG a s f o r LG 140 ( F i g u r e 29], t h e n o n - e v o k e d c a t e g o r y - p a r t i c u l a r l y f o r DG i n c l u d e s a number o f c e l l s w h i c h w e r e n o t e x c i t e d a t t h e 110 nA l e v e l . . B e c a u s e , h o w e v e r , . a c e i l i n g o f 110 nA was u s e d f o r p u r p o s e s o f c a l c u l a t i o n , t h e d i f f e r e n c e s , p a r t i c u l a r l y f o r DG may a p p e a r s m a l l e r t h a n t h e y s h o u l d b e . W h e t h e r t h e d i f f e r e n c e s a r e a t t r i b u t e d t o d i f f e r e n t r e c e p t o r m e c h a n i s m s o r t o p r e s e n t l y . u n k n o w n p r o p e r t i e s o f d i f f e r e n t . c e l l p o p u l a t i o n s o r n u c l e a r r e g i o n s c a n n o t be d e c i d e d a t t h i s s t a g e . An e x a m p l e o f t h e l a t t e r c o u l d be t h e p r e s e n c e o f an g l u t a m a t e s p e c i f i c enzyme o r membrane u p t a k e s y s t e m i n VL w h i c h i s n o t p r e s e n t t o t h e same degree:-.in t h e more d o r s a l n u c l e i . I t i s t e m p t i n g t o p o s t u l a t e t h a t a g l u t a m a t e r e c e p t o r and a n o n - s p e c i f i c d i a c i d i c a m i n o a c i d r e c e p t o r e x i s t , b u t t h e s i m i l a r i t y o f e f f e c t s o f LG and i t s s t e r e o i s o m e r DG on t h e two c e l l g r o u p w o u l d seem t o i n d i c a t e a l a c k o f s p e c i f i c i t y o f t h e g l u t a m a t e r e c e p t o r t h a t i s n o t g e n e r a l l y f o u n d i n enzyme and d r u g r e c e p t o r s . A c o n c e p t u a l o b j e c t i o n t o t h e p r o p o s a l o f g l u t a m i c a c i d a s a s y n a p t i c t r a n s m i t t e r [ K r n j e v i c : , : 1970] h a s b e e n i t s p r o p o s e d n o n - s p e c i f i c i t y o f a c t i o n . The a b o v e i n d i c a t i o n s t h a t s p e c i f i c i t i e s do e x i s t b e t w e e n t h e e x c i t a t o r y a m i n o a c i d s s h o u l d h e l p t o remove t h i s o b j e c t i o n . A t t e m p t s t o f i n d a g l u t a m a t e b l o c k i n g a g e n t h a v e shown t h a t some b l o c k i n g p r o p e r t i e s c a n be a t t r i b u t e d t o a = m e t h y l g l u t a m a t e , b u t n o t - t o N - m e t h y l g l u t a m a t e [ M c L e n n a n , M a r s h a l l and H u f f m a n , 1971]. 141 The two f o r m s o f a n t a g o n i s m f o u n d w i t h aMG a r e p u z z l i n g b u t a p o s s i b l y s i m i l a r v a r i a b i l i t y o f a c t i o n h a s b e e n o b s e r v e d i n t h i s s t u d y d u r i n g t h e i o n t o p h o r e t i c a p p l i c a t i o n o f a t r o p i n e . I f t h e a t r o p i n e a p p l i c a t i o n was s t o p p e d as. s o o n as ACh s e n s i t i v i t y h a s b e e n b l o c k e d , t h e ACh r e s p o n s e u s u a l l y r e t u r n e d w i t h i n a few m i n u t e s . " H o w e v e r , a prolonged a t r o p i n e a p p l i c a t i o n o f t e n b l o c k e d t h e ACh s e n s i t i v i t y f o r l o n g p e r i o d s o f t i m e a f t e r c e s s a t i o n o f a t r o p i n e d e l i v e r y . The b l o c k i n g o f LG e x c i t a t i o n was v e r y s p e c i f i c when c o m p a r e d t o ACh e x c i t a t i o n , b u t l e s s s p e c i f i c when c o m p a r e d w i t h e x c i t a t i o n b y DLH o r a s p a r t i c a c i d . More w o r k i s o b v i o u s l y n e c e s s a r y i n VL and i n o t h e r p a r t s o f t h e b r a i n t o d e t e r m i n e t h e n a t u r e o f t h e a n t a g o n i s m and i t s g e n e r a l a p p l i c a b i l i t y . F u r t h e r s t u d i e s on t h e s e p r o b e l m s a r e i n p r o g r e s s . I n t e r a c t i o n s b e t w e e n a n e s t h e t i c a g e n t s a nd e f f e c t s o f i o n t o p h o r e t i c a l l y a p p l i e d d r u g s , p a r t i c u l a r l y ACh, h a v e b e e n n o t e d p r e v i o u s l y [ P h i l l i s a n d T e b e c i s , 1967; B l o o m , C o s t a and S a l m o i r a g h i , 1965]. The s e l e c t i v e e f f e c t s o f s o d i u m t h i o p e n t a l i n d e l a y i n g t h e o n s e t o f ACh e x c i t a t i o n i s a n o v e l o b s e r v a t i o n . The d e p r e s s i o n o f s p o n t a n e o u s c e l l f i r i n g d u r i n g t h e l a t e n t p e r i o d ( F i g u r e 42) i s o f i n t e r e s t , and c o u l d a g a i n r e f l e c t a d u a l a c t i o n o f ACh, i . e . , an e x c i t a t i o n a n d a d e p r e s s i o n w h i c h a r e a t l e a s t p a r t i a l l y i n d e p e n d e n t . I n s p i t e o f t h e o b s e r v a t i o n o f B l o o m e t a l . t h a t f e w e r A C h - s e n s i t i v e c e l l s a r e f o u n d i n t h e c a u d a t e n u c l e u s o f c h l o r a l o s e a n e s t h e t i z e d t h a n o f 142 d e c e r e b r a t e c a t s , t h e p o t e n c y a n d s p e c i f i c i t y o f t h e e f f e c t s e e n i n t h i s s t u d y was s u r p r i s i n g . T hough o n l y one s p e c i f i c e x a m i n a t i o n o f t h i s e f f e c t was made i t i s t h o u g h t t o be o f g e n e r a l v a l i d i t y b e c a u s e o f t h e n o t a b l e a b s e n c e o f ACh e x c i t e d VL n e u r o n e s i n t h e c h l o r a l o s e a n e s t h e t i z e d p r e p a r a t i o n s . The s t u d i e s o f t h e c o r t i c o - C M p a t h w a y w e r e u n d e r t a k e n o r i g i n a l l y i n a n a t t e m p t t o ; d e f i n e e l e c t r o p h y s i o l o g i c a l l y t h e l a t e r a l p a r t o f t h e CM-Pf c o m p l e x w h i c h was o u t l i n e d on an a n a t o m i c a l b a s i s by M e h l e r [ 1 9 6 6 ] , The a t t e m p t a p p e a r s s u c c e s s f u l and i t i s h o p e d t h a t t h e r e s p o n s e may be u s e f u l t o o t h e r w o r k e r s i n a t t e m p t s t o d e f i n e f u r t h e r c h a r a c t e r i s t i c s o f t h i s n u c l e u s . The. p o s i t i v e c o r r e l a t i o n o f t h e e l e c t r o -p h y s i o l o g i c a l f i n d i n g s w i t h t h e r e s u l t s o f p r e v i o u s a n a t o m i c a l s t u d i e s i n d i c a t e t h a t , t h e s p i k e r e s p o n s e s a r e p r o b a b l y e v o k e d by. a d i r e c t p a t h w a y f r o m t h e c o r t e x . R i n v i k ' s [1968] o b s e r v a t i o n t h a t t h e f i b r e s o f t h e c o r t e x - C M p a t h w a y a r e o f c o n s i d e r a b l y f i n e r d i a m e t e r t h a n f i b r e s f r o m t h e c o r t e x t o r e l a y n u c l e i a c c o u n t s f o r t h e r e l a t i v e l y l o n g l a t e n c y o f t h e r e s p o n s e . The n e u r a l b a s i s o f t h e p o s i t i v e wave f i e l d r e s p o n s e i s unknown, b u t a s a" d e p r e s s i o n o f f i r i n g was some-t i m e s f o u n d f o l l o w i n g a c t i v a t i o n o f CM c e l l s f r o m t h e c o r t e x , and b e c a u s e t h e t r o u g h o f t h e p o s i t i v e wave was o f l o n g e r l a t e n c y t h a n t h e s p i k e r e s p o n s e s , i t i s t h o u g h t t h a t i t may r e p r e s e n t a p o s t e x c i t a t o r y h y p e r p o l a r i z a t i o n o f CM c e l l s , p e r h a p s s i m i l a r t o t h a t d e s c r i b e d i n c e l l s o f t h e r e l a y n u c l e i [ A n d e r s e n and A n d e r s s o n , 1 9 6 8 ] , 143 The a b u n d a n c e o f s t u d i e s o f VL w h i c h h a v e b e e n r e -p o r t e d i n t h e l a s t two y e a r s h a v e s t i l l l e f t many q u e s t i o n s u n a n s w e r e d . I t i s s u g g e s t e d t h a t a p r e r e q u i s i t e t o t h e u n d e r s t a n d i n g o f t h e r o l e s o f t h i s n u c l e u s w i l l be a c a r e f u l a n a l y s i s o f t h e t y p e s o f c e l l s w i t h i n VL and t h e r e l a t i o n -s h i p o f e a c h t y p e t o a f f e r e n t and e f f e r e n t p a t h w a y s . S u c h an a n a l y s i s h a s n o t y e t b e e n r e p o r t e d , b u t F r i g y e s i and M a c h e k 1 s o b s e r v a t i o n t h a t EN and SN s t i m u l a t i o n d i d n o t a c t i v a t e t h e same g r o u p s o f VL n e u r o n e s i n d i c a t e s t h a t some s y s t e m a t i c d i f f e r e n c e s , a t . l e a s t o f a f f e r e n t c o n n e c t i o n s -do e x i s t . 144 SUMMARY AND CONCLUSIONS S p o n t a n e o u s a nd BC and c o r t i c a l - e v o k e d r h y t h m i c b u r s t -i n g a c t i v i t y was f o u n d i n VL. N e i t h e r t h e b u r s t i n g a c t i v i t y n o r d e p r e s s i o n o f d r u g - i n d u c e d f i r i n g b y BC s t i m u l a t i o n was a f f e c t e d by i o n t o p h o r e t i c o r i n t r a v e n o u s s t r y c h n i n e . I n a d d i t i o n t o t h e s h o r t - l a t e n c y and l a t e - b u r s t i n g a c t i v i t y e v o k e d i n VL b y BC and c o r t i c a l s t i m u l a t i o n , b u r s t r e s p o n s e s o f n e u r o n e s t o BC, c o r t i c a l a n d EN s t i -m u l a t i o n w e r e f o u n d . The b u r s t r e s p o n s e s t o BC and c o r t i c a l s t i m u l a t i o n w e r e f o u n d i n a n e s t h e t i z e d a n i m a l s , b u t t h o s e t o EN s t i m u l a t i o n w e r e f o u n d i n d e c e r e b r a t e p r e p a r a t i o n s a l s o . S h o r t - l a t e n c y B C - e v o k e d r e s p o n s e s i n VL w e r e b l o c k e d by I.V. a t r o p i n e , b u t n o t by i o n t o p h o r e t i c a l l y a p p l i e d a t r o p i n e . DHBE b l o c k e d s u c h r e s p o n s e s o n o n l y one o c c a s i o n . The r e s u l t s i n d i c a t e t h a t t h e BC-VL p a t h w a y i s c h o l i n e r g i c a t l e a s t i n p a r t and t h e i n e f f e c t i v e n e s s o f i o n t o p h o r e t i c a l l y a p p l i e d b l o c k i n g a g e n t s may be due t o t h e h i g h s y n a p t i c e f f i c i e n c y o f t h i s s y s t e m . C o r t i c a l a n d E N - e v o k e d r e s p o n s e s i n VL w e r e n o t b l o c k e d by i o n t o p h o r e t i c o r I.V. a t r o p i n e n o r by i o n t o p h o r e t i c DHgE. I.V. DHBE d i d n o t a l t e r E N - e v o k e d c e l l r e s p o n s e s on t h e two o c c a s i o n s t e s t e d . The c o r t i c o - V L a n d EN-VL p a t h w a y s do n o t a p p e a r t o d e p e n d on c h o l i n e r g i c m e d i a -t i o n . 145 The b u r s t r e s p o n s e s o f VL n e u r o n e s t o s t i m u l a t i o n o f EN h a v e a w i d e l a t e n c y r a n g e , w h i c h c o u l d be due i n p a r t t o a c t i v a t i o n o f t h e s e c e l l s by d i r e c t a s w e l l a s i n d i r e c t p a t h w a y s . The c o n v e r s i o n o f a b u r s t r e s p o n s e t o a s i n g l e 2-3 msec l a t e n c y s p i k e by e x c i t a t o r y a g e n t s may r e f l e c t t h e p r e s e n c e o f 2-3 E P S P s r e s u l t i n g f r o m a c t i v a t i o n o f t h e d i r e c t p a t h w a y . I o n t o p h o r e t i c a l l y a p p l i e d ACh s o m e t i m e s d e p r e s s e d s y n a p t i c r e s p o n s e s o f VL n e u r o n e s t o EN s t i m u l a t i o n b u t n o t t o BC o r c o r t i c a l s t i m u l a t i o n . T h i s may r e f l e c t a p o s t s y n a p t i c d e p r e s s i o n w h i c h a f f e c t s t h e s y n a p t i c r e s p o n s e s o n l y o f w e a k e r a f f e r e n t s y s t e m s o r c o u l d b e due t o more s p e c i f i c a c t i o n s o f ACh o n EN-VL t r a n s -m i s s i o n . LG and DLH w e r e f o u n d t o h a v e d i f f e r e n t p a t t e r n s i n t h e i r a b i l i t y t o i n d u c e f i r i n g i n two g r o u p s o f n e u r o n e s . NMA was s i m i l a r i n p a t t e r n t o DLH w h i l e DG and ACh w e r e s i m i l a r t o LG. T h e s e f i n d i n g s i n d i c a t e d i f f e r e n c e s w h i c h c o u l d b e r e l a t e d t o d i f f e r e n t e x c i t a t o r y m e c h a n i s m s f o r a t l e a s t two g r o u p s o f amino a c i d s . aMG was f o u n d t o b l o c k w i t h c o n s i d e r a b l e s p e c i f i c i t y t h e f i r i n g o f t h a l a m i c n e u r o n e s i n d u c e d b y LG; b u t i t d i d n o t b l o c k t h e s h o r t - l a t e n c y e x c i t a t i o n o f VL n e u r o n e s by BC s t i m u l a t i o n . 146 9. T h i o p e n t a l , p e n t o b a r b i t a l a n d a - c h l o r a l o s e i n j e c t e d s y s t e m i c a l l y w e r e f o u n d t o a l t e r t h e a b i l i t y o f ACh t o i n d u c e f i r i n g i n VL n e u r o n e s , b u t h a d much l e s s e r e f -f e c t s o n f i r i n g c a u s e d b y DLH. 10. An e l e c t r i c a l r e s p o n s e i n d i c a t i v e o f a c o r t i c o t h a l a m i c p a t h w a y t e r m i n a t i n g p r e f e r e n t i a l l y i n t h e l a t e r a l p a r t o f t h e CM-Pf c o m p l e x was f o u n d . 1 1 . A n t i d r o m i c a c t i v a t i o n o f EN c e l l s f r o m VL o r CM a n d c o l l i s i o n b e t w e e n a c t i v a t i o n s f r o m t h e s e l o c i i n d i c a t e t h a t m o s t i f n o t a l l o f t h e EN n e u r o n e s s e n d i n g a x o n s t o VL h a v e c o l l a t e r a l s t o CM. 147 BIBLIOGRAPHY Adrian, E.D. (1941) Afferent discharges to the cerebral cortex from peripheral sense organs. J. Physiol. (Lond.) 100_, 159-191. Ahmed, A.; P.B. Marshall (1962) Relationship between a n t i -acetylcholine and a n t i -tremorine a c t i v i t y i n anti-parkinsonian and related drugs. B r i t . J . Pharmacol. 1_8, 247-254. Albe-Fessard, D.; A. Fessard (1953) Thalamic integrations and t h e i r consequences at the telencephalic l e v e l . Prog. Brain Res. 1, 115-148. Albe-Fessard, D.; G. Guiot, Y.Lamarre, G. A r f e l (1966) Acti v a t i o n of thalamocortical projections related to tremorogenic process. pp. 137-253 i n : "The Thalamus" Eds: D.P. Purpura; M.D. Yahr, Columbia Univ. Press, New York. Amassian, V.E.; H. Weiner (1966) Monosynaptic and poly-synaptic a c t i v a t i o n of pyramidal t r a c t neurones by thalamic stimulation, pp. 255-286, i n : "The Thalamus" Eds: D.P. Purpura; M.D. Yahr, Columbia Univ. Press, New York. Anden, N.E.; A. Dahlstrom, K. Fuxe, K. Larsson (1965) Further evidence for the presence of nigro-neostriatal dopamine neurones i n the r a t . Am. J . Anat. 116, 329-334. Andersen, P.; S.A. Andersson (1968) "Physiological Basis of the Alpha Rhythm" Appleton-Century-Crofts, New York. Andersen, P.; S.A. Andersson, T_,L0mo (1967) Nature of thalamo-cortical. spindles during spontaneous barbiturate spindle a c t i v i t y , J . Physiol. (Lond.) 192, 283-307. Andersen, P.; D.R. Curtis (1964a) The exci t a t i o n of thalamic neurones by acetylcholine, Acta Physiol. Scand., 61, 85-99. 148 Andersen, P.; D.R. C u r t i s (1964b) The pharmacology-of the s y n a p t i c and a c e t y l c h o l i n e - i n d u c e d e x c i t a t i o n of v e n t r o b a s a l thalamic neurones, A c t a P h y s i o l . Scand. 61, 100-120. Andersen, P.; J.C. E c c l e s (1962) I n h i b i t o r y phasing of neuronal d i s c h a r g e , Nature (Lond.) 196, 645-647. Andersen, P.; L. Olsen, K. Skrede, 0. Sveen (1969) Mechanism of t h a l a m o - c o r t i c a l rythmic a c t i v i t y with s p e c i a l r e f e r e n c e to the motor system, pp. 112-118. i n : " T h i r d Symposiom on Parkinson's Disease." Eds: F . J . G i l l i n g h a m ; I.M.L. Donaldson, E and S L i v i n g s t o n e L t d . , Edinburgh. Andersen P.; T.A. -Sears (1964) The r o l e of i n h i b i t i o n i n the phasing of spontaneous t h a l a m o - c o r t i c a l discharge, J . P h y s i o l . (Lond.) 173' 459-480. Angaut, P.; D. Bowsher (1965) C e r e b e l l o - r u b r a l c o n n e c t i o n i n the c a t . Nature (Lond.) 208, 1002-1003. Angaut, P.; G.. Guilbaud, M.C. -Reymond (-1968) An e l e c t r o -p h y s i o l o g i c a l study of the c e r e b e l l a r p r o j e c t i o n s to the nucleus v e n t r a l i s l a t e r a l i s of thalamus i n the c a t . I. N u c l e i f a s t i g i i e t i n t e r p o s i t u s . J . Cpmp. Ne u r o l . 134, 9-19. Appelberg, B. (1961) L o c a l i z a t i o n of f o c a l p o t e n t i a l s evoked i n the red nucleus and the v e n t r o l a t e r a l nucleus by e l e c t r i c a l s t i m u l a t i o n of the c e r e b e l l a r n u c l e i . . A c t a P h y s i o l . Scand. 5_1, 356-370. A s t r u c , J . (1964) C o r t i c o f u g a l f i b r e d egeneration f o l l o w -i n g l e s i o n s of area 8 ( f r o n t a l eye f i e l d ) i n Macaca mulatta. Anat. Record 148, 256. Auer, J . (1956) Terminal degeneration i n the diencephalon a f t e r a b l a t i o n of f r o n t a l c o r t e x i n the c a t . J . Anatomy _90, 30-41. Bloom, F.E.; E..Costa, G.C. S a l m o i r a g h i (1965) A n e s t h e s i a and the responsiveness of i n d i v i d u a l neurones of the caudate nucleus of the cat to a c e t y l c h o l i n e , n o r e p i n e p h r i n e and dopamine admi n i s t e r e d by micro-e l e c t r o p h o r e s i s . J . Pharmacol, and Exp. Ther. 150, 244-252. 149 Boakes, R.J.; P.D. Badley, I. Briggs, A. Dray (1969) Antagonism by LSD to effects of 5-HT on single neurones, Brain Res. 15_, 5 2 9 - 5 3 1 . Bowsher, D.; P. Angaut (1968) Ascending projections from the red nucleus i n the decerebellate cat. Experientia. 2 4 , 2 6 3 - 2 6 3 . Burgen, A.S.V.; L.M. Chipman (1951) Cholinesterase and succinic dehydrogenase i n the central nervous system of the dog. J. Physiol. (Lond.) 114, 29 6 - 3 0 5 . Carpenter, M.B.; G.M. B r i t t i n , J. Pines (1958) Isolated lesions of the f a s t i g i a l nuclei i n the cat. J . Comp. Neurol. 109, 65-89. Carpenter, M.B.; N.L. Strominger (1967) Efferent fibres of the subthalamic nucleus i n the monkey. A comparison of the efferent projections of the sub-thalamic nucleus, substantia nigra and globus-p a l l i d u s . Am. J . Anat. 1 2 1 , 41-72. Cohen, L.; W.W. Chambers, J.M. Sprague (1958) Experimental study of the efferent projections from the cerebellar nuclei to the brain-stem of the cat. J. Comp. Neurol. 109, 233-259. Cole, M.; W. J. Nauta, W.R. Mehler (1964) The ascending efferent projections of the substantia nigra. Trans. Am. Neurol. Assn. 89_, 74-78. Conde, H.; P. Angaut (1970) An electrophysiological study of the cerebellar projections to the nucleus v e n t r a l i s l a t e r a l i s thalami i n the cat. II. Nucleus Laberalis. Brain Res. 2j0, 107-119. Cooper, I.S.; K. Samra, L. Bergmann (196 8) The thalamic lesion which abolishes tremor and r i g i d i t y of Parkinsonism. A radiologico-clinico-anatomic c o r r e l a t i v e study. J. Neurol. S c i . 8, 68-84. Cordeau, J.P. (1966) pp. 3 8 4 - 3 8 6 , Discussion i n D.P. Purpura and M.D. Yahr (Eds.) "The Thalamus" Columbia Univ. Press, New York. 150 C o u r v i l l e , J . (1966) S o m a t o t o p i c a l o r g a n i z a t i o n o f t h e p r o j e c t i o n f r o m t h e n u c l e u s i n t e r p o s i t u s a n t e r i o r o f t h e c e r e b e l l u m t o t h e r e d n u c l e u s : an e x p e r i m e n t a l s t u d y i n t h e c a t w i t h s i l v e r i m p r e g n a t i o n m e t h o d s . E x p . B r a i n R e s . 2_,-191-215. C o y l e , J . T . ; S.H. S n y d e r (1969) A n t i p a r k i n s o n i a n D r u g s : I n h i b i t i o n o f d o p a m i n e u p t a k e i n t h e c o r p u s s t r i a t u m a s a p o s s i b l e mechamism o f a c t i o n . S c i e n c e 1 6 6 , 89 9 - 9 0 1 . C r a w f o r d , J.M.; D.R. C u r t i s (1966) P h a r m a c o l o g i c a l s t u d i e s on f e l i n e B e t z c e l l s . J . P h y s i o l . (Lond.) 186, 121 - 1 3 8 . C r o u c h , R.L. (1934) The n u c l e a r c o n f i g u r a t i o n o f t h e t h a l a m u s o f M a c a c u s r h e s u s . J . Comp. N e u r o l . 5 9 , 451- 4 8 5 . C u r t i s , D.R. (1964) p p . 144 - 1 9 0 . M i c r o e l e c t r o p h o r e s i s , i n : " P h y s i c a l T e c h n i q u e s i n B i o l o g i c a l R e s e a r c h . " E d . W.L. N a s t u k , V o l . 5, P a r t A. A c a d e m i c P r e s s , New Y o r k . C u r t i s , D.R.; R. D a v i s (1962) P h a r m a c o l o g i c a l s t u d i e s u p o n n e u r o n e s o f t h e l a t e r a l g e n i c u l a t e n u c l e u s o f t h e c a t . B r . J . P h a r m a c o l . C h e m o t h e r . 1_8, 217-246.. C u r t i s , D.R.; R. D a v i s (1963) The e x c i t a t i o n o f l a t e r a l g e n i c u l a t e n e u r o n e s by q u a t e r n a r y ammonium d e r i v a t i v e s , J . - P h y s i o l . (Lond.) 1 6 5 , 62-82. C u r t i s , D.R.; A.W. -Duggan, D. F e l i x , G.A.R. J o h n s t o n (1970) GABA, b i c u c u l l i n e and c e n t r a l i n h i b i t i o n . N a t u r e (Lond.) 2 2 6 , 1222-1224.. G u r t i s , D.R.; A.W. Duggan, G.A.R. J o h n s t o n (1969) G l y c i n e , s t r y c h n i n e , p i c r o t o x i n , a n d s p i n a l i n h i b i t i o n s . B r a i n R e s . 14, 759-762., C u r t i s , D.R.; R.M. E c c l e s (1958) The e x c i t a t i o n o f Renshaw c e l l s b y p h a r m a c o l o g i c a l a g e n t s a p p l i e d e l e c t r o -p h o r e t i c a l l y . J . P h y s i o l . (Lond.) 141,. 435 - 4 4 5 . C u r t i s , D.R. ; J.W-. P h i l l i s (1960) The a c t i o n o f p r o c a i n e and a t r o p i n e on s p i n a l n e u r o n e s . J . P h y s i o l . (Lond.) 1 5 3 , 17-34. C u r t i s , D.R.; R.-.W. R y a l l (1966a) The e x c i t a t i o n o f Renshaw c e l l s b y c h o l i n o m i m e t i c s . E x p . B r a i n R e s . 2_, 4 9 - 6 5 . 151 C u r t i s , D.R.; R.W. R y a l l (1966b) The a c e t y l c h o l i n e r e c e p t o r s o f Renshaw c e l l s . E x p . B r a i n R e s . 2, 6 6 - 8 0 . C u r t i s , D.R.; J.C. W a t k i n s (1960) The e x c i t a t i o n and d e p r e s s i o n o f s p i n a l n e u r o n e s by s t r u c t u r a l l y r e l a t e d a m i n o a c i d s . J . N e u r o c h e m . 6,.117-141.. C u r t i s , D.R.; J.C. W a t k i n s (1963) A c i d i c amino a c i d s w i t h s t r o n g e x c i t a t o r y a c t i o n s on mammalian n e u r o n e s . J . P h y s i o l . (Lond.) 166 , 1-14. D a v i s , R. (1966) A c e t y l c h o l i n e - s e n s i t i v e n e u r o n e s i n v e n t r o l a t e r a l t h a l a m i c n u c l e u s , pp. 1 9 3 - 1 9 5 , . i n : "The T h a l a m u s " E d s . D.P. P u r p u r a ; M.D. Y a h r , C o l u m b i a U n i v . P r e s s , .New-York. D e k a b a n , A. (1953) Human t h a l a m u s . An a n a t o m i c a l d e v e l o p -m e n t a l and p a t h o l o g i c a l s t u d y . I . D i v i s i o n o f : t h e human a d u l t t h a l a m u s i n t o n u c l e i b y u s e o f t h e c y t o -m y e l o - a r c h i t e c t o n i c m e t h o d . J . Comp. N e u r o l . 9 9 , 6 3 9 - 6 8 3 . D e s i r a j u , T.; G. B r o g g i , S . P r e l e v i c , M. S a n t i n i , D.P. P u r p u r a (1969) I n h i b i t o r y s y n a p t i c p a t h w a y s l i n k i n g , s p e c i f i c and n o n s p e c i f i c t h a l a m i c n u c l e i . B r a i n R e s . 1 5 , 5 4 2 - 5 4 3 . D e s i r a j u , T.; D.P. P u r p u r a (1969) S y n a p t i c c o n v e r g e n c e o f c e r e b e l l a r and l e n t i c u l a r p r o j e c t i o n s t o t h a l a m u s . B r a i n R e s . 1 5 , 5 4 4 - 5 4 7 . D o r m o n t , J . F . ; J . M a s s i o n (1970) D u a l i t y o f c o r t i c a l c o n t r o l on v e n t r o l a t e r a l t h a l a m i c a c t i v i t y . E x p . B r a i n R e s . 1 £ , 2 0 5 - 2 1 8 . D o r m o n t , J . F . ; C. .'.Ohye, D. A l b e - F e s s a r d (1968) C o m p a r i s o n o f t h e r e l a t i o n s h i p s o f t h e n u c l e u s e n t o p e d u n c u l a r i s a n d b r a c h i u m c o n j u n c t i v u m w i t h t h e t h a l a m i c n u c l e u s v e n t r a l i s l a t e r a l i s o f t h e c a t . pp. 1 0 8 - 1 1 1 , i n " T h i r d Symposium on P a r k i n s o n ' s D i s e a s e " E d s . F . J . - G i l l i n g h a m ; I.M.L. D o n a l d s o n . E and S L i v i n g s t o n e L t d . , E d i n b u r g h and L o n d o n . Duggan, A.W.; H. M c L e n n a n (1971) B i c u c u l l i n e and i n h i b i t i o n i n t h e t h a l a m u s . B r a i n Res 2J5, 1 8 8 - 1 9 1 . D u v o i s i n , R.C. (1967) C h o l i n e r g i c - a n t i c h o l i n e r g i c a n t a g o n i s m i n P a r k i n s o n i s m . A r c h . N e u r o l . 17_, 124-136. E c c l e s , J.C.;.M. I t o , J . S z e n t a g o t h a i (1967) "The C e r e b e l l u m as a N e u r o n a l M a c h i n e . " S p r i n g e r - V e r l a g , New Y o r k . 15 2 E h r i n g e r , H.; 0. H o r n y k i e w i c z (1960) V e r t e i l u n g v o n N o r a d r e n a l i n und Dopamin (3 H y d r o x y t y r a m i n ) i m G e h i r n d e s M e n s c h e n u n d i h r V e r h a l t e n b e i E r k r a n -k u n g e n d e s E x t r a p y r a m i d a l e n S y s t e m s . K l i n . W s c h r . 3 8 , 1 2 3 6 - 1 2 3 9 . E v a r t s , E.V.; W.T. T h a c h (1969) M o t o r m e c h a n i s m s o f t h e CNS: C e r e b e l l a r i n t e r r e l a t i o n s . A n n . R e v . . P h y s i o l . 3 1 , 451^498. F a u l l , R.L.M.; J . B . Carman (1968) A s c e n d i n g p r o j e c t i o n s o f t h e s u b s t a n t i a n i g r a i n t h e r a t . J . Comp. N e u r o l . 1 3 2 , 7 3 - 9 2 . F l o o d , S.; J . J a n s e n (1966) The e f f e r e n t f i b r e s o f t h e c e r e b e l l a r n u c l e i and t h e i r d i s t r i b u t i o n on t h e c e r e b e l l a r p e d u n c l e s i n t h e c a t . A c t a . A n a t . 6 3 , 137-166. F o x , C.A.; D.E. H i l l m a n , D.E. S i e g e s m u n d , L.A. S e t h e r (1966) The p r i m a t e g l o b u s p a l l i d u s and i t s f e l i n e and a v i a n h o m o l o g u e s : A G o l g i and e l e c t r o n m i c r o s c o p e s t u d y . , pp. 234-2 48. i n : " E v o l u t i o n o f t h e F o r e -b r a i n " E d s . R . - H a s s l e r ; H. S t e p h a n , S t u t t g a r t G e o r g T h i e m e V e r l a g . F r i g y e s i , T.L. (1969) I n t r a c e l l u l a r s t u d i e s o f t h e d u a l p r o j e c t i o n s y s t e m l i n k i n g c o r p u s s t r i a t u m and d o r s a l t h a l a m u s . EEG C l i n . N e u r o p h y s i o l . 2_6, 6 3 1 . F r i g y e s i , . T . L . ; J . Machek (1970) B a s a l g a n g l i a - d i e n c e p h a l o n s y n a p t i c r e l a t i o n s i n t h e c a t . I An i n t r a c e l l u l a r s t u d y o f d o r s a l t h a l a m i c n e u r o n e s d u r i n g c a p s u l a r and b a s a l g a n g l i a s t i m u l a t i o n . B r a i n R e s . 20, 20 1 - 2 1 7 . F r i g y e s i , T.L.; D.P. P u r p u r a (1966) A c e t y l c h o l i n e s e n s i t i v i t y o f t h a l a m i c s y n a p t i c o r g a n i z a t i o n s a c t i v a t e d by b r a c h i u m c o n j u n c t i v u m s t i m u l a t i o n . A r c h . I n t e r n . P h a r m a c o d y n . . 1 6 3, 1 1 0 - 1 3 2 . F u x e , K.; M. G o l d s t e i n , A. L j u n g d a h l (1970) A n t i p a r k i n s o n i a n d r u g s and c e n t r a l d o p a m i n e n e u r o n e s . . L i f e S c i . 9_, 811-824. G r e e n , J.D. (1958) A s i m p l e m i c r o e l e c t r o d e f o r r e c o r d i n g f r o m t h e CNS. N a t u r e (Lond.) 1 8 2 , 962. G r e e n f i e l d , J.G,. ; F.D. B o s a n q u e t (1953) B r a i n s t e m l e s i o n s i n p a r k i n s o n i s m . J . N e u r o l . N e u r o s u r g . and P s y c h i a t . 16, 2 1 3 - 2 2 6 . 153 Guzman, C.; M. Alcarez, A. .Fernandez (1958) Rapid procedure to l o c a l i z e electrodes i n experimental neurophysio-logy. Boletin del I n s t i t u t o de Estudios Medicos y Biologicos 16_, 29-31. Hassler, R. (1959) Anatomy of the Thalamus, pp. 230-290 i n : "Introduction to Stereotaxis with an atlas of the Human Brain" Ed: G. Schaltenbrand; P. Bailey, Georg Thieme Verlag, Stuttgart. Hebb, CO; A. S i l v e r (1956) Choline acetylase i n the central nervous system of man and some other mammals. J . Physiol. (Lond.) 134, 718-728. Ito, M..(1969) Neurones of Cerebellar Nuclei, pp. 309-327. i n : "The Interneuron" Ed. M.A.B. Brazier, Univ. of C a l i f . Press. Jasper, H.H.; C. Ajmone-Marsan (1954) A stereotaxic atlas of the diencephalon of the cat. National Res. Council (Ottawa). Jasper, H.H.; G. Bertrand (1966) Thalamic units involved_ in Somatic sensation on voluntary and involuntary ' movements i n man. pp. 365-390, i n : "The Thalamus" Eds. D.P. Purpura; M.D. Yahr, Columbia Univ. Press, New York. Johnson, T.N. (1961) Fiber connections between the dorsal thalamus and corpus striatum i n the cat. Exp. Neurol. 3_, 556-569. Kaelber, W.W.; C L . M i t c h e l l (1967) The centrum medianum-central tegmental fasciculus complex. A stimulation, lesion and degeneration study in the cat.- Brain 9 0, 83-100. Koelle, G.B. (1954) The histochemical l o c a l i z a t i o n of cholinesterase i n the central nervous system of the rat . J. Comp. Neurol 100, 211-228. K o l l e r , E.A.; M. Jenny (1969) A technique for standard decerebration by high-frequency coagulation. Brain Res . " 14_, 549-552 . Krnjevic, K. (1964) Micro-iontophoretic studies on c o r t i c a l neurones. Int. Rev. Neurobiol, 7, 41-98. 154 Krnjevic, K. (1970) Glutamate and y ' ^ i n o k u t y r i c acid i n the Brain. Nature, 2 28, 119-124. Krnjevic, K.; J.W. P h i l l i s (1963) Pharmacological properties of acetylcholine sensitive c e l l s i n the cerebral cortex. J. Physiol. (Lond.) 166, 328-350. Kusama, T.; K. Otani, E. Kawana (1966) Projections of the motor, somatic sensory, auditory.and v i s u a l cortices i n cats. Prog, i n Brain Res..21A, 292-322. Kuypers, H.G.J.M. (1970) In: Symposium on Cerebral and Cerebellar Motor Control. Brain Res. ~1_7, 539-552. L a l l e y , P.M.; G.V. Rossi,W.W. Baker (1970) Analysis of l o c a l cholinergic tremor mechanisms following se l e c t i v e neurochemical lesions. Exp. Neurol. 27, 258-275. Lamarre, Y.; J.P. Cordeau (1964) .Ac t i v i t e des neurones centraux chez l e singe porteur d 1un tremblement postural experimentale. J . Physiol. (Paris) 56, 589-591. Leblanc, F.E.; J.P. Cordeau (1969) Modulation of pyramidal t r a c t c e l l a c t i v i t y by v e n t r o l a t e r a l thalamic regions. It's possible role i n tremorogenic mechanisms. Brain Res. 14, 255-270. Maekawa, K.; D.P. Purpura (1967) Properties of spontaneous and evoked synaptic a c t i v i t i e s of thalamic ventro-basal neurones. J. -Neurophysiol. 3_0, 360-381. Marco, L.A.; T.S. Brown (1966) Thalamic i n h i b i t o r y i n t e r -neurones , Nature 210, 1388-1389. Massion, J . (1968) Etude d'une structure motrice thalamique, le-noyau v e n t r o l a t e r a l et de sa regulation par les afferences s e n s o r i e l l e s . D. Sc. Thesis, Univ. of Paris. Massion, J. (1969) Le noyau v e n t r o l a t e r a l , structure mqtrice thalamique. Laval Med. J . £0, 411-421. Massion, J . ; P. Angaut, D. Albe-Fessard (1965a) A c t i v i t y evoked i n the cat i n the region of the nucleus v e n t r a l i s l a t e r a l i s by various sensory s t i m u l i : I Macro-physiological study. EEG., C l i n . Neurophysiol. 19, 433-451. 155 Massion, J . ; P. Angaut, D. A l b e - F e s s a r d (1965b) A c t i v i t y evoked i n the c a t i n the r e g i o n of the nucleus v e n t r a l i s l a t e r a l i s by v a r i o u s sensory s t i m u l i : I I . M i c r o - p h y s i o l o g i c a l study. E E C , C l i n . N e u r o p h y s i o l . •19 , 452-469. McCance, I . ; J.W. P h i l l i s (1965) The l o c a t i o n of m i c r o e l e c -trode t i p s i n nervous t i s s u e . E x p e r i e n t i a 21, 108-109. McCance,.I.; J.W. P h i l l i s , R.A. Westerman (1968a) Physio-l o g i c a l and ph a r m a c o l o g i c a l s t u d i e s on the c e r e b e l l a r p r o j e c t i o n s t o the f e l i n e thalamus. Exp. Neurol. 21, 257-269. McCance I . ; J.W. P h i l l i s , R.A. Westerman (1968b) A c e t y l -c h o l i n e - s e n s i t i v i t y of thalamic neurones: i t s r e l a t i o n s h i p t o s y n a p t i c t r a n s m i s s i o n . Br. J . Pharmacol. Chemother. 3_2, 635-651. McCance, I . ; J.W. P h i l l i s , A.K. T e b e c i s , R.A. Westerman (1968) The pharmacology of a c e t y l c h o l i n e - e x c i t a t i o n of thalamic neurones, Br. J . Pharmacol. Chemother. 32, 652-662. ~~ McLardy, T. (1963) Thalamic microneurones.. Nature (Lond.), 199, 820-821. McLennan, H. (1970) I n h i b i t i o n s of long d u r a t i o n i n the c e r e b r a l c o r t e x . A q u a n t i t a t i v e d i f f e r e n c e between e x c i t a t o r y amino a c i d s . Exp. B r a i n Res. 10_, 417-426. McLennan, H. (1971) An anomalous a c t i o n of e x c i t a t o r y amino .acids i n the o l f a c t o r y b u l b . Canada P h y s i o l o g y 2_, 20. McLennan, H.; R.D. Huffman, K.C M a r s h a l l (1968a) E l e c t r o -p h y s i o l o g i c a l and p h a r m a c o l o g i c a l s t u d i e s of t h a l a m i c neurones. Proc. I n t . Un. P h y s i o l . S c i . V o l . V I I . McLennan, H.; R.D. Huffman, K.C M a r s h a l l (1968b) P a t t e r n s of e x c i t a t i o n of thalamic neurons by amino a c i d s and by a c e t y l c h o l i n e . Nature (Lond.) 219, 387-388. McLennan, H.; K.C. M a r s h a l l , R.D. Huffman (1971) Antagonism of the glutamate e x c i t a t i o n of thalamic neurones, submitted f o r p u b l i c a t i o n . Mehler, W.R. (1966) F u r t h e r notes on the c e n t r e median nucleus of Luys, pp. 109-127 i n "The Thalamus" ed. by D.P. Purpura and M.D. Yahr, Columbia Univ. P r e s s , New York. i 156 Mehler, W.R. (1969) Some neurological,species differences -a p o s t e r i o r i . Ann. N.Y. Acad. S.ci. 167, 424-468. Mettler, F.A. (1970) Nigrofugal connections i n the primate brain J. Comp. Neurol. 13 8, 291-320. Nashold, B.S. J r . (1959) Cholinergic stimulation of globus . p a l l i d u s i n man. Proc. Soc. Exp. B i o l . Med. 101, 68-69. Nastuk, W.L. (1953) Membrane poten t i a l changes at a single muscle endplate produced by tra n s i t o r y application of acetylcholine with an e l e c t r i c a l l y controlled microjet. Fed. Proc.l_2, 102. Nauta, W.J.H.; W.R.Mehler (1961) Some efferent connections of the lentiform. nucleus i n the monkey and cat. Anat. Record 139, 26 0. Nauta, W.J.H.; W.R. Mehler (1966) Projections:from the lentiform nucleus i n the monkey. Brain Res. 1, 1-38. Nauta, W.J.H.; D.G. Whitlock (1954) An anatomical analysis of the non-specific thalamic projection system, pp. 81-116 i n "Brain Mechanisms and Consciousness" ed. by J.F. Delafresnaye, Thomas. Niimi, K.;'K. Katayama, T. Kanaseki, K. Morimoto (1960) Studies on the derivation of the centre median nucleus of Luys. Tokushima J. Exp. Med. 1_, 261-268. Niimi, K.; S. K i s h i , M. M i l i , S. F u j i t a (1963) An experimental study of•the course and termination of the projection f i b r e s from c o r t i c a l areas 4 and 6 i n the cat. F o l i a Psychiat. et Neurol. Japon. 17, 167-216. N i s h i , S.; H. Soeda,,K. Koketsu (1965) E f f e c t s of a l k a l i -earth cations on frog spinal ganglion c e l l s . J. j Neurophysiol. 2_8, 457-472. Ohye, C.; J.F. Dormont, L. Rispal (1968) Etude electrophysio-logique des relations pallidothalamiques. J. Physiol. (Paris) 60_ suppl. No. 2, 374. O l i v i e r , A.; A. Parent, L.J. P o i r i e r (1970) I d e n t i f i c a t i o n of the thalamic nuclei on the basis of t h e i r c h o l i n -esterase content i n the monkey. J. Anat. 106, 37-50. 157 O l i v i e r , A . ; A. Parent, H. Simard, L . J . P o i r i e r (1968) I d e n t i f i c a t i o n of e x t r o p y r a m i d a l and r e l a t e d s t r u c -t u r e s on the b a s i s o f t h e i r content i n c h o l i n e s t e r a s e i n the monkey and the c a t . Anat. Record. 160, 402. Olsz e w s k i , J . (1952) The thalamus of the Macaca mulatta. S. Karger, B a s e l . P e t r a s , J.M. (1964) Some f i b r e connections of the p r e c e n t r a l c o r t e x (areas 4 and 6) wit h the diencephalon i n the monkey (Macaca mulatta) Anat. Record, 148, 322. P e t r a s , J.M. (1969) Some e f f e r e n t connections of the motor and somatosensory, c o r t e x of simian primates and f e l i d , c a n i d and p r o c y o n i d c a r n i v o r e s . Ann. New York, Acad. S c i . 167, 469-505. P h i l l i s , J.W. (1968) A c e t y l c h o l i n e s t e r a s e i n the f e l i n e c e rebellum. J . Neurochem. 15_, 691-698. P h i l l i s , J.W.; K. K r n j e v i c (1961) S e n s i t i v i t y of c o r t i c a l neurones to a c e t y l c h o l i n e . E x p e r i e n t i a 17_, 469. P h i l l i s , J.W.; A.K. Te b e c i s (1967a) The e f f e c t s of pento-b a r b i t o n e sodium on a c e t y l c h o l i n e e x c i t a t i o n and n o r a d r e n a l i n e i n h i b i t i o n of thalamic neurones. L i f e S c i . 6, 1621-1625. P h i l l i s , J.W.; A.K. Te b e c i s (1967b) The responses o f tha l a m i c neurones t o i o n t o p h o r e t i c a l l y a p p l i e d monoamines J . P h y s i o l . (Lond.) 192, 715-745. P h i l l i s , J.W.; A.K. T e b e c i s , D.H. York (1967) A study of c h o l i n o c e p t i v e c e l l s i n the l a t e r a l g e n i c u l a t e nucleus. J . P h y s i o l . (Lond.) 192, 695-713. P o i r i e r , L . J . (1960) Experimental and h i s t o l o g i c a l study of midbrain d y s k i n e s i a s . J . N e u r o p h y s i o l . 23 534-551. Pompeiano, O.; A. B r o d a l (1957) Experimental demonstration of a s o m a t o t o p i c a l o r i g i n o f r u b r o s p i n a l f i b r e s i n the c a t . J . Comp. Ne u r o l . 10 8, 225-251. Powell, T.P.S.;• M.W. Cowan (1956) A study of t h a l a m o - s t r i a t e r e l a t i o n s i n the monkey. B r a i n 79_, 364-390. Purpura, D.P.; T.L. F r i g y e s i , J.G.. McMurtry, T. S c a r f f (1966)-S y n a p t i c mechanisms i n thalamic r e g u l a t i o n of c e r e b e l l o - c o r t i c a l p r o j e c t i o n a c t i v i t y , pp. 153-172 i n "The Thalamus" ed. by D.P. Purpura and M.D. Yahr, Columbia Univ. P r e s s , New York. 158 Purpura, D.P.; J.G. McMurtry, K. Maekawa (1966) Synaptic events i n ventr o l a t e r a l -thalamic neurons during suppression of r e c r u i t i n g responses by brain stem r e t i c u l a r stimulation, Brain Res. 1, 63-76. Purpura, D.P.; T. Scarff, J.G. McMurtry (1965) I n t r a c e l l u l a r study of internuclear i n h i b i t i o n i n ventr o l a t e r a l thalamic neurones. J. Neurophysiol. 28_, 487-496. Ranson, S.W.; M. Ranson (1939) P a l l i d o f u g a l f i b r e s i n the monkey. Arch. Neurol. Psychiat. (Chicago) 42_, 1 0 5 9 -1067. Ranson, S.W.; S.W. Ranson J r . , M. Ranson (1941) Fibre connec-tions of corpus striatum as seen i n Marchi preparations. Arch. Neurol. Psychiat. (Chicago) £ 6 , 230 - 2 4 9 . Reinoso-Suarez, F. (1961) Topographischer Hirnatlas der Katze, E. Merck, Darmstadt. Rinvik, E. (196 8) The corticothalamic projection from the precruciate and coronal g y r i i n the cat. An experimental study with silver-impregnation methods. Brain Res. 10, 79-119. Rispal-Padel, L.; J. Massion (1970) Relations between the vent r o l a t e r a l nucleus and the motor cortex i n the cat. Exp. Brain Res. 10, 331 - 3 3 9 . Sakata, H. ; T. Ishijima, Y. Toyoda (1966) Single unit studies on ventr o l a t e r a l nucleus of the thalamus in cat: Its r e l a t i o n to the cerebellum, motor cortex and basal ganglia. Jap. J. Physiol. 16_, 42-60. Salmoiraghi, G.C.; C.N. Stefanis (1967) A c r i t i q u e of ionto-phoretic studies of central nervous system neurones. Int. Rev. Neurobiol. 1 0 , 1-30. Scheibel, M.E.; A.B. Scheibel (1966) The organization of the . n u c l e u s . r e t i c u l a r i s thalami: a Golgi study. Brain Res. 1, 43-62. Sheps, J.G. (1945) The nuclear configuration and c o r t i c a l connections of the human thalamus. J. Comp. Neurol. 83_, 1-56. Shute, C C D . ; P.R. Lewis (1963) Cholinesterase - containing systems of the brain of the rat. Nature> (Lond.) 199 1 1 6 0 - 1 1 6 4 . 159 Siegel, S. (1956) "Nonparametric S t a t i s t i c s for the Behavioral Sciences." McGraw - H i l l , New York. Smaha, L.A.; W.W. Kaelber, R.R. Maharry (1969) Efferent projections of the nucleus v e n t r a l i s l a t e r a l i s . An experimental study i n the cat. J. Anat. 104, 33-40. Snider, R.S.; W.T. Niemer (1961) "A Stereotaxic Atlas of the Cat Brain." Univ. of Chicago Press. Sourkes, T.L.; L. P o i r i e r (1965) Influence of the substantia nigra on the concentration of 5 - Hydroxytryptamine and dopamine of the striatum. Nature (Lond.) 207, 202-203. St r i c k , P.L. (1970) C o r t i c a l projections of the f e l i n e thalamic nucleus v e n t r a l i s l a t e r a l i s . Brain Res. 20, 130-134. Suh, T.H.; CH. Wang, R.K.S. Lim (1936) The e f f e c t of i n t r a -c i s t e r n a l application of acetyl•choline and the l o c a l i z a t i o n of the pressor centre and t r a c t . Chin. J. Physiol. 10, 61-78. Szabo, J. (1965) Synaptic arrangements i n the VA-VL thalamic nuclei of the cat. pp. 117-118. i n : "Abstracts of the 8th International Congress of Anatomy." Tebecis, A.K. (1970) Studies on cholinergic transmission i n the medical geniculate nucleus. Br .J'.. Pharmacol. 38, 138-147. Thomas, D.M.; R.P. Kaufman, J.M. Sprague,- W.W. Chambers (1956) Experimental studies of the vermal cerebellar pro-jections i n the brain stem of the cat. J. Anat. 90, 371-385. Tombol, T. (196 6) Short neurones and t h e i r synaptic r e l a t i o n s in the s p e c i f i c thalamic n u c l e i . Brain Res. 3_, 307-326. Toyaraa, K.; N. Tsukahara, K. Kosaka, K. Matsunami (1970) Synaptic ex c i t a t i o n of red nucleus neurones by fibres from interpositus nucleus. Exp. Brain Res. 1JL, 187-19 8. Uno, M.; M. Yoshida, I. Hirota (1970) The mode of cerebello-thalamic relay transmission investigated with i n t r a -c e l l u l a r recording from c e l l s of the ven t r o l a t e r a l nucleus of cat's thalamus. Exp. Brain Res. 10_, 121-139. 160 V o g t , C ; 0. V o g t (1941) T h a l a m u s s t u d i e n I - I I I , J . P h y s i o l , u. N e u r o l / 5 0 , 32-154. W a l k e r , A.E. • (1938), "The P r i m a t e T h a l a m u s " U n i v . o f C h i c a g o P r e s s . Y o s h i d a , M.; K. Y a j i m a , M. Uno (1966) D i f f e r e n t a c t i v a t i o n o f t h e two t y p e s o f t h e p y r a m i d a l t r a c t n e u r o n e s t h r o u g h t h e c e r e b e l l o - t h a l a m o c o r t i c a l p a t h w a y . E x p e r i e n t i a 2_2, 3 3 1 - 3 3 2 . Z e r n i c k i , B. (1968) P r e t r i g e m i n a l c a t . B r a i n R e s . 9_, 1-14. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0093283/manifest

Comment

Related Items