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Anatomical and histochemical studies of the globus pallidus and related basal ganglia nuclei Staines, William Alan


The anatomical organization of the connections of the major components of the basal ganglia was investigated in detail. A sensitive procedure for the simultaneous study of afferents and efferents was carried out on the striatum (CP), globus pallidus (GP), and substantia nigra (SN). Previously well characterized connections of the CP were confirmed, additional evidence for a projection to the CP from the ventromedial nucleus of the thalamus was obtained and a topographically organized projection to the CP from the GP was discovered. A similar study of the SN revealed a nigral projection to the ipsilateral lateral dorsal nucleus of the thalamus and nigral input from the contralateral posterior lateral hypothalamus. The projection of the GP to the SN was found to be linked topographically to the striatonigral and pallidostriatal pathways. A study of the connections of the GP confirmed a massive projection from the CP and provided further evidence of a reciprocal connection. In addition, pallidal innervations of the entopeduncular nucleus and reticular nucleus of the thalamus were indicated. Because of the potential importance of a pallidostriatal projection and the significant number of technical difficulties associated with its demonstration, additional experiments were carried out to confirm the presence of this pathway and to determine its anatomical relationship to other basal ganglia connections. Retrograde labelling of pallidostriatal neurons, studied with electron microscopy and in combination with lesions of the striatum, confirmed that pallidal neurons project either to or through the striatum. Evidence for possibly two groups of pallidal neurons that project to the CP was obtained, and it was observed that both of these cell groups were congruent with the striatopallidal terminal fields. Comparisons of the distribution of cells retrogradely labelled after tracer injections into the cortex and CP in combination with histochemistry for acetylcholinesterase demonstrated that the population of pallidal neurons projecting to the CP was distinct from that of peripallidal cholinergic neurons which may project through the striatum to the cortex. Double retrograde fluorescent tracing experiments indicated that pallidal neurons which project to the CP also have collateral projections to the substantia nigra and perhaps to the subthalamic nucleus. The application of a new technique for studying, efferent projections allowed the confirmation and morphological description of the projection of the globus pallidus to the striatum. The characteristic morphology of this projection was shared by pallidal efferents which project to the entopeduncular nucleus, the reticular nucleus of the thalamus, the subthalamic nucleus and the substantia nigra. The fine morphological detail afforded by this method of anterograde tracing was utilized in combination with a histochemical protocol to show that pallidostriatal terminals end in part on somatostatin-containing neurons in the CP.

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