UBC Theses and Dissertations
Information transmission in trigeminal afferents from teeth Farnsworth, Thomas J.
The purpose of this study was to measure the rate at which information is transmitted by periodontal mechanosensitive neurons. In addition, an attempt was made to develop a quantitative means for expressing their responsiveness, or sensitivity. The experiments were carried out on cats anaesthetized with sodium pentobarbital. The canine teeth were stimulated with computer-controlled forces, and the response of single first-order axons supplying the periodontal mechano-receptors were recorded following microdissection of the inferior dental nerve. Thirty mechanosensitive units, selected from twelve animals, were used in the study. For fifteen of these units a stimulus uncertainty of 5 bits, a stimulus duration of 500 msec, and an inter-stimulus interval of 500 msec was employed. Under these conditions, the mean channel capacity was found to be 3.2 bits/ stimulus ± S.E. = 0.09. The channel capacity for the other fifteen mechanosensitive units was measured using a 7 bit/stimulus uncertainty. This increased the channel capacity to a mean of 3.77 bits/stimulus, S.E. =± .14. It was found that this calculated rate of information transmission could be further significantly increased (P.> 0.0001) to 4.33 bits/stimulus, S.E. = ± 0.11, by treating the data so as to prevent aver-sampling of parts of the response.- .When the outputs of seven neurons innervating a single tooth were pooled, the maximum rate of information transmission for the group as a whole was found to be 6.84 bits/ stimulus. Comparison of the pooled data with the results of psychophysical experiments suggests that the rates of encoding of information in primary afferents far exceeded the ability of the CNS to receive such data. A computational method for the measurement of the sensitivity of the neurons was also developed. It is proposed that the sensitivity, or responsiveness, of neurons can be measured properly only by the use of information theory, and that it depends upon the particular method of coding chosen by the experimenter.
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