PRESYNAPTIC COMPONENT IN THE AFFERENT INHIBITION OBSERVED WITHIN TRIGEMINAL BRAIN-STEM NUCLEI OF THE CAT

Abstract

Depolarization of the central terminals of large myelinated trigeminal cutaneous afferent fibers was shown to be produced by excitation of adjacent cutaneous fibers. Characteristics of the "trigeminal tract" reflex, analogous to the dorsal root reflex, together with excitability changes in the central terminals, estimated using Wall''s technique, demonstrated the following characteristics: Primary afferent depolarization was evoked by tactile or electrical stimulation of a region of skin including and surrounding the "orthodromic" receptive field of the particular primary fiber. Following the conditioning stimulus depolarization commenced in less than 10 msec, was greatest at 20-40 msec and persisted for 200-300 msec. Primary afferent depolarization was maximal in the terminals within the rostral trigeminal nuclei and fell off rapidly caudal to the obex. Afferent inhibition of 2nd order trigeminal neurons, was effected by tactile or electrical stimulation of a similar "surround" area of skin. Its time course was similar to that of the primary afferent depolarization, and it was most effective in neurons within the rostral trigeminal subnuclei. It was concluded that presynaptic inhibition operates very effectively at the first trigeminal relay contributing largely to the afferent inhibition which occurs at this level.