Segmental influence of slowly-adapting cutaneous mechanoreceptors on gamma motoneurones revealed by cross-correlation of unit discharges in the cat.

Cross-correlations between the discharges of individual cutaneous afferents and gamma motoneurones have been constructed in the spinal, decerebrated cat. The discharges of single receptors in the sural nerve field from the heel were recorded in dorsal root ganglia. Background discharges of gamma motoneurones and the responses to heel stimulation were recorded from cut filaments of the muscle nerve to gastrocnemius medialis of the same leg. Slowly-adapting afferents were stimulated by steady application of a probe to the receptive field whereas rapidly-adapting afferents required continuous movement to sustain discharge of a receptor. Cross-correlation between the discharges of 17 out of 39 slowly-adapting, type-1 (SA1) mechanoreceptors and gamma motoneurones revealed sharp increases in probability of gamma motoneurone discharge that were delayed with respect to the afferent discharge. The peaks were of short duration with widths at half maximum in the range 2-7 ms and rise times of 1 to 4 ms. Deducting peripheral conduction times gave central delays of 3-6.5 ms for gamma motoneurone facilitation. These delays were comparable to those of gamma motoneurone excitation seen in response to electrical stimulation of the sural nerve at 1.5 to 4 times threshold. No short duration peaks were seen in correlograms between hair follicle (n = 29) or slowly-adapting type-2 (SA2) (n = 11) afferents and gamma motoneurones. It is concluded that a single impulse from a SA1 afferent from the hairy skin of the heel is able to facilitate the discharge of gamma motoneurones to the ipsilateral gastrocnemius muscle.(ABSTRACT TRUNCATED AT 250 WORDS)