Facilitation of individual gamma-motoneurones by the discharge of single slowly adapting type 1 mechanoreceptors in cats.

1. Cross-correlation of the discharges of individual neurones has been used to investigate the influence of identified cutaneous afferents on gamma-motoneurones below the level of complete spinal section in decerebrated cats. Discharges of single, sural nerve afferents from the heel were recorded in the dorsal root ganglia. Discharges of gamma-motoneurones were recorded from cut filaments of the muscle nerve to gastrocnemius medialis of the same leg. gamma-Motoneurones had a background discharge in the absence of intentional stimulation. 2. Correlograms involving slowly adapting afferents were formed during steady application of a probe to the receptive field for repeated periods of 10 s. Afferent synchronization was minimized by rejecting any period of probe movement. Correlograms involving rapidly adapting afferents required continuous movement of the probe to sustain afferent discharge. 3. Statistically significant primary peaks in correlations were observed for twenty-one pairings of gamma-motoneurones with seventeen out of thirty-nine slowly adapting, type 1 (SA1) mechanoreceptors. Primary peaks had widths at half-maximum in the range of 2-7 ms. No such short duration peaks were seen for fourteen pairings of gamma-motoneurones with eleven slowly adapting type 2 (SA2) receptors or for thirty-five pairings with twenty-nine hair follicle (HF) afferents. Broad correlations with peaks extending over tens of milliseconds were seen for HF afferents and could be generated in correlograms for slowly adapting afferents by moving the probe. 4. The short duration peaks were delayed with respect to the SA1 afferent discharges. Subtraction of peripheral conduction times gave central delays for the increased probability of gamma-motoneurone firing ranging from 2.7 to 6.5 ms (mean = 4.0 ms). These values were not significantly different from the central delays of gamma-motoneurone excitation in response to electrical stimulation of the sural nerve at a strength 1.2 times threshold. 5. The increase in probability of gamma-motoneurone discharge given a single SA1 afferent discharge ranged from 0.005 to 0.156 with a mean value of 0.052. The rise time of the peak ranged from 1 to 4 ms with a mean value of 1.9 ms (n = 9). 6. The properties of the correlogram peaks were not related to the axon conduction velocity of either the SA1 afferent or gamma efferent neurones. 7. The SA1 afferents that facilitated gamma-motoneurone discharge had axon conduction velocities in the range 29-81 m/s and could not be distinguished from SA1 afferents lacking correlations.(ABSTRACT TRUNCATED AT 400 WORDS)