Inhibition of gamma motoneurone discharge by contraction of the homonymous muscle in the decerebrated cat.

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Abstract
The autogenetic reflex effects of tendon organ stimulation upon .gamma. motor neurons were studied in the decerebrated cat. Tendon organs serving a muscle were excited by isometric twitch contractions of some motor units of that muscle. Contractions were elicited by electrical stimulation of the cut peripheal end of part of the ventral root supply, using a stimulus strength at or below maximum for .alpha. motor neuron axons. Afferent recordings in the same cats as were used to study reflexes of .gamma. motor neuron showed that tendon organs were powerfully excited during twitch contractions of their muscle. Early discharges were also elicited in both tendon organs and spindle primary afferents. Background discharges of 47 triceps surae .gamma. motor neurons, ranging in axonal conduction velocity from 17-40 m/s, were recorded from filaments of the muscle nerve. The initial response of 22 (47%) neurons to twitches of the triceps surae was inhibition of their discharge. The latency from time of stimulation of the ventral root ranged from 10-22 ms. The lowered probability of firing lasted an average of 25 ms and, occasionally, was seen as a short silent period. The inhibition was reduced in duration by decreasing the resting muscle length so that less active tension was developed during contraction. The inhibition was observed, in 1 instance, in the spinally transected decerebrated cat. Early discharges in tendon organ afferents may give rise to the onset of inhibition; tension dependent discharges of tendon organs may contribute further inhibition of .gamma. motor neurons. Sixteen .gamma. motor neurons were not affected by contraction but 17 showed a period of facilitation. The facilitation could occur after a period of inhibition (8 neurons) or as the sole response to contraction. In either case facilitation occurred mainly during relaxation of the muscle and may, in part, be attributable to increased spindle primary ending firing at this time. A comparison is made of the reflex effects of muscle afferent discharges on .alpha. and .gamma. motor neurons.