Closely coupled excitation of gamma-motoneurones by group III Muscle afferents with low mechanical threshold in the cat.

1. The reflex responses of gamma-motoneurones to discharges of muscle receptors innervated by Group III axons have been examined in hind-limb muscles of decerebrated and spinal cats.2. Electrical stimulation of the gastrocnemius medialis nerve at a strength sufficient to excite Group III axons caused excitation of gastrocnemius lateralis or soleus gamma-motoneurones. The excitation was more prominent in the spinal animal.3. Excitation of either silent or tonically firing gamma-motoneurones was a secure, driven type of response consisting of one or two spikes occurring at fairly fixed latency in response to a single stimulus. Eighteen out of thirty-nine gamma-motoneurones studied showed such excitation.4. Subtracting peripheral conduction times of the gamma-motoneurone impulse and the earliest component of the Group III volley from the latency of reflex excitation gave a range of central delays of 1.8-4.8 msec (mean 3.0 msec).5. Inhibitory Group III effects were also seen but were less pronounced than the excitation.6. The origin of the receptors connected to the Group III axons whose discharge causes driven excitation of gamma-motoneurones was investigated.7. Isometric twitch contractions of a muscle caused pronounced facilitation or excitation of homonymous and heteronymous gamma-motoneurones. Excitation occurred close to the peak or during relaxation of the twitch with a range in latency of 30-110 msec. The muscles studied were triceps surae, flexor digitorum and hallucis longus.8. Pressure or light taps applied to the gastrocnemius medialis muscle caused an increase in discharge frequency in twenty out of twenty-eight gamma-motoneurones of the same muscle (four were inhibited). The response to a steady stimulus adapted but could outlast it by many seconds. Pressure was more effective when applied to the proximal or distal parts of the muscle.9. Fifteen out of thirty-eight gastrocnemius medialis receptors having Group III axons (conduction velocities 5-27 m/sec) were found to discharge one, or occasionally two, spikes to twitch contractions of the parent muscle at latencies of 30-80 msec.10. Eight out of ten receptors with Group III axons that responded to contraction had low thresholds to pressure and taps, whereas the majority of the twenty-three Group III units which were insensitive to contraction had high thresholds to such stimuli. Receptive fields of all Group III units were confined to either the proximal third or distal third of gastrocnemius medialis or, in a few instances, the Achilles tendon.11. We conclude that discharges in Group III axons from receptors which respond to non-noxious, low threshold mechanical stimuli cause a tightly coupled excitation of gamma-motoneurones. The likely contribution of this reflex to the control of movement is discussed.