Closely coupled excitation of γ‐motoneurones by group III muscle afferents with low mechanical threshold in the cat

Publisher Website
Google Scholar
Abstract
The reflex responses of .gamma.-motoneurons to discharges of muscle receptors innervated by group III axons were examined in hind-limb muscles of decerebrated and spinal cats. Electrical stimulation of the gastrocnemius medialis nerve at a strength sufficient to excite group III axons caused excitation of gastrocnemius lateralis or soleus .gamma.-motoneurons. The excitation was more prominent in the spinal animal. Excitation of either silent or tonically firing .gamma.-motoneurons was a secure, driven type of response consisting of 1 or 2 spikes occurring at fairly fixed latency in response to a single stimulus. Of 39 .gamma.-motoneurons, studied, 18 showed such excitation. Isometric twitch contractions of a muscle caused pronounced facilitation or excitation of homonymous and heteronymous .gamma.-motoneurons. Excitation occurred close to the peak or during relaxation of the twitch with a range in latency of 30-110 ms. The muscles studied were triceps surae, flexor digitorum and hallucis longus. Pressure or light taps applied to the gastrocnemius medialis muscle caused in increase in discharge frequency in 20 of 28 .gamma.-motoneurons of the same muscle (4 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. Of 38 gastrocnemius medialis receptors having group III axons (conduction velocities 5-27 m/s) is discharged 1 or occasionally 2 spikes to twitch contractions of the parent muscle at latencies of 30-80 ms. Of 10 receptors with group III axons that responded to contraction 8 had low thresholds to pressure and taps, whereas the majority of the 23 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 proxi al third or distal third of gastrocnemius medialis or, in a few instances, the Achilles tendon. Discharges in group III axons from receptors which respond to non-noxious, low threshold mechanical stimuli cause a tightly coupled excitation of .gamma.-motoneurons. The likely contribution of this reflex to the control of movement is discussed.