Actions on gamma‐motoneurones elicited by electrical stimulation of group II muscle afferent fibres in the hind limb of the cat.

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Abstract
The reflex effects elicited by electrical stimulation of group II muscle afferent fibers were recorded with micro-electrodes in 98 hind-limb .gamma.-motoneurons of cats anesthetized with chloralose. .gamma.-Cells (81) were classified as either static or dynamic by means of stimulation in the mesencephalic area for dynamic control known to influence dynamic .gamma.-motoneurons selectively. A high responsiveness to activity in group II muscle fibers was found for the whole sample of .gamma.-cells. Group II muscle action on dynamic .gamma.-motoneurons was more frequent than that on static ones. Excitation from group II fibers outweighed inhibition. This was clear cut for flexor .gamma.-motoneurons. In extensor .gamma.-cells, excitation prevailed by a small margin only. For both static and dynamic extensor .gamma.-cells, excitation prevailed from both posterior biceps-semitendinosus and the gastrocnemius-soleus nerves, whereas inhibition was more frequent from the deep peroneal and quadriceps nerves. All the reflex effects studied were likely to be mediated via oligosynaptic pathways. The shortest latencies of excitatory effects were compatible with a disynaptic coupling. The fastest inhibitions were presumably trisynaptic. The present findings, supported by a parallel study of reflexes evoked by group III muscle afferents, strongly suggest that the reflexes on .gamma.-motoneurons are not organized in accordance with the concept of flexion reflex afferents as conceived for 3-motoneurons. The interpretation of the results suggests a particularly independent position for dynamic .gamma.-cells in relation to .alpha.- and static .gamma.-motoneurons. The results also furnish an argument against the concept of .alpha.-linkage.