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

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Abstract
The reflex actions evoked by electrical stimulation of group III muscle afferent fibers were investigated with microelectrode recordings from 93 .gamma.-motoneurons projecting to hind limb muscles of cats anesthetized with chloralose. For 78 of the 93 .gamma.-cells the frequency of occurrence and types of effects mediated via group II and group III muscle fibers were compared. Cells (77) tested at intensities which excited group III and 75 of the cells tested at intensities which excited both group II and group III afferent fibers were classified as either static or dynamic, using the method of mesencephalic stimulation. The responsiveness of the whole sample of .gamma.-motoneurons to inputs from group III muscle fibers was high and comparable to that found with group II fibers. Group III msucle fibers acted preferentially on static .gamma.-motoneurons. Group II fibers acted preferentially on dynamic .gamma.-motoneurons. Both excitatory and inhibitory effects were provoked by stimulation of group III fibers. Excitation was more frequent than inhibition. A strong dominance of excitation over inhibition was found in flexor muscles, and a weaker prevalence of excitation was also encountered in extensor muscles. This prevalence of excitation in extensor .gamma.-motoneurons is in contrast to the striking predominance of group III-evoked inhibition of extensor .alpha.-motoneurons as described by the flexion reflex afferents concept. A comparative survey is also given of the patterns of responses elicited in individual posterior biceps-semitendinosus and gastrocnemius-soleus .gamma.-cells by stimulation of group II and group III fibers. Reflexes from high-threshold muscle afferent fibers to .gamma.-motoneurons are organized differently from those to .alpha.-motoneurons. The pools of .gamma.-motoneurons should be considered as integrative systems intercalated between descending and reflex pathways on the 1 hand and the skeletomotor neurons on the other. The descending messages and the multisensory peripheral information, integrated in the fusimotor neurons, undergo final adjustment in the muscle spindle. The link to the skeletomotor neurons is formed by the primary spindle afferents. These constitute a final common input conveying integrated detailed polymodal feed-back to the central nervous system. This new concept is referred to as the final common input hypothesis.