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

The reflex actions evoked by electrical stimulation of group III muscle afferent fibres were investigated with micro-electrode recordings from ninety-three gamma-motoneurones projecting to hind-limb muscles of cats anaesthetized with chloralose. For seventy-eight of the ninety-three gamma-cells the frequency of occurrence and types of effects mediated via group II and group III muscle fibres were compared. Seventy-seven of the cells tested at intensities which excited group III and seventy-five of the cells tested at intensities which excited both group II and group III afferent fibres were classified as either static or dynamic, using the method of mesencephalic stimulation (Appelberg, 1981). The responsiveness of the whole sample of gamma-motoneurones to inputs from group III muscle fibres was high and comparable to that found with group II fibres. It was found that group III muscle fibres acted preferentially on static gamma-motoneurones. In contrast, group II fibres acted preferentially on dynamic gamma-motoneurones. Both excitatory and inhibitory effects were provoked by stimulation of group III fibres. Generally 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-motoneurones is in contrast to the striking predominance of group III-evoked inhibition of extensor alpha-motoneurones 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 fibres. These data further corroborate the view that reflexes from high-threshold muscle afferent fibres to gamma-motoneurones are organized differently from those to alpha-motoneurones. The functional implications of these findings are discussed. It is proposed that the pools of gamma-motoneurones should be considered as integrative systems intercalated between descending and reflex pathways on the one hand and the skeletomotor neurones on the other. The descending messages and the multisensory peripheral information, integrated in the fusimotor neurones, undergo final adjustment in the muscle spindle. The link to the skeletomotor neurones 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.