SUMMARY1. The reflex actions evoked by electrical stimulation of group III muscle afferent fibres were investigated with micro-electrode recordings from ninety-three ymotoneurones projecting to hind-limb muscles of cats anaesthetized with chloralose. For seventy-eight of the ninety-three y-cells the frequency of occurrence and types of effects mediated via group II and group III muscle fibres were compared.2. 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).3. The responsiveness of the whole sample of y-motoneurones to inputs from group III muscle fibres was high and comparable to that found with group II fibres.4. It was found that group III muscle fibres acted preferentially on static y-motoneurones. In contrast, group II fibres acted preferentially on dynamic y-motoneurones.5. Both excitatory and inhibitory effects were provoked by stimulation of group III fibres. Generally excitation was more frequent than inhibition.6. 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 y-motoneurones is in contrast to the striking predominance ofgroup III-evoked inhibition ofextensor a-motoneurones as described by the flexion reflex afferents concept. 7. A comparative survey is also given of the patterns of responses elicited in individual posterior biceps-semitendinosus and gastrocnemius-soleus y-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 y-motoneurones are organized differently from those to a-motoneurones.8. The functional implications of these findings are discussed. It is proposed that the pools of y-motoneurones should be considered as integrative systems intercalated

SUMMARY1. The reflex effects elicited by electrical stimulation of group II muscle afferent fibres were recorded with micro-electrodes in ninety-eight hind-limb y-motoneurones of cats anaesthetized with chloralose.2. Eighty-one of the y-cells were classified as either static or dynamic by means of stimulation in the mesencephalic area for dynamic control known to influence dynamic y-motoneurones selectively.3. A high responsiveness to activity in group II muscle fibres was found for the whole sample of y-cells.4. Group II muscle action on dynamic y-motoneurones was found to be more frequent than that on static ones.5. Excitation from group II fibres outweighed inhibition. This was clear cut for flexor y-motoneurones. In extensor y-cells, excitation prevailed by a small margin only. However, for both static and dynamic extensor y-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.6. 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.7. The present findings, supported by a parallel study of reflexes evoked by group III muscle afferents, strongly suggest that the reflexes on y-motoneurones are not organized in accordance with the concept of flexion reflex afferents as conceived for a-motoneurones.8. The interpretation of the results suggests a particularly independent position for dynamic y-cells in relation to a-and static y-motoneurones. Hence, the results also furnish an argument against the concept of a-y linkage.