1. Motoneurons innervating peroneal muscles were recorded intracellularly in anesthetized cats during sustained submaximal isometric contractions of peroneus brevis produced by repetitive electrical stimulation of motor axons in the distal portion of cut ventral root filaments. 2. In contrast with the inhibition previously observed during contractions of gastrocnemius medialis muscle in triceps surae motoneurons, the afferent input generated by peroneus brevis contraction elicited excitatory potentials in nearly all motoneurons supplying peroneus brevis, peroneus tertius, or peroneus longus muscles. 3. We ascribed the contraction-induced excitation of peroneal motoneurons to spindle afferents for two reasons. First, the amplitude of contraction-induced excitatory potentials increased when the ventral root stimulation strength was increased to recruit gamma-axons. Second, with stimulation strengths under gamma-threshold, peroneus brevis contraction still excited peroneal motoneurons, and we obtained evidence that activation of spindles by skeletofusimotor beta-axons could account at least partly for this excitation. 4. The lack of contraction-induced inhibition in peroneal motoneurons and the prevalence of contraction-induced excitation raised the possibility that, in contrast to the usual effects of tendon organ afferents, Ib afferents from peroneus brevis might exert an excitatory influence on homonymous motoneurons. The fact that electrical stimulation of group I afferents in the nerve to peroneus brevis only exceptionally evoked inhibition in peroneal motoneurons would appear compatible with this hypothesis. Furthermore, stimulation of cutaneous afferents, known to facilitate transmission in Ib pathways, only exceptionally revealed a weak contraction-induced inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)
1. Clarke's column neurons of the dorsal spinocerebellar tract (DSCT) were recorded intracellularly in anaesthetized cats during weak sustained contractions of triceps surae (TS) produced by direct electrical stimulation of the muscle. 2. Of 145 DSCT neurons, 77 (53%) were contraction sensitive suggesting that information about weak contraction of a limited number of muscles is widely distributed among DSCT neurons. Four types of effects were observed in individual neurons during TS contractions. 3. In the first group of 11 DSCT neurons (14% of the contraction-sensitive cells), the effect was excitation persisting throughout the duration of contractions. These responses were ascribed to actions of afferents from contraction-activated tendon organs. 4. In a second group of 15 neurons (20% of the contraction-sensitive cells), quickly declining excitatory potentials were recorded during sustained TS contractions. By analogy with previous observations of contraction-induced effects in motoneurons, the decline of excitation might be explained by contraction-induced presynaptic inhibition of group I afferents in Clarke's column. 5. Declining inhibitions, resembling those previously observed in homonymous and synergic motoneurons, were recorded in 49 % of contraction-sensitive DSCT neurons. This appears in keeping with the fact that interneurons mediating I b inhibition to motoneurons project axon collaterals to DSCT neurons. Presynaptic inhibition of I b fibres might therefore cause parallel reductions of inhibitory potentials in motoneurons and in DSCT neurons. 6. In a final group of 13 neurons, mixed excitatory and inhibitory effects were observed during TS contractions. Such DSCT neurons might monitor the excitability of Ib interneurons by integration of information about input to and output from these neurons. 7. The non-uniform patterns of DSCT responses to TS contractions suggest complex processing of information on ankle extensor activity in cerebellum. Phasic signalling of contraction onset is observed in many DSCT neurons while others carry messages about duration and strength of contraction.
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