Contraction-Induced Excitation In Cat Peroneal Motoneurons

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 perorteal 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 To pathways, only exceptionally revealed a weak contraction-induced inhibition.5. The functional significance of a positive feedback of peroneus brevis contraction on peroneal motoneurons is probably related to the complex biomechanical operations of peroneal muscles in preservation of foot contact with support in a variety of positions and movements.