Efferent and afferent activity in a gastrocnemius nerve branch during locomotion in the thalamic cat

SummaryThe firing patterns of α and γ efferent fibres and of group I and group II afferent fibres innervating the gastrocnemius muscle were observed during spontaneous locomotor movements in the thalamic cat. Multi-unit discharges of each kind of fibre were obtained by electronic sorting of the action potentials from the whole activity of a thin branch of gastrocnemius lateralis or medialis nerve. The main results were: (1) During the locomotor cycle the activity of the afferent and efferent populations was highly modulated. (2) α- and γ-motoneurones were co-activated within the locomotor cycle during ankle plantar-flexion. The γ discharge began to rise earlier and to fall later than did the α discharge. The amplitude of the γ discharge, unlike that of the α discharge, was largely independent of the vigour of walking. Between the cyclic discharges, most of γ populations were tonically active whereas α populations were silent. Subgroups of the α and γ populations were not usually activated according to the cell-size principle, but, the activation of the latest γ subgroup always preceded that of the earliest α subgroup. (3) Modulation of the group I and II afferent discharges was closely related to the cyclic length changes of the parent muscle. Fusimotor activation during the active shortening of gastrocnemius muscle prevented the afferent discharges from pausing. (4) The pattern of afferent and efferent activity during selective curarisation of the extrafusal junctions indicated that the discharge of static γ-motoneurones is modulated during the locomotor cycle. After curarisation of both extrafusal and intrafusal junctions, an efferent-discharge pattern of central origin persisted alternately in extensor- and flexor-muscle nerves (fictive locomotion). The durations of the fictive locomotor cycle and of the cyclic discharge in the sartorius nerve were increased as a consequence of the suppression of phasic afferent inputs to the C.N.S. (5) Maintained ankle dorsi-flexion slowed the fictive locomotor rhythm and elicited opposite effects, respectively excitation and depression, on the magnitude of the α and γ discharges. Maintained ankle plantar-flexion scarcely perturbed the duration of the fictive locomotor cycle, but the duration of the sartoriusnerve discharge lengthened at the expense of that of the gastrocnemius discharge. Both gastrocnemius α-and γ-motoneurones were depressed, the former considerably more than the latter. (6) The roles of the gastrocnemius afferents and γ-efferents during the locomotor cycle are discussed in the light of these results.