Library

Notes: Summary The activity in α and γ efferent axon populations and in group I and group II afferent fibre populations innervating a flexor muscle, the sartorius medialis, was 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 overall activity of a thin, intact branch of the sartorius medialis nerve. The following results were obtained: (1) The γ-motoneurones have a phasic behaviour characterized by a single discharge period during the hip flexion (swing phase of the step-cycle). (2) The γ-motoneurones are co-activated with the homonymous α-motoneurones. (3) Between rhythmic α and γ discharges, i.e. during the hip extension (stance phase of the step cycle), both α- and γ-motoneurones were normally silent. However, in 5 out of 17 experiments, a few units of the γ population fired at very low frequency. (4) Two observations indicate that the γ-motoneurones that are co-activated with the α-motoneurones by central locomotor commands are predominantly of the static type. In actual locomotion, the rhythmic fusimotor discharges over-compensate the depressor effect on the firing rate of the group II afferents of the unloading of muscle spindles by the active shortening of the parent muscle. In fictive locomotion, when the transmission of the excitation is blocked by selective curarization in alpha skeleto-motor junctions alone, the rhythmic fusimotor discharges elicit in-phase modulations not only of the group I but also of the group II fibres. The group II afferent population consists almost entirely of fibres arising from spindle secondary endings which are located primarily on intrafusal muscle fibres whose contraction is exclusively controlled by static fusimotor motoneurones. In the two experimental circumstances, the analysis of the group I fibre discharge does not allow to decide whether dynamic γ motoneurones are firing or silent during rhythmic γ discharge. (5) The group I and group II afferent discharges during the step-cycle showed two frequency peaks, one static-fusimotor dependent while the contracting muscle shortened during the hip flexion (swing) phase, the other length-change dependent while the relaxed muscle was rapidly stretched during the first part of the hip extension (stance) phase. Then, during the second part of hip extension when the muscle was slowly stretched in the absence of fusimotor drive, the firing rate of the spindle afférents decreased to a low level. The spindle sensory endings during the extension phase showed low dynamic and static responsiveness like deefferented spindles. (6) The results obtained in sartorius medialis (flexor) muscle are discussed in comparison with the results previously obtained in gastrocnemii (extensor) muscles (Bessou et al. 1986). The consequences of the predominant activation of the static or dynamic fusimotor system in functionally different muscles are considered with respect to the proprioceptive or motor role of musclespindles during muscle contraction.

Notes: Summary The 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.