Evidence of the co-activation of alpha-motoneurones and static gamma-motoneurones of the sartorius medialis muscle during locomotion in the thalamic cat.

The activity in alpha and gamma 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 gamma-motoneurones have a phasic behaviour characterized by a single discharge period during the hip flexion (swing phase of the step-cycle). (2) The gamma-motoneurones are co-activated with the homonymous alpha-motoneurones. (3) Between rhythmic alpha and gamma discharges, i.e. during the hip extension (stance phase of the step cycle), both alpha- and gamma-motoneurones were normally silent. However, in 5 out of 17 experiments, a few units of the gamma population fired at very low frequency. (4) Two observations indicate that the gamma-motoneurones that are co-activated with the alpha-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.(ABSTRACT TRUNCATED AT 250 WORDS)