Discharge patterns of gamma motoneurone populations of extensor and flexor hindlimb muscles during walking in the thalamic cat

Two monopolar recordings of the whole activity in a fine nerve branch innervating the gastrocnemius lateralis (GL) or the sartorius medialis (SM) muscle were obtained during spontaneous walking in thalamic cats. Using a special electronic device, the potentials of several groups of efferent (alpha and gamma) and afferent (I and II) fibres constituting the whole nerve activity were separated. In the present paper we compare the data obtained for an ankle extensor (GL) and a hip-knee flexor (SM) during the step cycle. In both muscles the gamma motoneurone population is activated in parallel with the alpha motoneurone population. Usually, between the cyclic locomotor discharges, the GL gamma neurones are tonically active whereas the SM gamma neurones are silent. During muscle contraction, the group I and II afferent discharges are both length and gamma dependent, but the prevailing factor is the muscle shortening for the GL afferents and the cyclic gamma drive for the SM afferents. Both dynamic and static fusimotor efferents appear to be activated during muscle contraction, but on indirect evidence it is suggested that dynamic action prevails in GL spindles whereas static action dominates in SM spindles.