Cat hindlimb motoneurons during locomotion. IV. Participation in cutaneous reflexes.

The responses of 11 individual motoneurons, the muscle to which each projected, plus all other muscles in the anterior thigh of the cat, were recorded following single non-noxious electrical stimuli to cutaneous nerves while the intact animal walked on a treadmill. The various excitatory and/or inhibitory responses were qualitatively similar for stimuli within the range 1.1-10 times threshold for group I fibers in the stimulated nerve (usually saphenous). Monarticular knee extensor muscles in the vastus group and their motoneurons were usually inhibited in the period 10- to 25-ms poststimulus. The faster contracting vastus medialis and lateralis muscles tended to have an excitatory rebound at approximately 25- to 40-ms poststimulus that was confined to the stance phase of the step cycle when these muscles were normally active. Biarticular hip flexor muscles rectus femoris and both the anterior and medial parts of sartorius and their motoneurons all had similar bimodal excitatory responses, including an early period 3- to 18-ms poststimulus and a later period 20- to 35-ms poststimulus. The short-latency excitatory responses appeared to be proportional to the normal recruitment of the muscles in the step cycle, whereas the long-latency responses tended to be phase advanced with respect to normal recruitment. Motoneurons projecting to muscles with two excitatory peaks tended to have similar excitatory responses at both latencies and occasionally responded at both latencies to a single stimulus.