Responses in human pretibial muscles to sudden stretch and to nerve stimulation.

Electromyograms have been recorded from the human tibialis anterior muscle during voluntary contraction. The accessibility of the nerve to this muscle (common peroneal) has permitted a comparison of reflex responses to low threshold electrical stimulation of the nerve with those to stretch of the muscle itself. Nerve stimulation elicited a reflex at monosynaptic latency (V1 at 28-29 ms) and a second response (V2 at 50-52 ms). A tendon tap induced two responses (M1 at 29 ms and M2 at 50 ms). The responses to a 'ramp' stretch were similar. The homology of V1 with M1, and of V2 with M2 is discussed. V2 and M2 probably correspond to the 'transcortical' reflexes described from other muscles. Neither V1, V2, M1 nor M2 were influenced by anaesthesia of the foot. M1 and M2 were both reduced in amplitude by a selective gamma-efferent block produced by local anaesthetic in the common peroneal nerve. It is concluded, that muscle spindles are the receptors predominantly responsible for M2 ('transcortical') responses. The amplitudes of M1 and M2, but not V1 and V2 were augmented by prior instruction to 'resist' the stimulus. This is interpreted as evidence for voluntary modulation of gamma-efferent activity at a constant force of contraction.