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

Abstract

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 γ-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 γ-efferent activity at a constant force of contraction.