NEUROMUSCULAR RESPONSE IN MAN TO REPETITIVE NERVE-STIMULATION

Paired stimulation has been used extensively in clinical neurophysiolo gy. We studied change in the sizes of compound muscle action potential s (CMAPs) and compound nerve action potentials (CNAPs) in humans after a single electrical stimulus to the peripheral nerve. For the paired stimuli, potentials elicited by the first stimuli were used as the tes t responses. When the interstimulus intervals were varied, the second potentials underwent refractoriness and then were facilitated up to 20 -30 ms, thereafter being depressed for 160-200 ms. When intensities we re graded at fixed intervals for motor fibers the maximal effect was o btained with liminal stimulation, but was no longer observed at supram aximal stimulation. When the intensity used to obtain M-responses was half the maximum, maximal facilitations were 35% (CNAP) and 17% (CMAP) of the first potential, the respective maximal depressions being 13% and 42%. When the sizes of the two CNAPs were equalized by adjusting t he second stimuli, the CMAP was facilitated (26%) up to 65 ms, thereaf ter being depressed (13%). These results must be taken into account wh en making clinical examinations that use aired stimulation.