Neuromuscular response in man to repetitive nerve stimulation

Paired stimulation has been used extensively in clinical neurophysiology. We studied change in the sizes of compound muscle action potentials (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 test 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 were graded at fixed intervals for motor fibers the maximal effect was obtained with liminal stimulation, but was no longer observed at supramaximal 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 the second stimuli, the CMAP was facilitated (26%) up to 65 ms, thereafter being depressed (13%). These results must be taken into account when making clinical examinations that use paired stimulation.