The Physiological Nature of the Slow Afterwave in Muscle Action Potentials

Lateva ZC, McGill KC. The physiological nature of the slow afterwave in muscle action potentials. 14th Ann Congr Electroenceph Clin Neurophysiol, 1997.

Both intramuscularly recorded motor-unit action potentials (MUAPs) and surface recorded compound muscle action potentials (CMAPs) have slow afterwaves which can contribute as much as half their measured duration. The origin of the slow afterwave is uncertain. It cannot be explained in terms of temporal or spatial dispersion of single-fiber potentials because it lasts considerably longer than the endplate-to-tendon propagation time. Nor is it merely a non-physiological artifact due to the amplifier's high-pass filter, because it does not match the filter's time constant. We present evidence to show that the slow afterwave is actually a manifestation of the slow second stage of muscle-fiber repolarization. Intracellular recordings in several species, including humans, have shown that the final 10-20% of repolarization occurs over an interval of tens of milliseconds in what is known as the negative afterpotential of the intracellular action potential. We investigated the slow afterwave in MUAPs and CMAPs from first dorsal interosseous, abductor pollicis brevis, brachial biceps, and tibialis anterior in five normal subjects, and using computer simulations. Our results show that the differences in the morphology of the slow afterwave at different intramuscular and surface recording sites are explained by volume conductor theory if the intracellular action potential has a negative afterpotential, and that the slow afterwave changes with double-pulse stimulation and decreasing temperature in ways that can be predicted from the results of intracellular studies of the negative afterpotential.