Refractory period of human muscle after the passage of a propagated action potential

Abstract

The recovery of excitability after the passage of a propagated action potential was studied in fibres in human muscle stimulated directly by paired stimuli variable in strength and time interval. The evoked action potentials were all-or-none and originated from two or three muscle fibres. With increasing strength of the testing stimulus the refractory period decreased until the stimulus exceeded threshold by about 50 per cent.The relative and absolute refractory periods of different fibre bundles fell into two discrete groups. With a repetition rate of paired stimuli of 1 per sec. the absolute refractory periods of one group varied between 2.2 and 3.1 msec. and of the other group between 3.4 and 4.6 msec. (34–36.5°C). The absolute irresponsive periods showed no sign of this grouping; they exceeded the absolute refractory periods slightly.The absolute refractory and irresponsive periods increased with increasing repetition rate of paired stimuli. This indicates that the recovery time after paired responses considerably exceeds the recovery time after a single action potential. Both these recovery periods were prolonged by anoxia.The maximum frequency of trains of stimuli to which the fibre bundle could respond corresponded to the absolute refractory period determined by paired stimuli; but the number of identical responses to successive stimuli decreased with increasing stimulation frequency.Near the absolute refractory period the conduction velocity of the action potential evoked by the stimulus was identical with the velocity of the conditioning action potential. At intervals between the stimuli of 6–10 msec. the conduction velocity of the second action potential exceeded that of the conditioning action potential by 10–20 per cent.The threshold was lower to the testing than to the conditioning stimulus at stimulus intervals of 5 to 30 msec. As additional evidence of facilitation subthreshold stimuli could evoke responses after a number of applications.