Lability of the Action Potential and the Recovery Cycle from Isolated Motor Axons of the Crab, Carcinus

Abstract

THE action potential evoked from an artificially produced excitable node of isolated crab axon membrane¹ shows similar changes to those described for the isolated toad node², when currents are applied during the action potential itself. The rising phase of the action potential can be modified only by very considerable anodal currents (some 40 times rheobase), but the repolarization phase shows changes when weak currents of either polarity are applied (Fig. 1). Repolarization is delayed by cathodal current and is hastened by anodal current, when these are applied during the falling phase of the action potential. The divergence of the voltage profile of repolarization, produced by a constant current pulse of either polarity, is greater the later it is applied, as would be expected from the form of the change in membrane resistance that accompanies the voltage change of the action potential. The changes in the rate of repolarization are proportional to the strength of the applied current and its duration, so that complete repolarization can be achieved with strong anodal current, that is, the later phase of the action potential can be abolished.