Kinetics of Change in Spike Height During Anodal Polarization of Isolated Single Nerve Fibers

¹ From the Department of Physiology, Washington University School of Medicine, Saint Louis, Missouri

The time dependent change in magnitude of the nodal action potential was analyzed as a function of time and of change in membrane potential during brief intervals of anodal polarization. The time course of this function involves an exponential term whose time constant decreases to a limiting value as extent of membrane hyperpolarization is progressively increased. While the time course of change in spike height cannot be correlated with any change in membrane potential induced by the polarization, the kinetics suggest that the sodium inactivation mechanism of Hodgkin and Huxley is sufficient to account for the results obtained. Brief direct current pulses elicit changes of membrane potential during activity which indicate that an augmented change in resistance of the hyperpolarized node is sufficient to account for the time dependent increase in spike height. These findings are consistent with the Hodgkin-Huxley scheme in that hyperpolarization of the squid axon membrane at constant membrane potential inhibits ‘sodium inactivation’ thereby allowing sodium conductance and sodium current to attain greater than normal values at peak time of activity. It is concluded that no change in emf of the membrane need be postulated in order to account for the time dependent increase in spike height during hyperpolarization of normal or depressed nodes.