Nerve Membrane Excitation without Threshold

Abstract

Evidence is presented to show that for a squid axon membrane the potential response, V, is a smoothly continuous function of a stimulating current, I. This makes it unlikely that an all-or-none or sharp transition phenomenon is a major factor in the processes by which ions cross the normal squid axon membrane and, probably, other excitable membranes. Spatially uniform V and I were first produced in the squid axon with internal and external electrode arrangements and later by isolating a short length of axon between external pools of sucrose. Under these simplified conditions, direct experiments and calculations based on the Hodgkin-Huxley empirical conductances agree in showing that the maximum response, R, is a continuous, single-valued function of the effect of the stimulus, S. The maximum value of DeltaR/DeltaS decreased steadily as the temperatures were increased from 25 degrees to 38 degrees C. Uncontrolled fluctuations prevented direct observations of DeltaR/DeltaS below 15 degrees C where calculations showed that it rose rapidly as the temperature decreased. Since the conductances are experimental parameters and since DeltaR/DeltaS as calculated from them remained finite and continuous event at 6.3 degrees C, this is experimental evidence against an all-or-none threshold excitation. However there is an all-or-none threshold for the initiation and propagation of an impulse along an axon where V and I are functions of both time and distance.