Nerve Membrane Excitation without Threshold

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<jats:p> Evidence is presented to show that for a squid axon membrane the potential response, <jats:italic>V</jats:italic> , is a smoothly continuous function of a stimulating current, <jats:italic>I</jats:italic> . 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. </jats:p> <jats:p> Spatially uniform <jats:italic>V</jats:italic> and <jats:italic>I</jats:italic> 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, <jats:italic>R</jats:italic> , is a continuous, single-valued function of the effect of the stimulus, <jats:italic>S</jats:italic> . The maximum value of Δ <jats:italic>R</jats:italic> /Δ <jats:italic>S</jats:italic> decreased steadily as the temperatures were increased from 25° to 38°C. Uncontrolled fluctuations prevented direct observations of Δ <jats:italic>R</jats:italic> /Δ <jats:italic>S</jats:italic> below 15°C where calculations showed that it rose rapidly as the temperature decreased. Since the conductances are experimental parameters and since Δ <jats:italic>R</jats:italic> /Δ <jats:italic>S</jats:italic> as calculated from them remained finite and continuous event at 6.3°C, this is experimental evidence against an all-or-none threshold excitation. </jats:p> <jats:p> However there is an all-or-none threshold for the initiation and propagation of an impulse along an axon where <jats:italic>V</jats:italic> and <jats:italic>I</jats:italic> are functions of both time and distance. </jats:p>

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