The Sodium Hypothesis For The Nerve Action Potential

Is it possible to specify ionic mechanisms that account for the action potential just as it was possible to do so for the resting potential? It is interesting to note that Julius Bernstein in 1902, when proposing his theory for the resting potential, also proposed a theory for the nerve action potential. Bernstein proposed that during a nerve action potential, the membrane suddenly became permeable to all ions. Bernstein predicted, based on this theory, that the membrane potential would shift from its resting level to a new value of about 0 mV. (Can you explain why?) However, from Fig. 3 it is clear that the potential changes during the action potential do not range from a value of —60 to 0 mV, but actually go well beyond 0 mV and approach a value of +55 mV. So, whereas Bernstein's hypothesis for the resting potential was nearly correct, his hypothesis for the action potential clearly missed the mark.

At the same time that Bernstein proposed theories for resting potentials and action potentials, Overton, another early physiologist, made some interesting observations about the critical role of Na+. Overton observed that Na+ in the extracellular medium was absolutely essential for cellular excitability. In general, in the absence of extracellular Na+, nerve axons cannot propagate information, and skeletal muscle cells are unable to contract.

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