= Solution
The intracellular recording by <Alan Hodgkin> and <Andrew Huxley> shows the membrane voltage rising from a negative resting value to a positive overshoot, with the outside held at zero reference. The <action potential> therefore does more than transiently abolish a resting polarization: it reverses the sign across the membrane. A simple hypothesis that excitation only makes an otherwise <potassium>-dominated membrane equally leaky cannot explain that positive overshoot.
The result helped motivate a transient, selective change in membrane permeability, later resolved into rapid inward <sodium> current followed by outward <potassium> current. The positive <sodium> <equilibrium> potential calculated above makes the overshoot physically plausible, while <potassium> conductance favours a negative resting voltage and <repolarization>. The 1939 voltage trace itself did not identify individual channel <proteins> or establish the full later voltage-dependent gating mechanism; those conclusions required ionic substitution and <voltage clamp>[voltage-clamp] evidence.<Action Potentials Recorded from Inside a Nerve Fibre by Alan Hodgkin and Andrew Huxley>[1939 intracellular recording and reversal of membrane voltage during excitation]{ref}
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