= Solution
<Alan Hodgkin> and <Andrew Huxley> recorded the <membrane potential> directly between the inside of a squid axon and the external solution. The crucial result was an <action potential> that passed through zero: the interior became positive, rather than merely losing its negative resting <membrane potential>. A theory in which excitation simply abolishes membrane selectivity and shorts out the resting <membrane potential> cannot account for that reversal. Their original report establishes the historical interpretation independently of the unavailable examination picture.<Action Potentials Recorded from Inside a Nerve Fibre by Alan Hodgkin and Andrew Huxley>[Nature 144 (1939), pp. 710–711: intracellular recording and reversal of the potential difference]{ref}
The later <Hodgkin-Huxley model> explains the rising phase through increased <sodium> permeability and the falling phase through <sodium> <ion-channel inactivation> and increased <potassium> permeability. Distinct <Nernst equilibrium potentials> make a selective conductance change capable of reversing the <membrane potential>. The recording established the phenomenon; it did not by itself reveal individual <ion channel> structures or all their gating kinetics.
\b[Source illustration TODO:] the source page's recording is missing in both the held PDF and its archive parent. No amplitudes, time intervals, or trace layout are reconstructed here; the surviving historical request is answered above.
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