Overshoot

One of the first of many contributions of the squid giant axon to neuroscience was to allow insertion of axial internal electrodes. In his "Chance and Design in Electrophysiology", Hodgkin relates that (during his visit with Cole and Curtis at the Marine Biological in Woods Hole in 1938) he and Curtis tried some "long-shot" experiments with trying to push electrodes into the cut end of a giant axon. "I think that we both came away with the idea that it might not be too difficult to record action potentials with and internal electrode; at all events, we both carried out the experiment with different partners in the following year."

Thus such axial electrodes were employed in independent experiments at

  • the Marine Lab in Plymouth, England, by Hodgkin and Huxley in 1939 and at
  • the Marine Biological Lab in Woods Hole, MA, USA, by Cole and Curtis in 1939 ( with an AC amplifier) and 1942 (with a DC amplifier).
    • Both groups found that the action potential overshot the resting potential by about 40 mV.

    This disclosed the serious flaw in the then popular Bernstein notion that nerve impulses represented simply a momentary reduction of the membrane resistance so that the potential would approach zero at the point of minimum resistance (or maximum conductance). These experiments showed that a new and more inclusive model for the action potential was required.

    Later in this same paper Hodgkin related that the fact that one of Curtis and Cole's action potential records was so very large (with an overshoot of 110mV, far exceeding the sodium equilibrium potential) contributed to the delay in the acceptance of the sodium hypothesis for a long time. There was never any confirmation of this observation and he and Huxley decided that it must have been an artifact of over-compensating the high frequency response of an electrical recording circuit. Later, in his memoirs, Cole conceded that this must have been the case.