12.2. Simplified mechanism of the action potential[[Student version, January 17, 2003]] 457
Figure 12.6: (Photomicrograph; oscilloscope trace.) Hodgkin and Huxley’s historic 1939 result. (a)Arecording
electrode (a glass capillary tube) inside a giant axon, which shows as a clear space between divisions marked 47 and
63 on the scale. (The axon in turn is contained in a larger glass tube.) One division of the horizontal scale equals
33 μm.(b)Action potential and resting potential recorded between the inside and outside of the axon. Below the
trace appears a time marker, showing reference pulses every 2ms.The vertical scale indicates the potential of the
internal electrode in millivolts, the sea water outside being taken as zero potential. Note that the membrane potential
actually changes sign for a couple hundred microseconds; note also the overshoot, or “afterhyperpolarization,” before
the potential settles back to its resting value. [Both panels from Hodgkin & Huxley, 1939.]
Idea 12.13 is certainly a falsifiable hypothesis. It predicts that changing the exterior concentration
of sodium, and hence the sodium Nernst potential, will alter the peak of the action potential.
Atthis exciting moment, most of British civilian science was interrupted for several years by the
needs of the war effort. Picking up the thread in 1946, Katz prepared axons with the exterior sea-
water replaced by a solution containing no sodium.^7 Though this change did nothing to the interior
of the axons, and indeed did not alter the resting potential very much, Katz found that eliminating
exterior sodium completely abolished the action potential, just as predicted by the hypothesis in
Idea 12.13. Later Hodgkin showed in more detail that reducing the external sodium to a fraction
of its usual concentration gave action potentials with reduced peak potentials (Figure 12.7), while
increasing it increased the peak, all in quantitative accord with the Nernst equation. Rinsing out
the abnormal solution and replacing it with normal sea water restored the normal action potential,
as seen in Figure 12.7.
Hodgkin and Katz then managed to get a quantitative estimate of the changes of the individual
conductances per area during an action potential. They found that they could explain the depen-
dence of the action potential on the sodium concentration ifgNa+increased about 500-fold from
its resting value. That is, the resting values,gK+≈ 25 gNa+≈ 2 gCl−(Equation 11.9 on page 420),
momentarily switch to
gK+≈ 0. 05 gNa+≈ 2 gCl−. (at the action potential peak) (12.14)This is a dramatic result, but how exactly do the membrane permeabilities change, and how does
(^7) In this and other modified-solution experiments, it’s important to introduce some other solute to match the
overall osmotic pressure across the cell membrane.