The Refractory Period

The existence of a refractory period would be expected from the gating scheme summarized in Figure 6-8. When the h gates of the voltage-sensitive sodium channels are closed (states C and D in Figure 6-8), the channels cannot conduct Na+ no matter what the state of the m gate might be. When the membrane is in this condition, no amount of depolarization can cause the cell to fire an action potential; the h gates would simply remain closed, preventing the influx of Na+ necessary to trigger the regenerative explosion. Only when enough time has passed for a significant number of h gates to reopen will the neuron be capable of producing another action potential.

Figure 6-8 The states of voltage-sensitive sodium and potassium channels at various times during an action potential in a neuron.

(a) At rest, neither channel is in a conducting state.

(b) During the depolarizing phase of the action potential, the sodium channels open, but the potassium channels have not yet responded to the depolarization. (c) During the repolarizing phase, sodium permeability begins to return to its resting level as h gates respond to the preceding depolarizing phase. At the same time, potassium channels respond to the depolarization by opening. (d) During the undershoot, sodium permeability returns to its usual low level; potassium permeability, however, remains elevated because ngates respond slowly to the repolarization of the membrane. The resting state of the membrane is restored after h gates and n gates return to their resting configurations. (Animation available at

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