During an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again. This period of resistance to restimulation is called the refractory period. It is divided into two phases—an absolute refractory period in which no stimulus of any strength will trigger a new action potential, and then a relative refractory period in which it is possible to trigger a new action potential, but only with an unusually strong stimulus (fig. 12.13).
The absolute refractory period lasts from the start of the action potential until the membrane returns to the resting potential—that is, for as long as the Na+ gates are open and subsequently inactivated. The relative refractory period lasts until hyperpolarization ends. During this period, K+ gates are still open. A new stimulus tends to admit Na+ and depolarize the membrane, but K+ diffuses out through the open gates as Na+ comes in, and thus opposes the effect of the stimulus. It requires an especially strong stimulus to override the K+ outflow and depolarize the cell enough to set off a new action potential. By the end of hyperpolarization, K+ gates are closed and the cell is as responsive as ever.
The refractory period refers only to a small patch of membrane where an action potential has already begun, not to the entire neuron. Other parts of the neuron can still be
stimulated while a small area of it is refractory, and even this area quickly recovers once the nerve signal has passed on.
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