Action potentials absolute refractory period

Figure 6. An action potential. The diagram illustrates the sequential changes in membrane potential that occur during an action potential. The refractory period is divided into absolute (solid line) and relative (dashed line). See text for details.

After an electrical impulse is initiated and conducted, there is a period of time during which cells and fibers cannot be depolarized again. This period of time is referred to as the absolute refractory period (Fig. 6-2),2 and corresponds to phases 1,2, and approximately half of phase 3 repolarization on the action potential. The absolute refractory period also corresponds to the period from the Q wave to approximately the first half of the T wave on the ECG (Fig. 6-2). During this period, if there is a premature stimulus for an electrical impulse, this impulse cannot be conducted, because the tissue is absolutely refractory. 

Because of the absolute refractory period of the voltagegated Na channels and the brief hyperpolarization resulting from efflux, the action potential Is propagated in one direction only, toward the axon terminus. 

Amiodarone prolongs the action potential in atrial muscle and increases the absolute and effective refractory periods. 

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