The absolute refractory period identifies the first period of time of an action potential. During this time, a second stimulus will not induce a second action potential. This means that regardless of the strength of the stimulus, a cell, or group of cells, is not able to respond.
The relative refractory period describes a time period following the absolute refractory period. Unlike the absolute period, it is possible for a second action potential to be initiated. This will only occur, however, if a certain threshold is reached by the second stimulus. In most instances, it requires a very strong stimulus to initiate a second action potential during the relative refractory period.
In the nervous system, the sodium potassium pump operates using action potentials generated by voltage differences caused by the influx of charged ions. The absolute refractory period is identified as the period directly after positively charged sodium ions rush into the cell in response to a certain stimulus, thereby decreasing the negative charge within the cell. The channels that these ions travel through close and will not reopen until the cell is repolarized. Therefore, another action potential cannot be initiated.
Following the absolute refractory period, the cells then enter the relative refractory period. This period is characterized by an efflux of potassium ions as the cell tries to repolarize itself by recreating the electrochemical gradient across the cellular membrane. Although the sodium channels are still closed, a strong enough stimulus can cause the channels to reopen, thereby creating a second action potential.