The period after the transmission of an impulse in a nerve or muscle in which the membrane of the axon or muscle fibre regains its ability to transmit impulses (see action potential). The absolute refractory period is the interval immediately following an action potential during which the sodium ion channels are inactivated and no stimulus, however large, will trigger a further impulse. This period can last from about 0.4 to 2 ms, depending on the type of neuron. During the ensuing relative refractory period, the ion channels continue to reactivate, but an action potential can be triggered only if the stimulus is abnormally large.