1. Neurophysiology. The time perod immediately following an action potential during which the membrane is incapable of responding to another. This is caused by inactivation of the sodium channels after opening. This interval lasts about 2 ms at neurons, and 200-500 ms at the heart muscle. The maximum frequency at which neurons can fire is thus limited to a few hundred Hertz. The absolute refractory period is the interval when another impulse cannot be produced, relative refractory period has a much higher threshold.
2. Medical engineering. The time period immediately following either cardiac pacemaker sensing or pacing, during which the pacemaker sensing is inactivated. It normally ranges from 200-500 ms in duration.