refractory period

The refractory period is the heart cell's enforced rest between beats — the window right after it fires when it simply will not respond to a new stimulus, no matter how strong. Think of a camera flash that needs time to recharge before it can fire again; until it is ready, pressing the button does nothing.

During the absolute refractory period the cell cannot be re-excited at all, because the sodium channels that drive depolarization are still inactivated and have not reset. This is followed by a relative refractory period, when an unusually strong stimulus can trigger a weak, slow response. In cardiac muscle the refractory period is long — it lasts almost the entire action potential, far longer than in nerve or skeletal muscle.

That long refractoriness is protective. It prevents the heart from being re-triggered too soon, which is why cardiac muscle cannot be driven into sustained tetanic contraction the way skeletal muscle can — the chambers must relax and refill between beats. It also acts as a natural speed limit on conduction and helps break up the self-sustaining electrical loops that cause re-entrant arrhythmias. Several antiarrhythmic drugs work precisely by lengthening the refractory period.