cardiac action potential

A cardiac action potential is the brief electrical pulse that runs through a heart cell each time it is activated — the cell's own miniature spark. It is the single event underlying every heartbeat, repeated billions of times across the heart. Picture the cell's membrane voltage swinging from a negative resting value, rapidly upward, holding for a moment, then sliding back down to rest.

In a typical working ventricular cell the cycle has distinct phases driven by ions crossing the membrane through channels: a fast upstroke (depolarization) as sodium rushes in; a brief early dip; a long, flat plateau held up by calcium flowing in balanced against potassium flowing out; then repolarization as potassium dominates and the voltage falls back to its resting baseline. That long plateau is what makes heart cells unusual — it is far longer than a nerve or skeletal-muscle spike and is the reason cardiac muscle cannot be driven into sustained tetanic contraction.

The exact shape differs by cell type. Pacemaker cells in the sinoatrial and atrioventricular nodes have no stable resting voltage and no fast sodium upstroke; instead their voltage drifts up on its own (their automaticity) and they depolarize via calcium. Because the action potential's plateau also sets the refractory period, drugs and electrolyte disturbances that alter these ion currents can lengthen or shorten it — which is why they can either calm or provoke arrhythmias.