pacemaker cells

Pacemaker cells are the heart's self-starters: the small population of specialized cells whose job is not to squeeze blood but to keep time. They fire spontaneously and rhythmically, and their steady beat sets the tempo that the rest of the heart follows.

What sets them apart from ordinary working muscle is that they never truly rest. A typical ventricular cell sits patiently at a stable negative voltage until something excites it; a pacemaker cell instead drifts upward on its own from its lowest point until it reaches threshold and fires — the property of automaticity. They also rely on calcium rather than a fast sodium current for their upstroke, which makes them depolarize more gradually. The densest, fastest population lives in the sinoatrial node, with slower groups in the atrioventricular junction and the ventricular conduction tissue.

Because the fastest pacemaker wins, the sinoatrial node normally dominates and suppresses the slower ones. The slower pacemakers act as a safety net: if the node fails, a junctional or ventricular pacemaker can take over, though at a slower escape rate. When pacemaker cells of the heart cannot do their job — for example in sick sinus syndrome or complete heart block — an artificial electronic pacemaker may be implanted to take over the timekeeping.