automaticity

Automaticity is the heart's ability to start its own beat — no nerve, no outside command required. Certain cardiac cells will fire rhythmically all on their own, like a clock that winds itself. This is why a heart removed from the body and kept alive in the lab keeps beating, and why a transplanted heart, cut off from its nerves, still pumps.

The mechanism is a slow, automatic upward drift in the cell's voltage during the resting interval. Instead of sitting still at a steady negative voltage, pacemaker cells leak ions in a way that gradually depolarizes them until they reach threshold and fire the next action potential. The steeper this drift, the faster the rate. The sinoatrial node has the steepest drift, so it normally outpaces and silences all the others — a phenomenon called overdrive suppression.

Automaticity is graded down the conduction system: the sinoatrial node fires fastest (about 60 to 100 per minute), the atrioventricular junction slower (40 to 60), and the ventricular Purkinje system slowest (20 to 40). This hierarchy provides built-in backups. But automaticity can also turn pathological — injured or irritated tissue may develop abnormal, enhanced automaticity and fire as an ectopic pacemaker, producing arrhythmias.