The cardiac rhythm, or heartbeat, results from coordinated contractions controlled by electrical signals.

These signals originate in the SA node, which is made of special noncontractile pacemaker cells. The SA node transmits the signals through the AV node and specialized structures, leading to ventricular contraction and blood circulation.

As the pacemaker impulses reach contractile cardiac muscle cells, their voltage-sensitive membrane channels regulate the ion movement to generate a five-phase action potential.

Phase 0 or depolarization involves rapid Na⁺ ion influx, followed by channel inactivation. This leads to partial repolarization in Phase 1.

In Phase 2, slow Ca²⁺ ion influx creates a plateau. Phase 3, or repolarization, involves Ca²⁺ channel inactivation and K⁺ ion outflow.

The rapid potassium efflux returns the membrane potential to its resting voltage in Phase 4.

The electrical waves produced by the heart are transmitted throughout the body and can be detected by an ECG.