Cardiac action potential

Every heartbeat starts with an electrical signal inside the heart muscle cell, the cardiomyocyte. That signal is called an action potential, and it is a sequence of changes in the voltage across the cell membrane. It lasts about 200 milliseconds and is divided into five numbered phases.

Phase 0 is the sudden climb. Fast sodium channels open, sodium pours into the cell, and the voltage jumps to about +52 millivolts. Then it starts to come back down, but not in one smooth drop. In phase 2 the fall stops and the voltage stays flat for a while. That flat stretch is called the plateau, and it is held up by the calcium moving into the cell.

That calcium does more than hold the plateau. It makes the sarcoplasmic reticulum, the cell's own calcium storehouse, release even more calcium, and that calcium binds a protein called troponin C, which unlocks the contraction machinery. This is where the electrical signal turns into a real squeeze of the muscle.

Not every cell in the heart works this way. Pacemaker cells, the ones that set the rhythm, have an action potential with only three phases: the two they lack are phase 1 and phase 2.