What period is defined as phases 0 to 3 in the cardiac cycle?

The effective refractory period is defined as phases 0 to 3 in the cardiac cycle. During this critical time, the heart muscle cells are in a state where they cannot be depolarized again, or can only be depolarized with a significant stimulus.

Phase 0 corresponds to rapid depolarization, where sodium channels open causing a swift influx of sodium ions. Phase 1 is the initial repolarization, with potassium ions moving out of the cell, and by phase 2, the plateau phase occurs, during which calcium channels are open to allow calcium ions to enter the cell, extending the depolarization. Finally, phase 3 represents rapid repolarization as potassium continues to exit the cell, and the cell prepares to return to its resting state. During phases 0 to 3, the myocardium is not able to respond to normal stimuli, thus defining the effective refractory period, which is vital in maintaining a proper rhythm during the cardiac cycle.

This distinction is important in understanding arrhythmias and the heart's electrical activity, as disturbances during these phases can lead to pathological conditions.