Myocardial action potential In myocardial action potential, phase 0 represents the rapid depolarization state. This uprise is caused by the opening of fast sodium channels that allow massive influx of sodium ions into the cytoplasm. In phase 1, there is rapid repolarization, caused by the opening of potassium channels, which allow the efflux of positively charged potassium ions outside the cells.
At the same time, fast sodium channels are suddenly closed. This phase is the shortest phase. In phase 2, more or less plateaus pattern is produced. The voltage-sensitive calcium channels are opened to facilitate the influx of positively charged calcium ions, to balance the repolarizing effects of potassium efflux. In phase 3, there is rapid repolarization of the cardiac cells.
The calcium channels are suddenly closed, which leaves the potassium efflux current unopposed. Finally, in phase 4, sodium re-enters into the cardiac cells, but this time through slow spontaneous permeation through the cell membrane. This slow process prepares the cells for the next action potential cycle, restarted with phase 1.