Which ion is primarily responsible for the rapid depolarization of cardiac myocytes?

The ion primarily responsible for the rapid depolarization of cardiac myocytes is sodium. During the process of depolarization, the cardiac action potential begins when sodium channels open, allowing a rapid influx of sodium ions into the cells. This influx leads to a significant change in the membrane potential, making the inside of the cell more positive compared to the outside. As the membrane potential reaches a certain threshold, it triggers further depolarization, which is essential for the initiation of the cardiac action potential and the subsequent contraction of the heart muscle.

Calcium does play a crucial role in cardiac myocyte contraction and in later phases of the action potential, particularly in the plateau phase, but it is the rapid entry of sodium that drives the initial depolarization. Potassium is involved in repolarization after the depolarization phase, while chloride does not have a significant role in the rapid depolarization of cardiac cells. Thus, sodium's role in initiating the rapid depolarization is pivotal in the cardiac cycle.