The rapid depolarization phase of the action potentials of myocardial contractile cells is due primarily to which ion(s)?

The rapid depolarization phase of myocardial contractile cells is due to the Na+ ions. Sodium ions are essential for regulation of blood and body fluids, communication of nerve impulses, heart activity, and metabolic purposes. Physiologically, it survives as an ion in the body. Sodium is needed by animals but is not needed by plants. The human prerequisite for sodium must be less than 500 mg per day.