NREMT Paramedic Exam 2026 – 400 Free Practice Questions to Pass the Exam

The primary ion responsible for the depolarization of myocardium is sodium. During the action potential of cardiac muscle cells, the depolarization phase occurs when sodium channels open, allowing an influx of sodium ions into the cell. This rapid movement of sodium ions into the intracellular environment causes the membrane potential to become more positive, leading to depolarization.

This process is crucial for initiating muscle contraction in the heart. As the voltage across the membrane changes due to sodium influx, it triggers further events in the cardiac electrical conduction system, including the opening of additional ion channels that sustain the cardiac action potential. In contrast, potassium is more involved in repolarization, returning the membrane potential to its resting state after depolarization. Calcium is important for muscle contraction but does not primarily initiate depolarization. Chloride's role in cardiac action potentials is minimal compared to sodium and potassium dynamics.