Which ion primarily drives the plateau phase of cardiac muscle action potentials?

• A. Sodium
• B. Potassium
• C. Calcium
• D. Chloride

Answer: (C)

The main idea here is what keeps the cardiac action potential from snapping back to resting value too quickly. After the rapid depolarization, the inward flow of calcium through voltage-gated calcium channels sustains a longer, steady depolarization—the plateau. This calcium entry counters the outward potassium current, so the membrane potential stays near its peak for a brief, extended period. That calcium influx is also the trigger for calcium-induced calcium release from the sarcoplasmic reticulum, which is essential for the muscle to contract. Once calcium channels inactivate and potassium keeps flowing outward, the cell repolarizes and the plateau ends. Sodium drives the quick rise in membrane potential at the start, potassium drives repolarization, and chloride doesn’t play the primary role in forming the plateau, making calcium the correct choice.