Physiology The plateau phase of the cardiac muscle action potential is due to: A The movement of fewer sodium ions across the cell membrane B The calcium channels remaining open longer than the sodium channels C The increased membrane permeability to potassium ion D A decrease in the amount of calcium diffusing across the membrane 💡 Explanation The plateau phase of the cardiac muscle action potential is due to: ✓ Correct Answer: B. The calcium channels remaining open longer than the sodium channels