The plateau phase of the cardiac action potential is due to which process?

• A. Sodium influx
• B. Ca2+ influx balancing K+ efflux
• C. Chloride influx
• D. No ion movement

Answer: (B)

The plateau phase is caused by a balance between inward calcium current and outward potassium current. Calcium enters the cell through L-type calcium channels and keeps the membrane depolarized, while potassium continues to exit. This ongoing Ca2+ influx offsets the outward K+ flow, so the membrane potential stays near a depolarized level for a short period, allowing time for the heart to contract and for that contraction to be sustained. The rapid depolarization that starts the action potential is driven mainly by sodium entry, not the plateau phase; chloride movements aren’t the primary driver of this plateau; and there is indeed ion movement during the plateau, not zero movement.