Which ion influx predominates during the plateau phase of the ventricular action potential?

• A. Na+
• B. K+
• C. Ca2+
• D. Mg2+

Answer: (C)

During the ventricular action potential plateau, the membrane remains depolarized because calcium ions continue to enter through L-type voltage-gated calcium channels. These channels open as the membrane potential rises and stay open long enough to provide a sustained inward Ca2+ current. This inward flow counters the outward potassium current that restarts as channels recover activity, preserving the depolarized state for a brief period. The calcium entering the cell also triggers calcium release from the sarcoplasmic reticulum, linking the electrical signal to contraction. Sodium influx has largely ceased as its channels inactivate, and magnesium does not contribute meaningfully to this phase.