Which phase of the non-pacemaker action potential is the plateau phase with high Ca++ currents?

• A. Phase 2 plateau
• B. Phase 3 repolarization
• C. Phase 0 rapid depolarization
• D. Phase 4 resting potential

Answer: (A)

In the non-pacemaker cardiac action potential, the plateau phase (Phase 2) is defined by a sustained calcium current. L-type calcium channels open and let Ca2+ enter the cell, which balances the outward potassium currents and keeps the membrane potential near 0 mV for an extended time. This Ca2+ influx is what maintains depolarization long enough to trigger calcium-induced calcium release from the sarcoplasmic reticulum, leading to muscle contraction and a longer refractory period. As the calcium channels inactivate and potassium currents dominate, the membrane repolarizes in Phase 3, returning to the resting potential in Phase 4.