Fast Na+ channels are responsible for which phase in non-pacemaker cardiac action potentials?

• A. Phase 0 depolarization
• B. Phase 1 repolarization
• C. Phase 2 plateau
• D. Phase 3 repolarization

Answer: (A)

Fast voltage-gated Na+ channels produce the rapid upstroke that starts the action potential in non-pacemaker ventricular myocytes. When these channels open, a large inward Na+ current drives a swift and steep depolarization, moving the membrane potential quickly from its resting level to a positive value. This sharp, initial depolarization is what we call Phase 0. After this, Na+ channels inactivate and the cell proceeds through the subsequent phases: a brief Phase 1 notch from transient outward K+ current, a Phase 2 plateau due to Ca2+ influx balanced by K+ efflux, and Phase 3 repolarization as K+ currents dominate, returning to Phase 4 resting potential. So the fast Na+ current is what underlies the Phase 0 depolarization.