Which phase of the non-pacemaker action potential corresponds to repolarization with high K+ currents and low Na+ currents?

• A. Phase 1 repolarization
• B. Phase 3 repolarization
• C. Phase 2 plateau
• D. Phase 4 resting potential

Answer: (B)

Ion movements during the cardiac action potential in non-pacemaker cells define the phases, and repolarization is driven by outward potassium currents. After the initial depolarization, the plateau is maintained by calcium entry with some opposing potassium flow. When the cell returns to rest, the outward potassium currents (the delayed rectifier channels, IKr and IKs) dominate and the calcium and sodium currents are minimal, allowing the membrane potential to fall back toward the resting level. This phase—repolarization with strong K+ efflux and little Na+ inflow—fits the final repolarization.