Regarding the ionic basis of the cardiac action potential, all of the following statements are correct EXCEPT:

Explanation

## Ionic Mechanisms of Cardiac Action Potential Phases ### Phase-by-Phase Breakdown | Phase | Duration | Primary Ions | Key Channels | Membrane Potential | |-------|----------|--------------|--------------|--------------------| | Phase 0 | ~1 ms | Na^+^ influx | Fast Na^+^ channels | −90 to +20 mV | | Phase 1 | ~10 ms | Na^+^ inactivation + K^+^ efflux | I_to (transient outward) | +20 to −10 mV | | Phase 2 | ~200 ms | Ca^2+^ influx vs K^+^ efflux | L-type Ca^2+^ channels | −10 to 0 mV (plateau) | | Phase 3 | ~150 ms | K^+^ efflux dominates | Delayed rectifier K^+^ | 0 to −90 mV | | Phase 4 | Variable | Depends on cell type | Various | Resting potential | ### Why Option 4 is INCORRECT **Key Point:** During Phase 3 (repolarization), sodium channels are already inactivated and sodium influx has essentially ceased. The repolarization is driven almost entirely by outward potassium currents (I_K and I_K1), NOT by continued sodium influx. **High-Yield:** The statement "sodium influx continues to depolarize the membrane" during Phase 3 is physiologically false. Sodium channels are inactivated by this point, and the membrane potential is becoming increasingly negative — conditions that keep sodium channels closed. Repolarization is a K^+^-driven process. ### Correct Statements (Options 1–3) **Option 1 (Phase 0):** ✓ Correct. Fast Na^+^ channels open rapidly when threshold is reached, causing the steep upstroke in atrial and ventricular myocytes. [cite:Guyton & Hall Ch 10] **Option 2 (Phase 1):** ✓ Correct. Early repolarization reflects sodium channel inactivation and activation of the transient outward current (I_to), which is prominent in ventricular epicardium. **Option 3 (Phase 2):** ✓ Correct. The plateau is maintained by a balance: L-type Ca^2+^ channels provide inward depolarizing current, while K^+^ channels (I_K) provide outward repolarizing current. When these balance, the membrane potential stays relatively stable. ### Clinical Pearl **Warning:** Confusing Phase 3 with Phase 0 is a common trap. Remember: Phase 0 = Na^+^ IN (depolarization); Phase 3 = K^+^ OUT (repolarization). Sodium is already "locked out" by inactivation during Phase 3.