Which phase of the non-pacemaker action potential is the rapid depolarization phase with high Na+ and Ca++ currents and low K+ currents?

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

Answer: (A)

The rapid upstroke of the non-pacemaker action potential is driven by a large inward current through fast voltage-gated sodium channels. When the resting potential is around −90 mV, these Na+ channels open, letting a flood of positively charged sodium into the cell and causing the membrane potential to swing quickly from negative toward positive. Because this inward current dominates and outward K+ currents are relatively small at this moment, the cell depolarizes very rapidly. Calcium entry via L-type channels becomes more important later, during the plateau phase, not the initial upstroke. A brief outward potassium current then causes the notch (phase 1), followed by calcium-dependent plateau (phase 2) and eventual repolarization (phase 3). Therefore, this phase is the rapid depolarization phase.