Which feature best distinguishes the depolarization phase of an action potential from the repolarization phase?

Explanation

## Depolarization vs. Repolarization Phases ### Depolarization Phase - **Duration:** ~1 ms in neurons - **Driving force:** Rapid **inward Na+ current** through voltage-gated Na+ channels - **Membrane voltage change:** Becomes **less negative** (more positive), moving from −70 mV toward +30 mV - **Ion channel status:** Na+ channels open (activated); K+ channels still mostly closed - **Result:** Overshoot occurs; membrane potential briefly becomes positive ### Repolarization Phase - **Duration:** ~2–3 ms in neurons - **Driving force:** Rapid **outward K+ current** through voltage-gated K+ channels - **Membrane voltage change:** Becomes **more negative** (less positive), returning from +30 mV back toward −70 mV - **Ion channel status:** K+ channels open (activated); Na+ channels inactivate - **Result:** Hyperpolarization may occur as K+ efflux overshoots ### Comparative Table | Feature | Depolarization | Repolarization | |---------|-----------------|----------------| | **Primary ion** | Na+ (inward) | K+ (outward) | | **Voltage direction** | Less negative → more positive | More positive → less negative | | **Voltage range** | −70 mV → +30 mV | +30 mV → −70 mV (or more negative) | | **Channel status** | Na+ channels open; K+ channels closed | Na+ channels inactivated; K+ channels open | | **Time course** | ~1 ms | ~2–3 ms | | **Driving mechanism** | Inward Na+ current | Outward K+ current | **Key Point:** Depolarization = **Na+ in, voltage up**; Repolarization = **K+ out, voltage down**. **High-Yield:** The **sequence of channel opening** is critical: Na+ channels open first (depolarization), then K+ channels open with a slight delay (repolarization). This delay is why the action potential has its characteristic shape. **Mnemonic:** **NARK** — Na+ (depolarization), After that K+ (repolarization).