Which phase of the action potential is most commonly associated with the opening of voltage-gated sodium channels?

Explanation

## Phases of the Action Potential and Ion Channel Activity **Key Point:** The depolarization or rising phase of the action potential is defined by the rapid inward movement of Na⁺ ions through voltage-gated sodium channels, which open when the membrane potential reaches threshold (~−55 mV). ### Timeline of Ion Channel Opening ```mermaid flowchart TD A[Resting Potential: −70 mV]:::outcome --> B[Stimulus: Membrane depolarizes]:::action B --> C{Threshold reached<br/>−55 mV?}:::decision C -->|Yes| D[Voltage-gated Na⁺ channels OPEN]:::action D --> E[Rapid Na⁺ influx<br/>Depolarization Phase]:::outcome E --> F[Membrane potential<br/>approaches +30 mV]:::outcome F --> G[Na⁺ channels INACTIVATE<br/>K⁺ channels OPEN]:::action G --> H[K⁺ efflux<br/>Repolarization Phase]:::outcome H --> I[Membrane returns to −70 mV]:::outcome I --> J[Hyperpolarization<br/>K⁺ channels still open]:::outcome ``` ### Phase-by-Phase Ion Channel Activity | Phase | Membrane Potential | Primary Channels | Ion Movement | Result | |-------|-------------------|------------------|--------------|--------| | **Depolarization (Rising)** | −70 → +30 mV | Na⁺ channels OPEN | Na⁺ influx | Rapid depolarization | | **Repolarization (Falling)** | +30 → −70 mV | Na⁺ inactivated; K⁺ channels OPEN | K⁺ efflux | Rapid repolarization | | **Hyperpolarization (Undershoot)** | −70 → −90 mV | K⁺ channels still open | Continued K⁺ efflux | Brief overshoot | | **Plateau (Cardiac only)** | ~0 mV | L-type Ca²⁺ channels open | Ca²⁺ influx = K⁺ efflux | Sustained depolarization | **High-Yield:** Na⁺ channel opening is **voltage-dependent** and occurs within milliseconds of threshold being reached. The channels have two gates: an **activation gate** (opens quickly at threshold) and an **inactivation gate** (closes after ~1 ms, causing Na⁺ channel refractoriness). **Mnemonic:** **DINS** — **D**epolarization = **I**nward (Na⁺), **N**a⁺ channels, **S**odium influx. **Clinical Pearl:** Drugs that block Na⁺ channels (e.g., local anesthetics, Class I antiarrhythmics) slow the rate of depolarization, reducing conduction velocity and excitability. This is why procainamide widens the QRS complex on ECG.