Which ion channel is responsible for the most common cause of the rapid depolarization phase (Phase 0) of the ventricular cardiac action potential?

Explanation

## Phase 0 Depolarization: The Sodium Influx **Key Point:** Phase 0 (rapid depolarization) of the ventricular action potential is driven almost entirely by **voltage-gated sodium (Na⁺) channels**, which open rapidly when the membrane potential reaches threshold (~−60 mV). ### Mechanism of Phase 0 The voltage-gated Na⁺ channels are responsible for: 1. **Rapid inward current** — Na⁺ influx at a rate of ~500 mV/s in ventricular myocytes 2. **Membrane potential swing** — from resting potential (−85 mV) toward +20 mV within ~1 ms 3. **Propagation of action potential** — the rapid depolarization allows electrical coupling to adjacent cells via gap junctions **High-Yield:** The slope of Phase 0 (dV/dt) is the **fastest** of any cardiac tissue and is directly proportional to the availability of Na⁺ channels. Drugs that block Na⁺ channels (Class I antiarrhythmics) slow Phase 0 and can be detected on the ECG as **QRS widening**. ### Why L-Type Calcium Channels Are NOT the Answer Although L-type Ca²⁺ channels do contribute to Phase 0 in **sinoatrial (SA) and atrioventricular (AV) node cells** (which lack fast Na⁺ channels), they are **NOT the primary driver in ventricular myocytes**. In ventricular tissue, Ca²⁺ channels open **after** Na⁺ channels and contribute to Phase 2 (plateau), not Phase 0. ### Comparison of Ion Channels in Phase 0 | Channel Type | Tissue | Phase 0 Role | Conductance | |---|---|---|---| | **Voltage-gated Na⁺** | Atria, ventricles, Purkinje | PRIMARY (fast depolarization) | Very high (500 mV/s) | | **L-type Ca²⁺** | SA node, AV node | Primary (slow depolarization) | Low (1–10 mV/s) | | **Inward rectifier K⁺** | All cardiac tissue | Repolarization (Phase 3) | Minimal during Phase 0 | | **ATP-sensitive K⁺** | All cardiac tissue | Hyperpolarization (stress) | Minimal during Phase 0 | **Clinical Pearl:** In **atrial fibrillation**, rapid ectopic foci fire repeatedly due to abnormal automaticity driven by Na⁺ channel reopening. In **Brugada syndrome**, loss-of-function mutations in the cardiac Na⁺ channel (SCN5A) reduce Phase 0 slope and predispose to ventricular fibrillation. **Mnemonic:** **FAST Na⁺, SLOW Ca²⁺** — Ventricular Phase 0 is fast (Na⁺-driven); nodal Phase 0 is slow (Ca²⁺-driven).