## Phase 2: The Plateau Phase **Key Point:** Phase 2 is the distinctive plateau phase of the ventricular action potential, where the membrane potential remains relatively stable near 0 mV due to a balance between inward L-type Ca²⁺ current and outward K⁺ current. ### Ionic Basis of the Plateau 1. **Inward current (depolarizing):** L-type calcium channels open and Ca²⁺ enters the cell 2. **Outward current (repolarizing):** Delayed rectifier K⁺ channels gradually open, allowing K⁺ efflux 3. **Balance:** These two currents are nearly equal, resulting in minimal net change in membrane potential 4. **Duration:** Phase 2 lasts 200–300 ms in ventricular muscle ### Functional Significance **High-Yield:** The prolonged Phase 2 plateau is crucial for: - Sustained contraction of ventricular muscle - Adequate time for calcium-induced calcium release (CICR) from the sarcoplasmic reticulum - Excitation-contraction coupling - Prevention of tetanic contraction (refractory period prevents summation) ### Phases of Cardiac Action Potential | Phase | Membrane Potential | Primary Ion Movement | Duration | Key Feature | |-------|-------------------|----------------------|----------|-------------| | 0 | −90 to +30 mV | Na⁺ influx | 1–2 ms | Rapid depolarization | | 1 | +30 to +10 mV | K⁺ efflux (transient outward) | Brief | Early repolarization, notch | | 2 | +10 to −10 mV | Ca²⁺ influx vs. K⁺ efflux (balanced) | 200–300 ms | **Plateau** | | 3 | −10 to −90 mV | K⁺ efflux (delayed rectifier) | 150–200 ms | Rapid repolarization | | 4 | −90 mV | K⁺ efflux (inward rectifier) | Variable | Resting potential | **Clinical Pearl:** Calcium channel blockers (verapamil, diltiazem) prolong Phase 2 and slow AV nodal conduction. Potassium channel blockers (amiodarone, sotalol) prolong Phase 2 and Phase 3, prolonging the QT interval. **Mnemonic:** **CAKEP** — **Ca²⁺** influx and **K⁺** efflux in equilibrium = **P**lateau (Phase 2).
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