Which feature best distinguishes the action potential of sinoatrial (SA) nodal pacemaker cells from that of ventricular myocytes?

## Comparison: SA Nodal Pacemaker Cells vs. Ventricular Myocytes ### SA Nodal Pacemaker Cell Action Potential **Key Point:** SA nodal cells **lack fast Na⁺ channels and a plateau phase**; depolarization is mediated by **L-type Ca²⁺ channels alone**. #### Phases of SA Nodal Action Potential 1. **Phase 4 (Diastolic Depolarization)** - Membrane potential: −60 to −50 mV (less negative than ventricular cells) - Slope: **Positive** (spontaneous depolarization) - Mechanism: Decreased K⁺ efflux (I_K1 inactivation) + increased Ca²⁺ influx (L-type channels) + If (funny current) - **No plateau phase** — this is the key discriminator 2. **Phase 0 (Depolarization)** - Slope: **Gentle** (0.5–1 V/s) — much slower than ventricular (100–200 V/s) - Mechanism: **L-type Ca²⁺ channels only** (no fast Na⁺ channels) - Membrane potential: −50 to +20 mV 3. **Phase 3 (Repolarization)** - Slope: Moderate - Mechanism: Inactivation of L-type Ca²⁺ channels + activation of K⁺ channels (I_K) ### Ventricular Myocyte Action Potential **Key Point:** Ventricular myocytes have **fast Na⁺ channels, a prominent plateau (Phase 2), and a stable resting potential**. #### Phases of Ventricular Action Potential 1. **Phase 4 (Resting Potential)** - Membrane potential: **−90 mV** (stable) - No spontaneous depolarization (no pacemaker activity) - Maintained by Na⁺/K⁺-ATPase and inward rectifier K⁺ channels (I_K1) 2. **Phase 0 (Rapid Depolarization)** - Slope: **Steep** (100–200 V/s) - Mechanism: **Fast Na⁺ channels** (primary inward current) - Membrane potential: −90 to +20 mV 3. **Phase 2 (Plateau)** - **Unique to ventricular and atrial myocytes** - Mechanism: Balanced L-type Ca²⁺ influx and K⁺ efflux - Duration: 200–300 ms 4. **Phase 3 (Repolarization)** - Slope: Steep - Mechanism: K⁺ efflux dominates ### Comparison Table | Feature | SA Nodal Pacemaker | Ventricular Myocyte | |---------|-------------------|---------------------| | **Resting membrane potential** | −60 to −50 mV (unstable) | −90 mV (stable) | | **Phase 4 behavior** | Spontaneous diastolic depolarization | Stable, no spontaneous activity | | **Depolarization mechanism** | L-type Ca²⁺ channels | Fast Na⁺ channels | | **Phase 0 slope** | Gentle (0.5–1 V/s) | Steep (100–200 V/s) | | **Plateau phase (Phase 2)** | **ABSENT** | **PRESENT** (200–300 ms) | | **Primary repolarizing current** | K⁺ efflux (I_K) | K⁺ efflux (I_K, I_Ks, I_K1) | | **Conduction velocity** | Slow (0.02–0.05 m/s) | Fast (0.3–0.4 m/s) | | **Automaticity** | Yes (intrinsic pacemaker) | No (requires external stimulus) | **High-Yield:** The **absence of a plateau phase in pacemaker cells** is the most clinically relevant discriminator. This is why pacemaker cells have shorter action potential durations and can fire spontaneously. **Clinical Pearl:** Calcium channel blockers (verapamil, diltiazem) preferentially slow SA nodal conduction because SA nodal cells depend entirely on L-type Ca²⁺ channels for depolarization. They have minimal effect on ventricular conduction (which relies on fast Na⁺ channels). **Mnemonic:** **"SA = Slow, Spontaneous, Sans plateau"** — SA nodal cells have slow depolarization (Ca²⁺-dependent), spontaneous Phase 4 depolarization, and no plateau phase.