Which electrophysiological feature best distinguishes the action potential of a ventricular myocyte from that of a sinoatrial (SA) node pacemaker cell?

## Ventricular Myocyte vs. SA Node Pacemaker Cell Action Potentials ### Ventricular Myocyte Characteristics **Key Point:** Ventricular myocytes have a stable resting membrane potential (approximately -85 to -90 mV) and exhibit rapid Phase 0 depolarization driven primarily by fast inward sodium current (I~Na~). They do NOT spontaneously depolarize during diastole. ### SA Node Pacemaker Cell Characteristics **Key Point:** SA node cells lack a stable resting potential and instead display spontaneous Phase 4 diastolic depolarization (slow diastolic depolarization). Phase 0 is driven by slow L-type calcium current (I~Ca,L~) rather than fast sodium current, resulting in a slower upstroke velocity. ### Comparative Action Potential Features | Feature | Ventricular Myocyte | SA Node Pacemaker | |---------|-------------------|-------------------| | Resting potential | -85 to -90 mV (stable) | -60 to -65 mV (unstable) | | Phase 4 behavior | Flat (no spontaneous depolarization) | Gradual spontaneous depolarization | | Phase 0 mechanism | Fast Na⁺ influx (I~Na~) | Slow Ca²⁺ influx (I~Ca,L~) | | Phase 0 slope | Steep (~200 V/s) | Shallow (~1–2 V/s) | | Threshold potential | -40 to -30 mV | -40 to -30 mV | | Plateau phase | Prominent (Phase 2) | Absent or minimal | | Refractory period | Long (~250 ms) | Short (~150 ms) | | Conduction velocity | ~0.3–0.4 m/s | ~0.05 m/s | ### Mechanism of Automaticity in SA Node 1. At end of repolarization, membrane potential reaches approximately -60 mV 2. L-type calcium channels open (If and I~Ca,L~ activate) 3. Slow inward calcium current gradually depolarizes the membrane during Phase 4 4. When threshold is reached, Phase 0 occurs via calcium influx 5. This cycle repeats spontaneously without external stimulus **Mnemonic:** **VENOM** — **V**entricular (Voltage-stable, Na⁺-driven), **E**arly (rapid Phase 0), **N**o (no Phase 4 depolarization), **O**utstanding (prominent plateau), **M**yocyte. **High-Yield:** The presence or absence of Phase 4 diastolic depolarization is the single most important discriminator. Ventricular cells have a flat Phase 4; pacemaker cells have a sloped Phase 4. This is tested frequently in NEET PG. **Clinical Pearl:** Beta-blockers and calcium channel blockers slow the slope of Phase 4 in pacemaker cells, reducing heart rate. They have minimal effect on ventricular myocyte Phase 0 (which is sodium-dependent) but can depress conduction in the AV node (which is calcium-dependent). ```mermaid flowchart TD A[Cardiac Action Potential]:::outcome --> B{Spontaneous Phase 4<br/>Diastolic Depolarization?}:::decision B -->|Yes| C[Pacemaker Cell<br/>SA/AV Node]:::outcome B -->|No| D[Non-pacemaker Cell<br/>Atrial/Ventricular Myocyte]:::outcome C --> E[Phase 0: Slow Ca²⁺ current<br/>Slope 1-2 V/s]:::action D --> F[Phase 0: Fast Na⁺ current<br/>Slope 200 V/s]:::action C --> G[Resting potential<br/>-60 to -65 mV]:::action D --> H[Resting potential<br/>-85 to -90 mV]:::action ``` [cite:Guyton & Hall Ch 10]