Considering the differences between ventricular myocyte and sinoatrial (SA) node action potentials, all of the following are true EXCEPT:

Explanation

## Comparison of Ventricular Myocyte vs. SA Node Action Potentials ### Structural and Functional Differences | Feature | Ventricular Myocyte | SA Nodal Cell | |---------|-------------------|---------------| | **Resting Potential** | −85 to −90 mV (stable) | No true resting potential; spontaneous diastolic depolarization | | **Phase 0 Depolarization** | Fast Na^+^ channels (rapid upstroke) | L-type Ca^2+^ channels (slower upstroke) | | **Phase 1 & 2** | Prominent; clear plateau | Absent; no plateau phase | | **Phase 3** | Rapid repolarization | Gradual repolarization | | **Phase 4** | Stable; no spontaneous activity | Spontaneous diastolic depolarization (pacemaker activity) | | **Conduction Velocity** | ~1 m/s (fast) | ~0.05 m/s (slow) | | **Automaticity** | No intrinsic automaticity | Intrinsic automaticity | ### Why Option 4 is INCORRECT **Key Point:** Ventricular myocytes and SA nodal cells are NOT functionally equivalent in terms of Phase 0 depolarization or conduction velocity. This is a critical distinction in cardiac electrophysiology. **High-Yield:** The SA node relies on L-type calcium channels (not fast sodium channels) for Phase 0 depolarization, resulting in a much slower upstroke (~0.5 V/s) compared to ventricular myocytes (~200 V/s). This difference in channel composition directly explains the slower conduction velocity in the SA node (~0.05 m/s vs. ~1 m/s in ventricles). **Mnemonic:** **FAST vs. SLOW** — Ventricles use **F**ast Na^+^ channels (fast conduction); SA node uses **S**low Ca^2+^ channels (slow conduction). ### Correct Statements (Options 1–3) **Option 1 (Resting Potential & Diastolic Depolarization):** ✓ Correct. Ventricular myocytes maintain a stable resting potential of −85 to −90 mV due to the Na^+^/K^+^-ATPase and high K^+^ permeability. SA nodal cells lack a stable resting potential and exhibit spontaneous diastolic (Phase 4) depolarization due to the "funny current" (I_f) and reduced K^+^ conductance. [cite:Guyton & Hall Ch 10] **Option 2 (Phase 0 Mechanism):** ✓ Correct. Ventricular myocytes use fast Na^+^ channels for the rapid upstroke, while SA nodal cells rely on L-type Ca^2+^ channels, which open more slowly and produce a gentler slope. **Option 3 (Phases 1 & 2):** ✓ Correct. Ventricular myocytes display a characteristic plateau (Phase 2) with distinct Phase 1 early repolarization. SA nodal cells lack these phases and transition directly from Phase 0 depolarization to Phase 3 repolarization. ### Clinical Pearl **Warning:** A common exam trap is to assume all cardiac cells use the same depolarization mechanism. The SA node's reliance on calcium channels (not sodium) is why calcium channel blockers slow the heart rate by slowing SA node conduction, whereas fast sodium channel blockers (Class I antiarrhythmics) primarily affect ventricular and atrial conduction but have less effect on SA node rate.