Compared with a ventricular myocyte, the sinoatrial nodal cell generates its phase 0 depolarization by a fundamentally different mechanism. Which statement is correct?
- A Phase 0 is carried by fast sodium channels, giving a maximal upstroke velocity near 200 V/s
- B Phase 0 is carried by the hyperpolarization-activated funny current entering the cell
- C Phase 0 is carried by L-type calcium channels, giving a slow upstroke and slow conduction ✓
- D Phase 0 is carried by transient outward potassium current leaving the cell
Correct answer: C. Phase 0 is carried by L-type calcium channels, giving a slow upstroke and slow conduction
Explanation
Nodal cells lack functional fast voltage-gated sodium channels because their maximum diastolic potential of about minus 55 to minus 65 mV inactivates them. Phase 0 therefore depends on L-type calcium channels, producing a slow upstroke of a few volts per second and slow conduction within the node. The funny current activates on hyperpolarization and contributes to phase 4 pacemaker depolarization, not phase 0, distinguishing it from the correct answer.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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