Learning Objectives

Analyze the three phases (0, 3, 4) of the pacemaker action potential in the SA and AV nodes. Distinguish how the Phase 0 upstroke differs (Ca2+ vs. Na+) from that of the ventricular muscle. Master how the Phase 4 diastolic depolarization slope determines heart rate (HR). Correlate autonomic nervous system inputs (sympathetic vs. parasympathetic) with molecular changes in Phase 4.

Phases of the Pacemaker Potential

This “slow-response” action potential occurs exclusively in the SA node (primary pacemaker) and AV node.

• Phase 4 (Diastolic Depolarization): The critical phase. A slow, spontaneous inward Na+/K+ current (I_f, “funny current”), different from fast Na+ channels, drives the potential toward threshold. This phase is responsible for automaticity. The slope of Phase 4 directly determines Heart Rate (HR).
• Phase 0 (Upstroke): Caused by Ca2+ influx through L-type voltage-gated Ca2+ channels. Fast Na+ channels are permanently inactivated due to the less negative (more depolarized) maximum diastolic potential. This Ca2+-driven upstroke produces a slow conduction velocity, which is crucial for the **AV nodal delay**, ensuring ventricular filling.
• Phase 3 (Repolarization): K+ efflux driven by the opening of K+ channels, while Ca2+ channels close. This restores the maximum diastolic potential.

Autonomic Regulation of Heart Rate

HR is adjusted by altering the speed of Phase 4 depolarization:

• Sympathetic (Catecholamines): Increases HR by increasing the chance that funny channels (HCN) are open, steepening the Phase 4 slope.
• Parasympathetic (ACh/Adenosine): Decreases HR by decreasing the funny current (I_f), flattening the Phase 4 slope and hyperpolarizing the cell.

Clinical Application: Pacemaker vs. Myocardial

A high-yield board question often asks to compare these two:

Activity