U01.07.018 Pacemaker

The pacemaker action potential occurs in the sinoatrial (SA) node and atrioventricular (AV) node, differing from the ventricular action potential in key ways. These differences are crucial for the automaticity and conduction of electrical impulses in the heart.

Key Phases of Pacemaker Action Potential

Unlike the ventricular action potential, the pacemaker action potential lacks Phases 1 and 2. The primary phases are:

Phase 0 – Upstroke (Depolarization)

Cause: Opening of voltage-gated Ca²⁺ channels.
Effect: Slow conduction velocity.
Key Point: Fast voltage-gated Na⁺ channels remain permanently inactivated due to the less negative resting potential of pacemaker cells.
Clinical Relevance: The slow conduction in the AV node helps delay impulse transmission from the atria to the ventricles, allowing for proper ventricular filling.

Phase 3 – Repolarization

Cause:
- Inactivation of Ca²⁺ channels.
- Activation of K⁺ channels → K⁺ efflux.
Effect: Brings the membrane potential back towards a more negative resting potential.

Phase 4 – Slow Spontaneous Diastolic Depolarization

Cause: If channels (“funny current“) generate a slow mixed Na⁺/K⁺ inward current.
Key Features:
- Different from the INa current in ventricular phase 0.
- Determines heart rate (HR)—a steeper slope of phase 4 leads to a faster HR.
Modulators:
- Acetylcholine (ACh) & adenosine → ↓ Phase 4 slope → ↓ HR.
- Catecholamines → ↑ Phase 4 slope → ↑ HR.
- Sympathetic stimulation → Increases the probability of If channel opening → ↑ HR.

Comparison of Phases in Pacemaker vs. Ventricular Action Potential

Phase	Pacemaker Action Potential (SA/AV Node)	Ventricular Action Potential
Phase 0	Ca²⁺ influx (slow depolarization)	Na⁺ influx (rapid depolarization)
Phase 1	Absent	K⁺ efflux (initial repolarization)
Phase 2	Absent	Ca²⁺ influx (plateau phase)
Phase 3	K⁺ efflux (repolarization)	K⁺ efflux (repolarization)
Phase 4	If (Na⁺/K⁺ mixed inward current)	Stable resting membrane potential

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