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pacemaker potential

Physiology / CardiologyCardiovascular

Summary

Pacemaker potential (phase 4 spontaneous depolarization) is the slow, spontaneous diastolic depolarization of SA node (and AV node) cells that drives automaticity until threshold is reached and an action potential fires. It is caused by the "funny" current (If, HCN channels, Na+ inflow), followed by transient T-type Ca2+ influx; the upstroke (phase 0) is mediated by L-type Ca2+ channels, not fast Na+ channels. The SA node has the fastest intrinsic rate (60-100 bpm) and therefore sets the heart rate.

Detail

**Cells involved:** SA node (primary pacemaker, intrinsic ~60-100 bpm), AV node (~40-60 bpm), His-Purkinje system (~20-40 bpm). The fastest phase 4 slope dominates (overdrive suppression of lower pacemakers).

**Phases of the nodal action potential (no true phase 1 or 2 plateau):** - **Phase 4 (pacemaker potential):** Unstable resting membrane potential (about -60 mV) that slowly depolarizes. (1) **If (funny current):** HCN channels open with hyperpolarization after repolarization, allowing slow inward Na+ (and some K+) current. (2) **T-type Ca2+ channels** open late in phase 4 and add further depolarization. (3) Decay of delayed rectifier K+ current (IK) contributes as well. - **Phase 0 (upstroke):** Threshold (about -40 mV) opens **L-type voltage-gated Ca2+ channels** → Ca2+ influx. This upstroke is slower than in myocytes, since there are no fast Na+ channels. This explains the slow conduction velocity of the SA and AV nodes (and the AV nodal delay). - **Phase 3 (repolarization):** L-type Ca2+ channels inactivate, and K+ channels open → K+ efflux.

**Autonomic modulation (high-yield):** - **Sympathetic (β1, Gs → ↑cAMP):** ↑If and ↑Ca2+ current → steeper phase 4 slope → ↑ heart rate (positive chronotropy). - **Parasympathetic (vagal, M2, Gi → ↓cAMP):** ↓If, and ↑K+ efflux through ACh-activated K+ channels (IK,ACh) → more negative maximum diastolic potential and a flatter phase 4 slope → ↓ heart rate (negative chronotropy). - Changing the threshold (e.g., by Ca2+ channel blockers) also shifts the rate, but the main physiologic changes are in slope and maximum diastolic potential.

**Clinical significance/pharmacology:** - **Ivabradine:** selectively blocks If (HCN) → ↓ heart rate without negative inotropy; used in chronic HFrEF and stable angina (side effect: visual phosphenes/luminous phenomena). - **β-blockers and non-dihydropyridine CCBs (verapamil, diltiazem):** ↓ phase 4 slope (β-blockers) and ↓ phase 0 upstroke/AV conduction (CCBs), so they are used for rate control in AF. - **Adenosine:** activates K+ channels (via A1 receptors) → hyperpolarizes cells → transient AV block; used for SVT. - **Atropine:** blocks M2 receptors → ↑ HR (used for symptomatic bradycardia). - **Sick sinus syndrome / sinus node dysfunction:** failure of SA automaticity → bradycardia, pauses, or tachy-brady syndrome; AV node or ventricular escape rhythms take over, and a permanent pacemaker may be needed. - **Ectopic/escape rhythms** and **enhanced automaticity** (e.g., from ↑ catecholamines, hypokalemia, ischemia, digoxin toxicity) cause abnormal phase 4 depolarization in non-pacemaker cells. - **Contrast with ventricular myocyte AP:** stable resting potential (about -90 mV, via IK1), phase 0 from fast Na+ channels, phase 2 plateau from Ca2+ influx balanced by K+ efflux. Myocytes lack spontaneous phase 4 depolarization.

**High-yield pearls:** Phase 4 = automaticity. SA node upstroke = Ca2+ (not Na+). Ivabradine = If blocker. Vagal stimulation = decreased slope and hyperpolarization. Overdrive suppression keeps latent pacemakers quiet.

Sources

  • First Aid for the USMLE Step 1 (Cardiovascular: Cardiac physiology, Pacemaker action potential)
  • Costanzo Physiology (Cardiac electrophysiology)
  • Guyton and Hall Textbook of Medical Physiology (Rhythmical excitation of the heart)
  • Boron & Boulpaep Medical Physiology (Cardiac electrophysiology)
  • Pathoma / Sketchy Medical (Cardiac action potentials)

Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.