SA Node Action Potential: Ionic Mechanisms & Pacemaker Function

The sinoatrial (SA) node is the heart's primary pacemaker. Its action potential (AP) differs fundamentally from ventricular myocytes—no stable resting potential, spontaneous diastolic depolarization.

Phases of SA Node Action Potential

1. Phase 4: Spontaneous Diastolic Depolarization (Pacemaker Potential)
   • Membrane potential: ~ -60 mV (maximum diastolic potential).
   • Three key currents:
     • I_f ("funny" current): Hyperpolarization-activated, mixed Na⁺/K⁺ inward current (HCN channels). Activates at ~ -50 mV. Major contributor.
     • I_Ca,T (T-type Ca²⁺): Activates at ~ -50 mV. Contributes to late Phase 4.
     • ↓ I_K (delayed rectifier K⁺): Deactivation of repolarizing K⁺ current reduces outward flow.
   • Net inward current → slow depolarization to threshold (~ -40 mV).
2. Phase 0: Upstroke (Depolarization)
   • Threshold reached → L-type Ca²⁺ channels (I_Ca,L) open (not Na⁺ channels as in ventricles).
   • Slow Ca²⁺ influx → slower upstroke (lower dV/dt_max) → slower conduction velocity (0.05 m/s vs. 1 m/s in ventricle).
   • Peak: ~ +10 to +20 mV.
3. Phase 3: Repolarization
   • L-type Ca²⁺ channels inactivate.
   • I_K (delayed rectifier K⁺) activates → K⁺ efflux → repolarization.
   • I_f deactivates as membrane hyperpolarizes.
4. Phase 1 & 2: Absent / Minimal – No fast Na⁺ spike, no plateau (I_Ca,L inactivates quickly, no sustained Ca²⁺ window current).

Autonomic Modulation

• Sympathetic (β₁-AR → cAMP → PKA): ↑ I_f, ↑ I_Ca,L, ↓ I_K → steeper Phase 4, higher threshold → ↑ HR.
• Parasympathetic (M₂-R → Gᵢ → ↓ cAMP + Gβγ → I_K,ACh): ↓ I_f, ↓ I_Ca,L, ↑ I_K (via I_K,ACh) → flatter Phase 4, lower max diastolic potential → ↓ HR.

Clinical Relevance

• Sick sinus syndrome: SA node dysfunction → bradycardia, pauses, chronotropic incompetence.
• Drug effects: β-blockers, CCBs (non-DHP), digoxin, ivabradine (I_f blocker) ↓ HR. Atropine ↑ HR.
• ECG: SA node AP not directly visible. P wave = atrial depolarization triggered by SA node.

The SA node's unique ionic choreography makes it the heart's metronome.