A patient develops complete heart block. The ventricular escape rhythm at 35 bpm originates from the Purkinje fibres. This escape rate (35 bpm) reflects the intrinsic automaticity of Purkinje fibres because the SA node (60–100 bpm) and AV node (40–60 bpm) are non-conducting. The mechanism of phase 4 spontaneous depolarization in Purkinje fibres involves which ion current?

• A ICaL — L-type calcium current providing the upstroke of the action potential
• B If (funny current) — a mixed Na+/K+ inward current activated at negative potentials, encoded by HCN4 channels ✓
• C IKr — rapid delayed rectifier potassium current driving repolarization
• D INa — fast sodium current responsible for phase 0 upstroke

Explanation

Phase 4 spontaneous depolarization (pacemaker potential) in all cardiac pacemaker cells (SA node, AV node, Purkinje fibres) is primarily generated by the If ('funny') current — a mixed inward Na+/K+ current through HCN (hyperpolarization-activated cyclic nucleotide-gated) channels that activate when the membrane is hyperpolarized (after repolarization). Sympathetic stimulation increases cAMP, shifting If activation to less negative potentials, accelerating the pacemaker rate. Ivabradine selectively inhibits If. ICaL provides the upstroke in nodal tissue; INa provides the fast upstroke in working myocardium and Purkinje fibres.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.