In the sinoatrial node action potential, the 'funny current' (If) responsible for spontaneous diastolic depolarisation is carried predominantly by:

• A Inward Na⁺ through fast Nav1.5 channels, activated at –70 mV
• B Outward K⁺ through KATP channels, suppressed by high intracellular ATP
• C Inward Ca²⁺ through T-type channels exclusively
• D Inward Na⁺ and K⁺ through HCN channels, activated by hyperpolarisation ✓

Explanation

The funny current (If) flows through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, principally HCN4 in the SA node. These channels open at hyperpolarised potentials and are non-selectively permeable to Na⁺ and K⁺, with net inward current producing slow diastolic depolarisation. cAMP binding to HCN4 accelerates activation, explaining sympathetic heart rate increase. Ivabradine selectively blocks HCN channels to reduce heart rate.

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

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Written and medically reviewed by the StethoPrep medical team.