A researcher studying ion currents in sinoatrial node pacemaker cells records a slowly depolarizing 'funny current' (If). Which ion channels predominantly carry this current during the diastolic depolarization phase?

• A HCN (hyperpolarization-activated cyclic nucleotide-gated) channels permeable to Na+ and K+ ✓
• B L-type calcium channels that open at -40 mV
• C T-type calcium channels that provide the upstroke
• D Delayed rectifier K+ channels closing during phase 4

Explanation

The 'funny current' (If) flows through HCN channels, which open on hyperpolarization and carry a mixed inward Na+/K+ current that slowly depolarizes the pacemaker membrane during phase 4. Sympathetic stimulation increases cAMP, which shifts the HCN channel activation curve to more positive potentials, accelerating If and thus increasing heart rate. L-type Ca2+ channels contribute to the later rapid depolarization of the SAN action potential upstroke, while T-type Ca2+ channels contribute to the mid-portion of diastolic depolarization.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.