Parasympathetic activation also opens which channels resulting in hyperpolarization?

• A. K+ channels
• B. Na+ channels
• C. L-type Ca2+ channels
• D. Cl- channels

Answer: (A)

Parasympathetic stimulation lowers heart rate by increasing potassium conductance in nodal tissue. Acetylcholine from the vagus binds to M2 muscarinic receptors on pacemaker cells, activating Gi proteins that open G-protein–gated inwardly rectifying K+ channels. The resulting K+ efflux hyperpolarizes the cell, moves the resting potential further from the threshold, and reduces the slope of phase 4 depolarization. This makes it harder for pacemaker cells to reach threshold and slows atrioventricular conduction. Sodium channels opening would depolarize, and L-type calcium channels contribute to depolarization and the plateau rather than hyperpolarization; chloride channels aren't the primary mediators of this parasympathetic effect in the heart.