Which autonomic receptor mediates heart rate decrease via parasympathetic stimulation?

• A. Nicotinic receptor
• B. Muscarinic (M2) acetylcholine receptor
• C. Beta-1 adrenergic receptor
• D. Alpha-1 adrenergic receptor

Answer: (B)

Parasympathetic control of heart rate is achieved when acetylcholine acts on M2 muscarinic receptors in the SA and AV nodes. Activation of these receptors slows the rate of spontaneous depolarization and slows conduction by opening GIRK potassium channels and reducing calcium and cAMP signaling. The overall effect is a slower heart rate and slower AV nodal conduction.

Nicotinic receptors are located in autonomic ganglia and at the neuromuscular junction, not in the heart to mediate this effect. Beta-1 adrenergic receptors drive sympathetic increases in heart rate and contractility, while alpha-1 receptors mainly affect vascular smooth muscle.