A cardiac surgeon performs an endarterectomy on the internal carotid artery. During the procedure, the surgeon infiltrates the carotid sinus with a local anesthetic to prevent bradycardia. This reflex bradycardia is mediated by baroreceptor activation leading to increased vagal (parasympathetic) outflow acting on which receptor on the SA node?

• A Alpha-1 adrenoceptor
• B Beta-1 adrenoceptor
• C Nn nicotinic receptor
• D M2 muscarinic receptor ✓

Explanation

Baroreceptor-mediated vagal stimulation releases acetylcholine that acts on M2 muscarinic receptors on the sinoatrial node, causing bradycardia and decreased conduction velocity. M2 receptors are the dominant cardiac muscarinic subtype. Alpha-1 and beta-1 receptors mediate sympathetic stimulation (increasing heart rate); Nn receptors are ganglionic, not present at the SA node.

Reference: Ganong Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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