What do muscarinic receptors control?

• A. Fight or flight responses
• B. Only pupil dilation
• C. Peristalsis, gland secretion, pupil constriction, vasodilation, and heart rate reduction
• D. Only gastric acid secretion

Answer: (C)

Muscarinic receptors respond to acetylcholine as part of the parasympathetic (rest-and-digest) system, producing a set of coordinated bodily responses. Activation in the gut increases peristalsis and gland secretion, in the eye causes pupil constriction (miosis) via the sphincter pupillae, in the heart slows the heart rate, and in the vasculature (through endothelial M3 receptors) leads to vasodilation. Together, these effects—enhanced GI motility and secretions, pupil constriction, vasodilation, and bradycardia—describe the classic parasympathetic actions mediated by muscarinic receptors. In contrast, fight-or-flight responses are driven by the sympathetic system, and pupil dilation is a sympathetic effect, not muscarinic. Gastric acid secretion is only one part of the muscarinic actions, not the full range.