A 22-year-old heroin addict is brought comatose with respiratory rate of 4 per minute and bilateral pinpoint pupils. Intravenous naloxone restores respiration and consciousness. The pupils are pinpoint because morphine acts on which structure?
- A Superior cervical ganglion
- B Edinger-Westphal nucleus, stimulating parasympathetic outflow ✓
- C Iris sphincter muscle directly
- D Locus coeruleus, inhibiting sympathetic tone
Correct answer: B. Edinger-Westphal nucleus, stimulating parasympathetic outflow
Explanation
Opioids act centrally on mu receptors in the Edinger-Westphal nucleus of the oculomotor complex, enhancing parasympathetic outflow to produce bilateral miosis. The classic opioid toxidrome is the triad of coma, respiratory depression, and pinpoint pupils, reversed by naloxone. Direct action on the iris sphincter is incorrect because the effect disappears after death and is abolished by central lesions, confirming a supranuclear site of action.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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