Which nucleus is involved in parasympathetic output to the pupil?

• A. Edinger-Westphal nucleus
• B. Dorsal motor nucleus
• C. Salivatory nucleus
• D. Nucleus ambiguus

Answer: (A)

Pupil constriction is driven by parasympathetic input that travels with the oculomotor nerve from a midbrain nucleus containing preganglionic parasympathetic neurons. This nucleus, the Edinger-Westphal, sends fibers to the ciliary ganglion, where they synapse on postganglionic fibers that innervate the sphincter pupillae muscle. When activated, these fibers cause the pupil to constrict and help with near focus by relaxing the ciliary muscle.

The other nuclei listed have different roles: the dorsal motor nucleus of the vagus sends parasympathetic output to thoracic and abdominal viscera; the salivatory nucleus provides parasympathetic input to salivary glands; and the nucleus ambiguus supplies motor neurons to the muscles of the pharynx and larynx via the vagus. None of these contribute to pupil constriction, so Edinger-Westphal is the one involved in parasympathetic output to the pupil.