Which nerve carries parasympathetic input to the eye that affects pupil size?

Which nerve carries parasympathetic input to the eye that affects pupil size?

• A. Optic nerve (CN II).
• B. Trochlear nerve (CN IV).
• C. Trigeminal nerve (CN V).
• D. Oculomotor nerve (CN III).

Answer: (D)

Parasympathetic control of pupil size is carried by the oculomotor nerve. The parasympathetic fibers originate in the Edinger-Westphal nucleus, travel with the oculomotor nerve to the ciliary ganglion, and then postganglionic fibers reach the sphincter pupillae muscle to constrict the pupil (miosis) and aid near focus via accommodation. The optic nerve carries sensory visual information, not parasympathetic output; the trochlear and trigeminal nerves do not convey parasympathetic innervation to the pupil. So this nerve is the one that modulates pupil size through parasympathetic input.

Parasympathetic control of pupil size is carried by the oculomotor nerve. The parasympathetic fibers originate in the Edinger-Westphal nucleus, travel with the oculomotor nerve to the ciliary ganglion, and then postganglionic fibers reach the sphincter pupillae muscle to constrict the pupil (miosis) and aid near focus via accommodation. The optic nerve carries sensory visual information, not parasympathetic output; the trochlear and trigeminal nerves do not convey parasympathetic innervation to the pupil. So this nerve is the one that modulates pupil size through parasympathetic input.