Which cranial nerve carries the efferent limb of the pupillary light reflex?

Which cranial nerve carries the efferent limb of the pupillary light reflex?

• A. CN II
• B. CN III
• C. CN IV
• D. CN VI

Answer: (B)

The essential idea is that constriction of the pupil in response to light is driven by parasympathetic output that travels from the brain to the iris. Light signals are detected by the retina and carried onward by the optic nerve, which provides the sensory (afferent) part of the reflex. But the actual signal that makes the pupil constrict follows a different route: the parasympathetic neurons originate in the Edinger–Westphal nucleus, travel with the oculomotor nerve, synapse in the ciliary ganglion, and then reach the sphincter pupillae muscle via short ciliary nerves. Activation of the sphincter pupillae tightens the pupil, causing constriction. So the nerve carrying this efferent, constricting signal is the oculomotor nerve. The other nerves listed are involved in moving the eye, not in transmitting the constriction signal to the pupil.

The essential idea is that constriction of the pupil in response to light is driven by parasympathetic output that travels from the brain to the iris. Light signals are detected by the retina and carried onward by the optic nerve, which provides the sensory (afferent) part of the reflex. But the actual signal that makes the pupil constrict follows a different route: the parasympathetic neurons originate in the Edinger–Westphal nucleus, travel with the oculomotor nerve, synapse in the ciliary ganglion, and then reach the sphincter pupillae muscle via short ciliary nerves. Activation of the sphincter pupillae tightens the pupil, causing constriction. So the nerve carrying this efferent, constricting signal is the oculomotor nerve. The other nerves listed are involved in moving the eye, not in transmitting the constriction signal to the pupil.