In the pupillary light reflex, which statement best describes the direct response?

• A. Direct response is constriction of the pupil in the eye that receives light.
• B. Direct response is constriction of the opposite pupil.
• C. Direct response involves dilation.
• D. Direct response only occurs in the pupil opposite the stimulated eye.

Answer: (A)

The direct pupillary light reflex is the constriction of the pupil in the eye that is exposed to the light. When light hits the retina, signals travel via the optic nerve to the midbrain, where the pretectal area sends information to both Edinger–Westphal nuclei. Parasympathetic fibers from these nuclei travel with the oculomotor nerves to the sphincter pupillae muscle, causing the pupil of the illuminated eye to constrict. Because the pathway projects bilaterally, the other pupil also constricts (the consensual response), but the term “direct” specifically refers to the eye that received the light. The option describing dilation is incorrect since the reflex is a constriction driven by parasympathetic activity, not sympathetic. It’s also not limited to the opposite pupil; the direct response is in the stimulated eye, with a concurrent consensual response in the other eye.