Which cranial nerves and their roles comprise the near triad?

• A. CN II provides parasympathetic constriction and accommodation; CN III provides afferent visual input; convergence by CN II.
• B. CN II provides vision input only; CN IV primarily mediates near response.
• C. CN III provides parasympathetic constriction and accommodation; CN II provides vision input for focusing; convergence is mediated by CN III.
• D. CN III has no role in accommodation.

Answer: (C)

The near triad is the coordinated set of responses when you focus on a near object: accommodation (ciliary muscle contracts to thicken the lens), convergence (eyes turn inward via the medial rectus muscles), and pupil constriction (sphincter pupillae). The motor control for accommodation and pupil constriction, as well as convergence, comes from the oculomotor nerve. Its parasympathetic fibers travel from the Edinger-Westphal nucleus to the ciliary ganglion and then to the ciliary muscle and sphincter pupillae, enabling both lens focusing and pupil constriction; the same nerve also innervates the medial rectus to drive convergence. The sensory input that helps trigger this reflex comes from the retina and is carried by the optic nerve, CN II, which provides vision input rather than performing the motor actions. The trochlear nerve isn’t involved in the near response. So the correct description is that the near triad relies on CN III for parasympathetic constriction, accommodation, and convergence, with CN II providing the vision input to initiate the reflex.