Which cranial nerve primarily mediates most eye movements, eyelid elevation, and pupil constriction?

• A. Oculomotor (CN III)
• B. Trochlear (CN IV)
• C. Abducens (CN VI)
• D. Trigeminal (CN V)

Answer: (B)

The nerve that coordinates most eye movements, lifts the upper eyelid, and constricts the pupil is the oculomotor nerve. It supplies the majority of the extraocular muscles (medial, superior, and inferior rectus; and inferior oblique), allowing a wide range of gaze directions. It also innervates the levator palpebrae superioris to raise the eyelid. In addition, it carries parasympathetic fibers to the sphincter pupillae, causing pupil constriction, and to the ciliary muscle, enabling lens accommodation for near vision. The other nerves have more limited roles: the trochlear nerve moves the superior oblique muscle, the abducens nerve moves the lateral rectus for outward gaze, and the trigeminal nerve mainly provides facial sensation and motor to muscles of mastication, not eye movements.