Oculomotor

The oculomotor nerve, or CN III, is the third cranial nerve. It provides motor innervation to most of the extraocular muscles and the levator palpebrae superioris, and carries parasympathetic fibers to the pupil and the lens.

Anatomy and pathways: The oculomotor nucleus, which gives rise to the somatic motor fibers, sits in the midbrain near the cerebral aqueduct. Parasympathetic fibers originate from the Edinger-Westphal nucleus. The fibers travel ventrally and emerge from the midbrain at the interpeduncular fossa, then pass between the posterior cerebral artery and the superior cerebellar artery. After traversing the subarachnoid space, the nerve enters the cavernous sinus and enters the orbit through the superior orbital fissure. In the orbit, it divides into a superior division (to levator palpebrae superioris and superior rectus) and an inferior division (to the medial rectus, inferior rectus, and inferior oblique muscles) plus the parasympathetic fibers to the sphincter pupillae and ciliary muscle via the ciliary ganglion.

Function: Somatic motor fibers innervate the four rectus muscles, the inferior oblique, and the levator palpebrae superioris. Parasympathetic fibers control pupil constriction (sphincter pupillae) and accommodation (ciliary muscle).

Clinical significance: Oculomotor dysfunction causes ophthalmoplegia and ptosis, with the eye often resting in a down-and-out position due to unopposed lateral rectus and superior oblique. Diplopia is common. Pupillary involvement suggests involvement of peripheral parasympathetic fibers (as in compressive lesions like a posterior communicating artery aneurysm), while pupil-sparing patterns are typical of microvascular ischemia affecting predominantly central somatic fibers. Evaluation includes testing eye movements, eyelid position, pupil reactivity, and accommodation. Causes include ischemic nerve palsy, aneurysm, trauma, demyelinating disease, and mass effect.