The oculomotor nerve (CN III) is a mixed cranial nerve containing motor fibers and parasympathetic fibers.

The general somatic efferent (motor) fibers of the oculomotor nerve supply the levator palpebrae superioris muscle and four of the other six extraocular muscles: medial rectus muscle, superior rectus muscle, and inferior rectus muscle, and the inferior oblique muscle.

The oculomotor nerve also conveys preganglionic parasympathetic fibers to synapse in the ciliary ganglion located within the bony orbit, which then provides postganglionic parasympathetic fibers to intra-ocular muscles (sphincter pupillae muscle and ciliary muscle), which enable pupillary constriction and accommodation.

The oculomotor nerve fibers are axons that originate from neurons in two nuclei located in the posteromedial aspect of the midbrain at the level of the superior colliculus:

Oculomotor nucleus - a somatic motor nucleus which provides the oculomotor nerve with general somatic efferent fibers to the extrinsic muscles of the eye;
Accessory nuclei of the oculomotor nerve - parasympathetic nuclei, among which the clinically most important one is the Edinger-Westphal nucleus; the accessory nuclei give rise to preganglionic parasympathetic (general visceral efferent) fibers of the oculomotor nerve innervating intrinsic muscles of the eye.

The oculomotor nerve appears outside on the anterior aspect of the midbrain, emerging on the oculomotor sulcus passing along the interpeduncular fossa between the cerebral peduncles. The third cranial nerve then travels forward below the posterior cerebral artery and above the superior cerebellar artery. In the subarachnoid space, it goes medial to the trigeminal nerve (CN V) and then pierces the dura mater and continues in ventral direction inside the lateral wall of the cavernous sinus.

The oculomotor nerve leaves the cranial cavity and enters the orbit through the superior orbital fissure dividing into two - superior and inferior - divisions that run beneath the trochlear nerve (CN IV) and ophthalmic nerve (CN V1).

The superior division of the oculomotor nerve contains only motor fibers. The division passes above the optic nerve (CN II) to supply the superior rectus muscle, also giving off a branch that innervates the levator palpebrae superioris muscle.
The inferior division of the oculomotor nerve contains motor and parasympathetic fibers. This division separates into medial, central and lateral branches.
- The medial branch enters the ocular surface of the medial rectus muscle innervating it.
- The central branch enters the ocular surface of the inferior rectus muscle.
- And the lateral branch of the inferior division of the oculomotor nerve enters the orbital surface of the inferior oblique muscle and communicates with the ciliary ganglion to distribute parasympathetic fibers to the sphincter pupillae (providing constriction of the pupil) and to the ciliary muscle (accommodation of the lens).

Note: Injury to this nerve can result in a dilated pupil if the parasympathetic fibers are damaged.

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Oculomotor nerve (CN III)