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The oculomotor nerve is the third (CN III) cranial nerve (TA: nervus oculomotorius or nervus cranialis III). It is a mixed nerve containing motor, parasympathetic and sympathetic fibers. It arises from the midbrain and passes through the cavernous sinus to the orbit where it is responsible for the movements of four of the six extraocular muscles (superior, medial and inferior rectus, and inferior oblique muscles) as well as levator palpebrae superioris and superior tarsal muscle.
Nucleus and intraparenchymal portion
There are two cranial nerve nuclei whose neurons contribute axons to the oculomotor nerve:
The oculomotor nucleus contributes somatic efferent fibers to the oculomotor nerve. It lies in the midbrain anterior to the periaqueductal grey matter at the level of the superior colliculus anterior to the cerebral aqueduct. The fibers run through the tegmentum, red nucleus and medial aspect of the substantia nigra.
The Edinger-Westphal nucleus contributes parasympathetic fibers to the oculomotor nerve which synapse at the ciliary ganglion. It is located immediately posterior to the oculomotor nucleus and fibers course through the tegmentum in a similar course to those from the oculomotor nucleus.
The nerve emerges from the medial aspect of the cerebral peduncle to enter the interpeduncular cistern. Traveling forwards it passes below the posterior cerebral artery and above the superior cerebellar artery (where it may be compressed by a posterior communicating artery aneurysm), before piercing the dura mater to enter the cavernous sinus.
Cavernous sinus portion
Within the cavernous sinus the oculomotor nerve is located uppermost, above the trochlear nerve in the lateral wall of the sinus. As it passes through the cavernous sinus it receives sympathetic fibers from the carotid plexus originating from the superior cervical ganglion.
It enters the orbit via the superior orbital fissure as two branches: superior division and inferior division, with the nasociliary nerve (a branch of the ophthalmic division of the trigeminal nerve) between them and the abducens nerve (CN VI) below all three. These four branches pass through the tendinous ring.
The superior division, the smaller of the two, runs above the optic nerve and gives branches to superior rectus, levator palpebrae superioris and superior tarsal muscle muscles. The former two it supplies with motor fibers while the superior tarsal muscle (sometimes considered part of levator palpebrae superiors) receives sympathetic fibers derived from the internal carotid artery.
The inferior division supplies the inferior rectus, medial rectus (this branch passes below the optic nerve), and the inferior oblique. It also gives off the parasympathetic root to the ciliary ganglion.
The oculomotor nerve is readily identified on MRI as it emerges from the midbrain in the interpeduncular fossa. On coronal images, it can be followed forwards between the superior cerebellar artery below and the posterior cerebral artery above into the cavernous sinus. A thin sleeve of CSF can be seen surrounding it for some distance in the cavernous sinus on most studies.