The oculomotor nerve is the third of the cranial nerves and arises from the midbrain. It is responsible for the movements of four of the six extraocular muscles, the other two being innervated by the trochlear and abducens nerves.
There are two cranial nerve nuclei whose neurons contribute axons to the oculomotor nerve:
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 (where it may be compressed by a posterior communicating artery aneurysm) and above the superior cerebellar artery, before piercing the dura mater to enter the cavernous sinus.
Cavernous sinus portion
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 and levator palpebrae superioris muscles which it supplies with motor fibers. Additionally, it also supplies the latter with 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.