oculomotor nerve (CN III)

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.

Gross anatomy

Nuclei

There are two cranial nerve nuclei whose neurons contribute axons to the oculomotor nerve:

  • The oculomotor nucleus 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 a small nucleus located between the oculomotor nucleus and the periaqueductal grey matter.
  • Cisternal portion

    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

    Within the cavernous sinus the oculomotor nerve is located uppermost, above the trochlear nerve in the lateral wall of the sinus.

    Orbital 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.

    Superior division

    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.

    Inferior division

    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.

    Related pathology