Oculomotor nerve (III)

At a glance

English

Oculomotor nerve (III)

Latin (TA2)

Nervus oculomotorius (III)

System

Nerve

Region

Head

Side

Paired, left and right

About the oculomotor nerve (iii)

Cranial nerve III, the oculomotor nerve, does most of the work of moving the eye. Its somatic fibres supply four of the six extraocular muscles (superior, medial and inferior recti and inferior oblique) and the levator palpebrae superioris that raises the upper lid. Its parasympathetic fibres reach two muscles inside the eye: sphincter pupillae, which narrows the pupil, and the ciliary muscle, which lets the lens thicken for near focus.

It starts from two nuclei in the upper midbrain, level with the superior colliculus, in the grey matter in front of the aqueduct: the main oculomotor nucleus and the parasympathetic accessory (Edinger-Westphal) nucleus. The fibres sweep forwards through the tegmentum, the red nucleus and the substantia nigra and emerge in the interpeduncular fossa on the inner side of the cerebral peduncle. The nerve then passes forwards between the posterior cerebral artery above and the superior cerebellar artery below, close to the posterior communicating artery, and pierces the dura beside the posterior clinoid process between the free and attached edges of the tentorium. It runs high in the outer wall of the cavernous sinus, uppermost of the nerves bound for the orbit, where sympathetic fibres from the carotid plexus join it.

Just before or within the superior orbital fissure it divides, inside the common tendinous ring. The smaller superior division passes above the optic nerve to superior rectus and levator palpebrae; the sympathetic fibres it carries reach the superior tarsal muscle. The larger inferior division sends twigs to medial rectus, beneath the optic nerve, and to inferior rectus, while its longest branch runs forward between inferior and lateral recti to inferior oblique. That branch also sends a short, thick parasympathetic root to the ciliary ganglion, whose short ciliary nerves carry the pupil and lens fibres into the eyeball.

A complete palsy leaves the eye down and out, the lid drooping and the pupil wide and unreactive. The pupil fibres lie near the surface of the nerve, so compression by an aneurysm of the posterior communicating artery or by the uncus in brain herniation tends to dilate the pupil early, and a palsy with a dilated pupil must be treated as an aneurysm until shown otherwise. Small-vessel disease, as in diabetes, damages the core of the nerve and usually spares the pupil.

Course and supply

Arises from

The oculomotor nucleus and accessory (Edinger-Westphal) nucleus in the midbrain; the nerve emerges in the interpeduncular fossa, medial to the cerebral peduncle.

Ends in

Divides at the superior orbital fissure into a superior division to superior rectus and levator palpebrae and an inferior division to the medial and inferior recti and inferior oblique, with a parasympathetic root to the ciliary ganglion.

Supplies

Motor to superior, medial and inferior recti, inferior oblique and levator palpebrae superioris; parasympathetic, via the ciliary ganglion, to sphincter pupillae and the ciliary muscle for pupil constriction and accommodation.