The Oculomotor Nerve (CN III)
The oculomotor nerve is the third cranial nerve (CNIII). It offers motor and parasympathetic innervation to many of the ocular structures.
In this article we shall look at the anatomical course, functions and clinical correlations of the oculomotor nerve.
Motor: Innervates a number of the extraocular muscles.
Parasympathetic: Supplies the sphincter pupillae and the ciliary muscles of the eye.
Sympathetic: No direct function, but sympathetic fibres run with the oculomotor nerve to innervate the superior tarsal muscle (helps to raise the eyelid).
The oculomotor nerve originates from the anterior aspect of the midbrain. It moves anteriorly, passing below the posterior cerebral artery, and above the superior cerebellar artery. The nerve pierces the dura mater and enters the lateral aspect of the cavernous sinus. Within the cavernous sinus, it receives sympathetic branches from the internal carotid plexus. These fibres do not combine with the oculomotor nerve – they merely travel within its sheath.
The nerve leaves the cranial cavity via the superior orbital fissure. At this point, it divides into superior and inferior branches. Once within the orbital cavity, both branches innervate accessory structures of the eye:
- Superior branch: Motor innervation to the superior rectus and levator palpabrae superioris. Sympathetic fibres run with the superior branch to innervate the superior tarsal muscle.
- Inferior branch: Motor innervation to the inferior rectus, medial rectus and inferior oblique. Parasympathetic fibres to the ciliary ganglion, which ultimately innervates the sphincter pupillae and ciliary muscles.
The oculomotor nerve innervates the majority of the extra-ocular muscles. These muscles move the eyeball and upper eyelid.
- Superior rectus – Elevates the eyeball
- Levator palpabrae superioris – Raises the upper eyelid.
There are sympathetic fibres that run with the superior branch of the oculomotor nerve. They innervate the superior tarsal muscle, which acts to keep the eyelid elevated after the levator palpabrae superioris has raised it.
- Inferior rectus – Depresses the eyeball
- Medial rectus – Adducts the eyeball
- Inferior oblique – Elevates, abducts and laterally rotates the eyeball
There are two structures in the eye that receive parasympathetic innervation from the oculomotor nerve:
- Sphincter pupillae – Constricts the pupil, reducing the amount of light entering the eye.
- Ciliary muscles – Contracts, causes the lens to become more spherical, and thus more adapted to short range vision.
The parasympathetic fibres travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The fibres are carried from the ganglion to the eye via the short ciliary nerves.
Clinical Relevance: Oculomotor Nerve Lesion
There are three main anatomical causes of an oculomotor nerve lesion:
- Increasing intracranial pressure – this compresses the nerve against the temporal bone.
- Aneurysm of the posterior cerebral artery.
- Cavernous sinus infection or trauma.
Note: there are other pathological causes of oculomotor nerve palsy such as diabetes, multiple sclerosis, myasthenia gravis and giant cell arteritis.
The oculomotor nerve is the major nerve supplying the ocular and extraocular muscles. The clinical signs of CN III injury are all associated with the eye:
- Ptosis (drooping upper eyelid) – due to paralysis of the levator palpabrae superioris.
- Eyeball resting in the ‘down and out‘ location – due to the paralysis of the superior, inferior and medial rectus, and the inferior oblique. The patient is unable to elevate, depress or adduct the eye.
- Dilated pupil – due to the unopposed action of the dilator pupillae muscle.