Oculomotor nerve

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Nerve: Oculomotor nerve
Image:Gray777.png
Nerves of the orbit, and the ciliary ganglion. Side view.

Plan of oculomotor nerve.
Latin	N. Oculomotorius
Gray's	subject #198 884
Innervates	Superior rectus, Inferior rectus, Medial rectus, Inferior oblique, Ciliary
MeSH	A08.800.800.120.600

The oculomotor nerve is the third of twelve paired cranial nerves. It controls most of the eye movements (cranial nerves IV and VI also do some), constriction of the pupil, and holding the eyelid open.

The oculomotor nerve arises from the anterior aspect of mesencephalon (midbrain), the oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the ciliary muscle (affecting accommodation), and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle. In addition, it supplies parasympathetic fibres - which originate in the Edinger-Westphal nucleus - to the eye via the ciliary ganglion, and thus controls pupil constriction.

1 Anatomy of the oculomotor nerve
2 Testing the oculomotor nerve
3 Additional images
4 See also
5 External links

[edit] Anatomy of the oculomotor nerve

On emerging from the brain, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid.

It passes between the superior cerebellar (below) and posterior cerebral arteries (above), and then pierces the dura mater in front of and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli.

It runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal.

It then divides into two branches, which enter the orbit through the superior orbital fissure, between the two heads of the lateral rectus.

Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami.

The superior ramus or the superior division, the smaller, passes medialward over the optic nerve, and supplies the superior rectus and Levator palpebræ superioris.

The inferior ramus or the inferior division, the larger, divides into three branches.
- One passes beneath the optic nerve to the medial rectus.
- Another, to the inferior rectus.
- The third and longest runs forward between the inferior recti and lateralis to the inferior oblique.

From the last a short thick branch is given off to the lower part of the ciliary ganglion, and forms its short root.

All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the inferior oblique, which enters the muscle at its posterior border.

[edit] Testing the oculomotor nerve

Cranial nerves III, IV and VI are usually tested together. The examiner typically instructs the patient to hold his head still and follow only with the eyes a finger or penlight that circumscribes a large "H" in front of the patient. By observing the eye movements and eyelids, the examiner is able to obtain more information about the extraocular muscles, the levator palpebrae superioris muscle, and cranial nerves III, IV, and VI.

Since the oculomotor nerve controls most of the eye muscles, it may be easier to detect damage to it. Damage to this nerve, termed oculomotor nerve palsy is known by the down n' out symptoms. The affected eye will be looking downward, because the superior oblique (innervated by CN IV), is unantagonized by the paralyzed superior rectus and inferior oblique and looking outwards, because the lateral rectus (innervated by CN VI) is unantagonized by the paralyzed medial rectus. There will also be strabismus.

They may show a degree of ptosis, or drooping of the eyelid, because the levator palpebrae superioris muscle (eyelid lifting muscle) is also innervated by the oculomotor nerve.

The oculomotor nerve also controls the constriction of the pupils. This can be tested in two main ways. By moving a finger towards a person's face to induce accommodation, as well as them going cross-eyed, their pupils should constrict.

Shining a light into their eyes should also make their pupils constrict. Both pupils should constrict at the same time, independent of what eye the light is actually shone on.