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|Nerve: Oculomotor nerve|
Nerves of the orbit. Seen from above.
Inferior view of the human brain, with the cranial nerves labelled.
|Innervates||Superior rectus, Inferior rectus, Medial rectus, Inferior oblique, Levator palpebrae, sphincter pupillae (parasympathetics), ciliaris muscle (parasympathetics)|
|From||oculomotor nucleus, Edinger-Westphal nucleus|
|To||superior branch, inferior branch|
The oculomotor nerve is the third cranial nerve. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the levator palpebrae superioris muscle. The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in control of eye movement.
The oculomotor nerve originates from the third nerve nucleus at the level of the superior colliculus, in the midbrain. The third nerve nucleus is located lateral to the cerebral aqueduct, on the pre-aqueductal grey matter. The fibers from the two third nerve nucleus located laterally on either side of the cerebral aqueduct then pass through the red nucleus. From the red nucleus fibers then pass via substantia nigra exiting through the interpeduncular fossa.
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 anterior 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 (the trochlear, ophthalmic and maxillary 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.
The inferior branch of the oculomotor nerve 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.
The oculomotor nerve (CN III) arises from the anterior aspect of mesencephalon (midbrain). There are two nuclei for the oculomotor nerve:
- The oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the striated muscle in levator palpebrae superioris and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle.
- The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion, and thus controls the sphincter pupillae muscle (effecting pupil constriction) and the ciliary muscle (affecting accommodation).
Sympathetic postganglionic fibres also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, e.g., to the smooth muscle of superior tarsal (Mueller's) muscle.
The oculomotor nerve include axons of type GSE, general somatic efferent, which innervate skeletal muscle of the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles.
The nerve also includes axons of type GVE, general visceral efferent, which provide preganglionic parasympathetics to the ciliary ganglion.
Paralysis of the oculomotor nerve, i.e., oculomotor nerve palsy, can arise due to:
- direct trauma,
- demyelinating diseases (e.g., multiple sclerosis),
- increased intracranial pressure (leading to uncal herniation)
- microvascular disease, e.g., diabetes.
In people with diabetes and older than 50 years of age, an oculomotor nerve palsy, in the classical sense, occurs with sparing (or preservation) of the pupillary reflex. This is thought to arise due the anatomical arrangement of the nerve fibers in the oculomotor nerve; fibers controlling the pupillary function are superficial and spared from ischemic injuries typical of diabetes. On the converse, an aneurysm which leads to compression of the oculomotor nerve affects the superficial fibers and manifests as a third nerve palsy with loss of the pupillary reflex (in fact, this third nerve finding is considered to represent an aneurysm—until proven otherwise—and should be investigated).
Cranial nerves III, IV, and VI are usually tested together as part of the cranial nerve examination. 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 movement 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 also known by the down 'n out symptoms, because of the position of the affected eye (lateral, downward deviation of gaze).
The oculomotor nerve also controls the constriction of the pupils and thickening of the lens of the eye. This can be tested in two main ways. By moving a finger toward a person's face to induce accommodation, as well as his going cross-eyed, his pupils should constrict.
Shining a light into one eye should result in equal constriction of the other eye. The neurons in the optic nerve decussate in the optic chiasm with some crossing to the contralateral optic nerve tract. This is the basis of the "swinging-flashlight test".
Loss of accommodation and continued pupillary dilation can indicate the presence of a lesion on the oculumotor nerve.
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This article uses anatomical terminology; for an overview, see anatomical terminology.
- Goodwin J. Oculomotor Nerve Palsy. eMedicine.com. URL:http://emedicine.medscape.com/article/1198462-overview. Accessed on: January 16, 2009.
|Wikimedia Commons has media related to Nervus oculomotorius.|
- MedEd at Loyola GrossAnatomy/h_n/cn/cn1/cn3.htm
- oph/183 at eMedicine - "Oculomotor nerve palsy"
- Oculomotor Nerve at the US National Library of Medicine Medical Subject Headings (MeSH)
- NeuroNames hier-479
- Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)
- cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (III)
- Notes on Oculomotor Nerve