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| Oculomotor nerve | |
|---|---|
 Nerves of the orbit. Seen from above. | |
 Inferior view of the human brain, with the cranial nerves labelled. | |
| Details | |
| From | Oculomotor nucleus,Edinger-Westphal nucleus |
| To | Superior branch,inferior branch |
| Innervates | Superior rectus,inferior rectus,medial rectus,inferior oblique,levator palpebrae superioris,sphincter pupillae (parasympathetics),ciliaris muscle (parasympathetics) |
| Identifiers | |
| Latin | nervus oculomotorius |
| MeSH | D009802 |
| NeuroNames | 488 |
| TA98 | A14.2.01.007 |
| TA2 | 6187 |
| FMA | 50864 |
| Anatomical terms of neuroanatomy | |
| Cranial nerves |
|---|
|
Theoculomotor nerve, also known as thethird cranial nerve,cranial nerve III, or simplyCN III, is acranial nerve that enters theorbit through thesuperior orbital fissure and innervatesextraocular muscles that enable mostmovements of the eye and that raise the eyelid. The nerve also contains fibers that innervate theintrinsic eye muscles that enable pupillary constriction and accommodation (ability to focus on near objects as in reading). The oculomotor nerve is derived from thebasal plate of theembryonicmidbrain. Cranial nervesIV andVI also participate in control ofeye movement.[1]
The oculomotor nerve originates from the third nervenucleus at the level of thesuperior colliculus in the midbrain. The third nerve nucleus is located ventral to thecerebral aqueduct, on the pre-aqueductalgrey matter. The fibers from the two third nerve nuclei located laterally on either side of the cerebral aqueduct then pass through thered nucleus. From the red nucleus fibers then pass via thesubstantia nigra[citation needed] to emerge from the substance of the brainstem at the oculomotor sulcus (a groove on the lateral wall of theinterpeduncular fossa).[2]
On emerging from thebrainstem, the nerve is invested with a sheath ofpia mater, and enclosed in a prolongation from thearachnoid. It passes between thesuperior cerebellar (below) andposterior cerebral arteries (above), and then pierces the dura mater anterior and lateral to theposterior clinoid process, passing between the free and attached borders of thetentorium cerebelli.[citation needed]
It traverses thecavernous sinus, above the other orbital nerves receiving in its course one or two filaments from thecavernous plexus of the sympathetic nervous system, and a communicating branch from theophthalmic division of thetrigeminal nerve. As the oculomotor nerve enters the orbit via thesuperior orbital fissure it then divides into a superior and an inferior branch.[1]
Thesuperior branch of the oculomotor nerve or thesuperior division, the smaller, passes medially over theoptic nerve. It supplies thesuperior rectus andlevator palpebrae superioris.
Theinferior branch of the oculomotor nerve or theinferior division, the larger, divides into three branches.
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 themesencephalon (midbrain). There are two nuclei for the oculomotor nerve:
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 includes 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. (Innervates all the extrinsic muscles except superior oblique and lateral rectus.)
The nerve also includes axons of type GVE,general visceral efferent, which provide preganglionic parasympathetics to the ciliary ganglion. From the ciliary ganglion postganglionic fibers pass through the short ciliary nerve to the constrictor pupillae of the iris and the ciliary muscles.
Paralysis of the oculomotor nerve, i.e.,oculomotor nerve palsy, can arise due to:
In people withdiabetes 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 to the anatomical arrangement of thenerve fibers in the oculomotor nerve; fibers controlling the pupillary function are superficial and spared fromischemic 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).[3]
Cranial nerves III, IV, and VI are usually tested together as part of thecranial 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 theeye movement andeyelids, the examiner is able to obtain more information about theextraocular muscles, thelevator palpebrae superioris muscle, and cranial nerves III, IV, and VI. Loss of function of any of the eye muscles results inophthalmoparesis.
Since the oculomotor nerve controls most of the eye muscles, it may be easier to detect damage to it. Damage to this nerve, termedoculomotor nerve palsy, is known by itsdown and out symptoms, because of the position of the affected eye (lateral, downward deviation of gaze).
The oculomotor nerve also controls the constriction of thepupils 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 induceaccommodation, their pupils should constrict.
Shining a light into one eye should result in equal constriction of the other eye. Fibers from the optic nervescross over in the optic chiasm with some fibers passing to the contralateral optic nerve tract. This is the basis of the "swinging-flashlight test".
Loss ofaccommodation and continued pupillary dilation can indicate the presence of a lesion on the oculomotor nerve.
This article incorporates text in thepublic domain frompage 884 of the 20th edition ofGray's Anatomy(1918)
