The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation (ability to focus on near objects as in reading). The oculomotor nerve is derived from the basal plate of the embryonicmidbrain. Cranial nerves IV and VI also participate in control of eye movement.[1]
Structure
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 ventral to the cerebral aqueduct, on the pre-aqueductal grey matter. The fibers from the two third nerve nuclei located laterally on either side of the cerebral aqueduct then pass through the red nucleus. From the red nucleus fibers then pass via the substantia nigra[citation needed] to emerge from the substance of the brainstem at the oculomotor sulcus (a groove on the lateral wall of the interpeduncular fossa).[2]
It traverses the cavernous sinus, above the other orbital nerves receiving in its course one or two filaments from the cavernous plexus of the sympathetic nervous system, and a communicating branch from the ophthalmic division of the trigeminal nerve. As the oculomotor nerve enters the orbit via the superior orbital fissure it then divides into a superior and an inferior branch.[1]
The third and longest runs forward between the inferior recti and lateralis to the inferior oblique.
From the third one, 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.
Nuclei
The oculomotor nerve (CN III) arises from the anterior aspect of the mesencephalon (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.
Function
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.
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 to 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).[3]
Examination
Eye muscles
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. Loss of function of any of the eye muscles results in ophthalmoparesis.
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 its down and 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, their pupils should constrict.
Shining a light into one eye should result in equal constriction of the other eye. Fibers from the optic nerves cross 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 of accommodation and continued pupillary dilation can indicate the presence of a lesion on the oculomotor nerve.
Additional images
Map of the oculomotor nerve.
Median sagittal section of brain.
Plan of oculomotor nerve.
Pathways in the Ciliary Ganglion.
Cross-sectional anatomy of the midbrain showing location of the nucleus of the oculomotor nerve and the Edinger-Westphal nucleus
^ abVilensky, Joel; Robertson, Wendy; Suarez-Quian, Carlos (2015). The Clinical Anatomy of the Cranial Nerves: The Nerves of "On Olympus Towering Top". Ames, Iowa: Wiley-Blackwell. ISBN978-1-118-49201-7.[page needed]
