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Oculomotor Nerve is the third cranial nerve (CN III). It enters the orbit via the superior orbital fissure and innervates extrinsic eye 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 embryonic midbrain. Cranial nerves IV and VI also participate in the control of eye movement.[rx]
The oculomotor nerve (the third cranial nerve; CN III) has three main motor functions:
- Innervation to the pupil and lens (autonomic, parasympathetic)
- Innervation to the upper eyelid (somatic)
- Innervation of the eye muscles that allow for visual tracking and gaze fixation (somatic)
Like all other nerve fibers in the human body, the oculomotor nerve can become impaired in disease states which can lead to lifelong impairment in normal vision. Dysfunction can also be indicative of more serious underlying diseases, such as an aneurysm or a neoplasm.
The oculomotor nerve originates from 2 nuclei in the midbrain[rx]:
- Oculomotor nucleus
- Accessory parasympathetic nucleus (Edinger-Westphal nucleus)
The oculomotor nerve exits the brainstem near midline at the base of the midbrain just caudal to the mammillary bodies. It passes through the cavernous sinus and proceeds through the supraorbital fissure to reach the orbit of the eye
The third cranial nerve has both somatic and autonomic fibers. Somatic (voluntary) nerve fibers are bundled deep inside the nerve, while the autonomic (involuntary) fibers surround the somatic fibers around the outside of the nerve. Knowing the spatial layout of these fibers will help one understand the various forms of presentation in third nerve palsies.
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 exiting through the interpeduncular fossa.
On emerging from the brainstem, 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 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.[rx]
The superior branch of the oculomotor nerve or the superior division, the smaller, passes medially over the optic nerve. It supplies the superior rectus and levator palpebrae superioris.
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 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.
The oculomotor nerve (CN III) arises from the anterior aspect of the 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 (affecting 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.
Somatic (voluntary) functions of the oculomotor nerve include elevation of the upper eyelid via innervation of the levator palpebrae superioris muscle. Other essential functions include coordination of eye muscles for visual tracking and gaze fixation. These functions of eye movement occur through innervation of four eye muscles:
- Superior rectus muscle – elevates the eye while looking straight ahead (primary position)
- Medial rectus muscle – adducts the eye from a primary position
- Inferior rectus muscle – moves the eye down from a primary position
- Inferior oblique muscle – elevates the eye when the eye is adducted from a primary position
Blood Supply and Lymphatics
The somatic and autonomic components of the oculomotor nerve have differentiated vascular supplies. The vasa vasorum supplies the inner somatic (voluntary) nerve fibers while pia mater blood vessels supply the outer autonomic nerve fibers.[rx]
Lymphatic drainage of the orbit of the eye is not yet well understood. There is morphological evidence for lymphatic structures within the lacrimal gland and arachnoid; however, distinctive lymphatic structures within other regions of the orbit are currently unknown.[rx]
Both the oculomotor nucleus and the Edinger-Westphal nucleus are located in the medial midbrain, which is supplied by the paramedian branches of the upper basilar artery and the proximal posterior cerebral artery.
The oculomotor nerve helps to adjust and coordinate eye position during movement. Several movements assist with this process: saccades, smooth pursuit, fixation, accommodation, vestibular-ocular reflex, and optokinetic reflex.
Saccades are rapid, jerky motions of the eye. This type of motion typically occurs when moving a gaze between objects. When tracking an object, we use smooth pursuit to keep our eyes focused on an object as it moves. When we want to stare at an object, we fixate on that object. Control of each of these actions is by either vestibule-ocular or optokinetic reflexes.
The vestibule-ocular reflex adjusts eye position during fast movements of the head. Head motion activates cells within the semicircular canals. Information about the motion is transmitted to the ipsilateral vestibular nucleus and forwarded to the oculomotor nucleus. Therefore, if the head moves to the left, the eyes move to the right to keep the gaze steady.
The optokinetic reflex adjusts eye position in response to changes in the visual field. This reflex initiates from visual information. Information about the visual field is transmitted from the parieto-occipital eye field through the pontine nuclei to the vestibulo-cerebellum. It then moves through the vestibular nuclei to the oculomotor nucleus where eye movement initiates. This reflex allows the eyes to follow large objects in the visual field.
The last action is accommodation. This action helps to keep the gaze focused when both vestibular and visual stimuli change. It requires the interaction of circuits between the vestibule-ocular and optokinetic reflexes.[rx][rx]
Deficits within the peripheral or central vestibular system will result in inappropriate quick gaze deviation movements of the eyes “nystagmus.” The direction of the last phase of the nystagmus can assist clinicians in the diagnosis of a patient’s pathology.
The oculomotor nerve controls several muscles:
- Levator palpebrae superioris – raises the upper eyelid
- Superior rectus muscle – rotates the eyeball backward, “looking up”
- Medial rectus muscle – adducts the eye, “looking towards your nose”
- Inferior rectus muscle – rotates the eyeball forwards, “looking down”
- Inferior oblique muscle – rotates the eyeball backward when the eye is adducted
- Ciliary muscle – controls lens shape to focus on up-close objects
- Sphincter pupillae – constricts the pupil
Control of other eye muscles is by the trochlear (CN IV) and abducens (CN VI) nerves.
The trochlear nerve (4th cranial nerve) controls:
- Superior oblique muscle – rotates the eyeball forward when the eye is adducted
The abducens nerve (6th cranial nerve) controls:
- Lateral rectus muscle – abducts the eye, “looking towards your ear on the same side”
There are two primary functions of the autonomic parasympathetic (involuntary) oculomotor nerve. It constricts the pupil (miosis) by innervating the smooth muscle (sphincter pupillae) near the pupil. It also innervates the ciliary muscles. These muscles connect the iris to the choroid. Contraction of the muscle alters the curvature of the lens which allows individuals to focus the lens on near objects.[rx]
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 parasympathetic to the ciliary ganglion. From the ciliary ganglion postganglionic fibers pass through the short ciliary nerve to the constrictor papillae of the iris and the ciliary muscles.
The oculomotor nerve innervates many of the extraocular muscles. These muscles move the eyeball and upper eyelid.
- Superior rectus – elevates the eyeball
- Levator palpebrae superiors – raises the upper eyelid.
Additionally, there are sympathetic fibers that travel with the superior branch of the oculomotor nerve. They innervate the superior tarsal muscle, which acts to keep the eyelid elevated after the levator palpebrae superiors have 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 pre-ganglionic parasympathetic fibers travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The post-ganglionic fibers are carried to the eye via the short ciliary nerves.
Somatic motor function
These nerve axons will arise from the oculomotor nucleus and innervate skeletal muscles associated with the eye. There are seven extrinsic eye muscles (muscles that lay outside of the eye itself) that move the superior eyelid and the eyeball.
Visceral motor function
The visceral motor axons of the oculomotor nerve are part of the autonomic nervous system, specifically the parasympathetic division. They will arise from the Edinger-Westphal nucleus and innervate two separate intrinsic muscles within the eye. These will constrict the pupil and cause accommodation of the lens of the eye respectively.