Oculomotor Nerve Definition:
is the 3rd of 12 paired cranial nerves. 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. It is somatovisceral nerve.
Oculomotor Nerve Nuclei:
- 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 fibres 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.
Oculomotor Nerve Superficial Origin:
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, receiving in its course one or two filaments from the cavernous plexus of the sympathetic, and a communicating branch from the ophthalmic division of thetrigeminal.
Oculomotor Nerve Course:
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 the pass via substantia nigra exiting through the interpeduncular fossa. The oculomotor nerve via superior orbital fissure then innervate (terminating) to the:
1) Superior Rectus muscle (extraocular muscle)
2) Inferior Rectus muscle (extraocular muscle)
3) Medial Rectus muscle (extraocular muscle)
4) Inferior Oblique (extraocular muscle)
5) Levator palpebrae superioris (muscle to upper eye lid)
Oculomotor Nerve Divisions:
Superior and inferior rami
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:
- superior branch of oculomotor nerve:
the smaller, passes medialward over the optic nerve.It supplies the Superior rectus and Levator palpebrae superioris.
- inferior branch of oculomotor nerve:
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.
Oculomotor Nerve Testing:
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 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.
Oculomotor nerve palsy:
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)
- due to a space-occupying lesion (e.g., brain cancer) or a
- spontaneous subarachnoid haemorrhage (e.g., berry aneurysm), and
- 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).
A complete Oculomotor nerve palsy will result in a characteristic down and out position in the affected eye. The eye will be displaced outward and displaced downward; outward because the lateral rectus (innervated by the sixth cranial nerve) maintains muscle tone in comparison to the paralyzed medial rectus. The eye will be displaced downward, because the superior oblique (innervated by the fourth cranial or trochlear nerve), is unantagonized by the paralyzed superior rectus andinferior oblique. The affected individual will also have a ptosis, or drooping of the eyelid, and mydriasis (pupil dilation).
It should be borne in mind, however, that the branched structure of the oculomotor nerve means that damage sustained at different points along its pathway, or damage caused in different ways (compression versus loss of blood supply, for example), will result in different muscle groups or, indeed, different individual muscles being affected, thus producing different presentation patterns.
Compressive oculomotor nerve damage could result in compression of the parasympathetic fibers before any disruption of the motor fibers occurs, since the parasympathetic fibers run on the outside of the nerve. Therefore, one could have lid ptosis and mydriasis (a “blown” pupil) as a result of parasympathetic fiber compression before the “down and out” position is seen.
Oculomotor Nerve Videos:
Oculomotor nerve explained: origin, course, innervation, lesion n tests
Cranial Nerve 3 Palsy
An excerpt from a neuro-ophthalmology lecture. This section covers the oculomotor nerve (CN3).
3rd Nerve Palsy
A short video demonstrating how to assess a patient’s 3rd nerve palsy.
Oculomotor Nerve Nuclei ,Origin,Course,Branches,Testing and palsy