The oculomotor nerve is the third of twelve paired cranial nerves. It controls most of the eye's movement, constriction of the pupil, and maintains an open eyelid. (Note: cranial nerves IV and VI also participate in control of eye movement.)
Path
Nuclei
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 fibres 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 levator palpebrae superioris.
Emergence from brain
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 the trigeminal.
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
inferior branch of oculomotor nerve
Testing the oculomotor nerve
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.
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 towards a person's face to induce
accommodation, as well as them going cross-eyed, their pupils should constrict.
Shining a light into their eyes should also make their pupils constrict. Both pupils should constrict at the same time, independent of what eye the light is actually shone on.
Pathology
Paralysis of the oculomotor nerve, i.e.
oculomotor nerve palsy, is a rare condition. It 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, classically, 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. Conversely, a subarachnoid haemorrhage, which leads to compression of the oculomotor nerve, usually affects the superficial fibers and manifests as a palsy with loss of the pupillary reflex.
Additional images
References
See also
Anisocoria
Oculomotor nucleus
External links
- "Oculomotor nerve palsy"
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Category:Cranial nerves
