The oculomotor nerve is the third cranial nerve (CN III). It provides motor and parasympathetic innervation to some of the structures within the bony orbit. In this article we shall look at the anatomy of the oculomotor nerve - its anatomical course, functions and clinical correlations.
The oculomotor nerve originates from the oculomotor nucleus - located within the midbrain of the brainstem, ventral to the cerebral aqueduct. It emerges from the anterior aspect of the midbrain, passing inferiorly to the posterior cerebral artery and superiorly to the superior cerebellar artery. The nerve then pierces the dura mater and enters the lateral aspect of the cavernous sinus. Within the cavernous sinus, it receives sympathetic branches from the internal carotid plexus. These fibres do not combine with the oculomotor nerve – they merely travel within its sheath. The nerve leaves the cranial cavity via the superior orbital fissure. At this point, it divides into superior and inferior branches.
The oculomotor nerve innervates many of the extraocular muscles. These muscles move the eyeball and upper eyelid.
There are two structures in the eye that receive parasympathetic innervation from the oculomotor nerve-
The pre-ganglionic parasympathetic fibres travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The post-ganglionic fibres are carried to the eye via the short ciliary nerves.
Oculomotor nerve palsy is a condition resulting from damage to the oculomotor nerve. The most common structural causes include-
In the case of oculomotor nerve palsy, assessment of the affected individual by a healthcare professional is recommended, to identify the underlying cause and appropriate treatment. Upon assessment, it may be necessary to undertake further investigations such as a CT scan or an MRI to help identify the cause of the nerve palsy and the extent of any damage.
In some cases, it is possible that treatment is not necessary as symptoms may improve in time. However, it is important to monitor changes to the affected eye and seek further medical advice if required. When treatment is necessary, this will depend on the underlying cause. Common treatments for this condition include the use of anti-inflammatory medications, such as corticosteroids, as well as surgery that may help to reduce any pressure on the nerve.
The oculomotor nerve plays a major role in the movement of the eye and upper eyelid. It is one of the twelve cranial nerves, and supplies both motor and parasympathetic innervation. It begins in the midbrain, and exits the cranial cavity via the superior orbital fissure. At this point, it divides into two branches: a superior branch and an inferior branch.
Within the orbit, the superior branch provides motor innervation to the superior rectus and levator palpabrae superioris, while the inferior branch provides motor innervation to the inferior rectus, medial rectus and inferior oblique. Additionally, the inferior branch provides pre-ganglionic parasympathetic fibres to the ciliary ganglion, which ultimately innervates the sphincter pupillae and ciliary muscles.
The oculomotor nerve innervates many of the extraocular muscles, which are responsible for the movement of the eyeball and upper eyelid. The superior branch provides motor innervation to the superior rectus and levator palpabrae superioris. This branch also contains sympathetic fibres which innervate the superior tarsal muscle, which acts to keep the upper eyelid elevated after the levator palpabrae superioris has raised it. The inferior branch provides motor innervation to the inferior rectus, medial rectus and inferior oblique.
The oculomotor nerve also sends out parasympathetic fibres to two structures within the eye. The sphincter pupillae muscle constricts the pupil, reducing the amount of light entering the eye, while the ciliary muscle contracts and causes the lens to become more spherical, and thus more adapted to short range vision.
The pre-ganglionic parasympathetic fibres first travel in the inferior branch of the oculomotor nerve. Within the orbit, they then branch off and synapse in the ciliary ganglion. The post-ganglionic fibres are carried to the eye via the short ciliary nerves.
Oculomotor nerve palsy is a condition resulting from damage to the oculomotor nerve. It has several possible structural causes, including raised intracranial pressure (which can compress the nerve against the temporal bone), posterior communicating artery aneurysms, and cavernous sinus infection or trauma. Other pathological causes include diabetes, multiple sclerosis, myasthenia gravis and giant cell arteritis.
As the oculomotor nerve provides both motor and parasympathetic innervation to structures within the bony orbit, damage to this nerve will result in various clinical features associated with the eye. These include ptosis (drooping of the upper eyelid) due to paralysis of the levator palpabrae superioris and unopposed activity of the orbicularis oculi muscle, and a 'down and out' position of the eye at rest due to paralysis of the superior, inferior and medial rectus, and the inferior oblique (and therefore the unopposed activity of the lateral rectus and superior oblique). Additionally, patients will be unable to elevate, depress or adduct the eye, and the pupil will be dilated due to the unopposed action of the dilator pupillae muscle.
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