Oculomotor nerve

Number: III

Name: Oculomotor

Sensory, motor, or both: Mainly motor

Origin/Target: Anterior aspect of Midbrain

Nuclei: Oculomotor nucleus, Edinger-Westphal nucleus

Function: Innervates the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique, which collectively perform most eye movements. Also innervates the sphincter pupillae and the muscles of the ciliary body.

Description:

The oculomotor nerve (CN III) is primarily a motor nerve responsible for eye movements, pupil constriction, and eyelid elevation. It contains somatic motor and parasympathetic fibers originating from the oculomotor (main motor) and Edinger-Westphal nuclei in the midbrain.

Nuclei:

• Main Motor Nucleus: Located near the cerebral aqueduct at the superior colliculus level—innervates all extrinsic eye muscles except the superior oblique and lateral rectus. Fibers pass through the red nucleus and exit the midbrain at the interpeduncular fossa, receiving input from bilateral corticonuclear fibers, tectobulbar fibers from bilateral corticonuclear fibers, tectobulbar fibers, and the medial longitudinal fasciculus. Click here to read more about the main motor nucleus.

• Accessory Parasympathetic Nucleus (Edinger-Westphal Nucleus): Situated behind the main motor nucleus, it sends preganglionic fibers to the ciliary ganglion. From here postganglionic fibers then innervate the constrictor pupillae and ciliary muscles, facilitating pupil constriction and lens accommodation for near vision. Click here to read more about the accessory parasympathetic nucleus.

Course of the Oculomotor Nerve:

Emerging from the anterior midbrain, the oculomotor nerve travels between the posterior cerebral and superior cerebellar arteries, enters the cavernous sinus's lateral wall, and divides into superior and inferior branches that pass through the superior orbital fissure into the orbit.

Functions:

The oculomotor nerve innervates the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles, enabling most eye movements and eyelid elevation. Parasympathetic fibers mediate pupil constriction and lens accommodation.

Clinical implications:

Oculomotor nerve lesions can cause ptosis, a "down and out" eye position, dilated pupil, and absent pupillary reflex. Conditions like diabetes or aneurysms may affect the nerve, necessitating an understanding of its anatomy for diagnosis.