Focused anatomy guide
Oculomotor Nerve
The oculomotor nerve is the third cranial nerve, often written as CN III. It carries motor signals from the brain to several muscles around the eye and parasympathetic signals to muscles inside the eye. Together, these pathways help coordinate eye position, eyelid movement, pupil size, and near vision.
Origin and course
The nerve begins in the midbrain, close to the level of the superior colliculus. It emerges from the front of the midbrain, travels forward through the skull, and passes along the lateral wall of the cavernous sinus. It then enters the orbit through the superior orbital fissure.
Inside the orbit, the nerve divides into superior and inferior branches. The inferior branch carries preganglionic parasympathetic fibres to the ciliary ganglion. From there, short ciliary nerves carry signals to muscles that control the pupil and lens.
Its nuclei
Two closely related groups of nerve cells contribute to CN III. The somatic motor nucleus controls muscles that move the eye and raise the eyelid. The nearby Edinger–Westphal nucleus contributes parasympathetic fibres involved in pupil constriction and accommodation.
Functional components
CN III is a motor nerve rather than a sensory nerve. Its general somatic efferent fibres supply most of the extraocular muscles. Its general visceral efferent, or parasympathetic, fibres supply two intraocular muscles.
Extraocular muscles move the eyeball or eyelid. Intraocular muscles adjust the pupil and lens inside the eye.
Functions
The nerve supports several connected actions: moving the eye upward, downward, and inward; raising the upper eyelid; narrowing the pupil; and changing the lens for near vision.
Motor functions
CN III supplies the superior, inferior, and medial rectus muscles, the inferior oblique muscle, and the levator palpebrae superioris. The lateral rectus is supplied by the abducent nerve, and the superior oblique is supplied by the trochlear nerve.
The medial rectus turns the eye inward. The superior and inferior recti help move the eye vertically and also contribute to rotation. The inferior oblique assists upward movement when the eye is turned inward. The levator palpebrae superioris raises the upper eyelid.
Parasympathetic functions
Parasympathetic fibres from CN III reach the sphincter pupillae and ciliary muscles. Contraction of the sphincter pupillae narrows the pupil. Contraction of the ciliary muscle changes the lens so that the eye can focus on a nearby object.
Conjugate eye movements
Conjugate movements are coordinated movements of both eyes in the same direction. When someone looks to one side, the medial rectus of one eye works with the lateral rectus of the other. A network called the medial longitudinal fasciculus links the relevant cranial-nerve nuclei and helps these movements stay coordinated.
Visual reflexes
CN III participates in reflexes that adjust the eyes to changing visual conditions. These responses involve information arriving through the optic nerve and signals leaving the midbrain through the oculomotor pathways.
Accommodation reflex
When attention shifts from a distant object to a nearby one, the eyes converge, the pupils become smaller, and the lenses become more rounded. This combination improves near focus and is known as the accommodation response.
Light reflex
Bright light normally causes the illuminated pupil to constrict. The other pupil also constricts through a consensual response. Connections in the midbrain allow visual information from one eye to influence both Edinger–Westphal nuclei.
Conditions affecting the oculomotor nerve
Problems involving CN III can interfere with eye alignment, eyelid elevation, pupil size, or near focusing. Causes may include pressure on the nerve, reduced blood supply, inflammation, injury, or disorders affecting the midbrain. The pattern of symptoms depends on which fibres or nearby structures are involved.
Oculomotor nerve palsy
A significant injury to the nerve can weaken several eye muscles. A person may experience double vision, a drooping upper eyelid, or an eye that rests farther outward than usual. If parasympathetic fibres are affected, the pupil may become larger and less responsive to light.
Ophthalmoplegia
Ophthalmoplegia means weakness or paralysis of eye movement. It can involve the muscles controlled by the motor fibres, the intraocular muscles controlled by parasympathetic fibres, or both. Medical assessment is important when these symptoms appear suddenly.
Sudden double vision, a new drooping eyelid, unequal pupils, severe headache, or unusual eye movement needs prompt medical attention. This page is educational and cannot diagnose or treat a condition.
Summary
The oculomotor nerve is CN III, a motor cranial nerve with somatic and parasympathetic components. It begins in the midbrain and enters the orbit through the superior orbital fissure.
It supplies most of the muscles that move the eye, the muscle that raises the upper eyelid, and the muscles that constrict the pupil and adjust the lens. It also works with other cranial nerves in coordinated eye movements and visual reflexes.
Damage to the nerve may cause double vision, eyelid drooping, abnormal eye position, or changes in pupil responses. Symptoms should be assessed by a qualified healthcare professional.
Further reading
Continue with the cranial nerves, compare the trochlear nerve and abducent nerve, or read about the optic nerve.