The Anatomy of the Oculomotor Nerve

The oculomotor nerve enables most of your eye movements, some aspects of vision, and raising the eyelid. It's the third cranial nerve and works with cranial nerves four (trochlear) and five (trigeminal) to coordinate eye movement. The oculomotor nerve contains both motor and parasympathetic fibers, which classifies it as a mixed nerve.

Anatomy

You have 12 cranial nerves that originate in the brain and brainstem and mainly perform functions in your face and throat. This sets them apart from the rest of your nerves, which branch out from the spinal column and travel throughout your body.

Cranial nerves come in pairs, with generally symmetrical courses on each side of your head. However, each pair is typically referred to collectively as a single nerve, or, when it's necessary to distinguish one from the other, as the right or left nerve.

Structure

The oculomotor nerve begins at the brainstem, which is a structure low in the back of your brain that connects the brain to the spinal column. In the brainstem, two clusters of neurons called nuclei give rise to the oculomotor nerve.

They're called:

1. Oculomotor nucleus
2. Accessory nuclei of the oculomotor nerve

Each of these nuclei supplies the nerve with a different type of fiber.

As it travels through your head toward the eyes, the oculomotor nerve branches out to innervate (supply nerve function to) various muscles.

Its major branches are:

• Superior branch
• Inferior branch

These branches further divide before reaching their destinations.

The superior branch splits into:

• Superior rectus
• Levator palpabrae superioris

The inferior branch gives off:

• Inferior oblique
• Medial rectus
• Inferior rectus
• Short ciliary nerves

Location

From where it emerges from the nuclei in the brainstem, the oculomotor nerve passes in front of the cerebral aqueduct and emerges from the midbrain, then passes between two arteries—the superior cerebellar artery and the posterior cerebral.

Next, it pierces the dura mater, which is the outermost membrane surrounding the brain and spinal cord, and moves into the cavernous sinus (a sinus cavity), which is about level with your ear.

Inside the cavernous sinus, it's joined by sympathetic fibers from the internal carotid plexus (a network of nerves). These fibers don't become part of the oculomotor nerve, but they do travel alongside it within its sheath.

The oculomotor nerve then leaves the cranial cavity through what's called the superior orbital fissure. Your "eye socket," is the orbit and the superior orbital fissure is a hole in the bone, behind the eye and on the inside of the eye socket.

Once the oculomotor nerve is inside the orbit, it divides into its superior and inferior branches.

Anatomical Variations

Anatomical variations of the oculomotor nerve are rare. The most common one results in a condition called congenital oculomotor palsy. It's caused by compression of the nerve at the junction of the posterior communicating artery and the internal carotid artery.

Symptoms of congenital oculomotor palsy include:

• A pupil that's "fixed" (doesn't change size in response to light) on the same side as the compression
• Ptosis (drooping of the upper eyelid) on the same side as the compression
• Decreased visual acuity (sharpness and clarity of vision), usually due to ambylopia because of eye motility disruption or droopy eyelid not allowing vision to develop well

Function

As a mixed nerve, the oculomotor nerve supplies motor function and parasympathetic function. It does not have any sensory function, which has to do with sensation.

Motor Function

Motor function means movement, and the oculomotor nerve is responsible for much of the movement associated with your eyes.

The muscles innervated by the superior branch and its offshoots are around your eye inside the orbital.

They are:

• Superior rectus: Moves the eye upward
• Levator palpabrae superioris: Raises the upper eyelid

The sympathetic fibers from the internal carotid plexus that travel with the oculomotor nerve provide motor function to the superior tarsal muscle, which keeps the eyelid open once the levator palpabrae superioris raises it.

The inferior branch and its offshoots innervate:

• Inferior rectus: Moves the eyeball downward; rotates the top of the eye outward
• Medial rectus: Moves the eyeball toward the nose
• Inferior oblique: Moves your eye up and outward

Parasympathetic Function

Parasympathetic function has to do with the parasympathetic nervous system, whose functions tend to oppose and balance those of the sympathetic nervous system."

The sympathetic nervous system takes over during stressful or dangerous situations and is responsible for "fight or flight" functions, such as increasing your adrenaline levels and dilating your eyes. When the parasympathetic nervous system is in control, it's often referred to as "rest and digest" mode. It lowers your heart rate to conserve energy, aids with optimal function of your intestines, and returns your pupils to their normal size.

The parasympathetic fibers from the oculomotor nerve innervate two muscles inside the iris:

• Sphincter pupillae: Constricts (shrinks) the pupil
• Ciliary muscles: Change the curvature and thickness of your lens so you can focus on objects at different distances

Associated Conditions

The oculomotor nerve can be damaged or paralyzed in numerous ways. This is called acquired oculomotor palsy and is different from congenital oculomotor palsy, which was discussed above.

Acquired oculomotor palsy can be caused by:

• Trauma to the eye or anywhere along the path of the nerve
• Pressure from tumors, lesions, or aneurysms
• Brain herniation
• Diseases that destroy the myelin sheath that encases the nerve, such as multiple sclerosis
• Diseases that impact small blood vessels, such as diabetes or hypertension, due to inadequate blood supply to the nerve
• Meningitis that affects the brain stem

Symptoms of Oculomotor Nerve Palsy

Symptoms of damage to the oculomotor nerve include:

• Ptosis
• The eye pointing downward and out
• Seeing double (diplopia)
• Permanently dilated pupil
• Inability to shift focus to objects at different distances

Treatment

Depending on the cause, immediate treatment of oculomotor nerve palsy is typically conservative. Depending on the specific symptoms and the part(s) of the nerve that's damaged, it may include:

• Eye patch
• Opaque contact lens to block vision in the affected eye
• Blurred glasses lens on the side of the affected eye
• Botulinum toxin (Botox) injection
• Prisms in glasses lens on the side of the affected eye

Conservative treatment results in a full recovery in about 63% of people with acquired oculomotor palsy. If this approach hasn't lead to much improvement after six months, surgery may be considered.

Surgery involves cutting and repositioning the muscles so that functional muscles can take over for those that aren't working properly.