Cranial nerve · one in each half of the brain
Oculomotor nerve (III)
Also called Cranial nerve III, CN III, Third nerve
The main eye mover: it aims your eye, lifts your eyelid, and shrinks your pupil.
The oculomotor nerve does the most work of the three nerves that move your eyes. It controls four of the six small muscles that steer each eye, plus the muscle that holds your upper eyelid open. It also makes the pupil smaller and helps your eye focus on things up close. It starts in the midbrain, the top part of the brainstem.
Where it is
It leaves the front of the midbrain, between the two thick stalks that carry signals down to the body, then runs forward along the base of the skull into the eye socket.
What it does
Steers the eye
It moves the eye up, down, and inward toward the nose, using four of the six eye muscles.
Holds the eyelid up
It powers the muscle that lifts your upper eyelid and keeps it open all day.
Shrinks the pupil
Its calming "rest and digest" fibers tighten a ring of muscle in the colored part of the eye, making the pupil smaller in bright light.
Focuses up close
When you look at your phone, it turns both eyes inward, rounds the lens to focus, and narrows the pupils, all at once.
Think of it as the camera operator for each eye: it swings the camera, lifts the lens cap, adjusts the aperture, and pulls focus.
Works with
- MidbrainIts starting cells sit in the midbrain, at about the level of the superior colliculus.
- Superior colliculusThis eye-aiming center helps decide where to look next, and the oculomotor nerve helps carry out the move.
- Optic nerve (II)In the pupil reflex, the optic nerve senses the light and the oculomotor nerve shrinks the pupil in response.
- Trochlear nerve (IV)Together with the trochlear and abducens nerves, it steers the eye in every direction.
- Abducens nerve (VI)To glance sideways, one eye's abducens and the other eye's oculomotor nerve fire together so both eyes turn as a pair.
- AcetylcholineIt uses acetylcholine to tell the eye muscles to contract.
When it goes wrong
How doctors check it
The doctor asks you to "follow my finger" as it traces a big H in the air, and shines a light into each eye to watch the pupil shrink.
Third nerve palsy
If the nerve stops working, the eyelid droops and the eye drifts down and outward, causing double vision. The pupil may also stay large. Diabetes and blocked tiny blood vessels are common causes.
An emergency sign
A suddenly enlarged pupil with a droopy lid and a bad headache can mean a bulging blood vessel (aneurysm) or brain swelling is pressing on the nerve. This needs emergency care.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- One nerve does three jobs every time you read: it turns your eyes inward, thickens the lens to focus, and shrinks your pupils. Doctors call this the near response.
- Emergency teams shine lights into the eyes after a head injury because a pupil that suddenly stays wide can be an early sign that swelling is pressing on this nerve.
By the numbers
Where the name comes from
From the Latin oculus, "eye", and motor, "mover". Quite simply, the "eye mover".
Learn it in a lesson
- The brainstem and cranial nervesLife support and the nerves of the head. A short guided lesson in 3D.
Quick quiz
3 questions. Your answers are saved and come back in your daily review.
Which of these is a job of the oculomotor nerve (III)?
Show the answer
Steers the eye. It moves the eye up, down, and inward toward the nose, using four of the six eye muscles.
Steers the eye. It moves the eye up, down, and inward toward the nose, using four of the six eye muscles.
The oculomotor nerve (III) is part of which larger structure?
Show the answer
Cranial nerves. It belongs to the cranial nerves.
Cranial nerves. It belongs to the cranial nerves.
Which part fits this description? "Like the camera operator for each eye: it swings the camera, lifts the lens cap, adjusts the aperture, and pulls focus."
Show the answer
Oculomotor nerve (III). It is the oculomotor nerve (III). The main eye mover: it aims your eye, lifts your eyelid, and shrinks your pupil.
Oculomotor nerve (III). It is the oculomotor nerve (III). The main eye mover: it aims your eye, lifts your eyelid, and shrinks your pupil.
These parts now come back in your daily review, just before you would forget them.
Practice it and its neighbors in 3DSources
- Neuroanatomy, Cranial Nerve 3 (Oculomotor), StatPearls, NCBI Bookshelf
- Cranial Nerve III Palsy (Oculomotor Palsy), StatPearls, NCBI Bookshelf
- Cranial Nerve Testing, StatPearls, NCBI Bookshelf
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Explore the oculomotor nerve (III) in 3D
Turn it around, zoom inside and see how it connects to the rest of the brain, then test yourself with quick quizzes.
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