Cranial nerve · one in each half of the brain
Optic nerve (II)
Also called Cranial nerve II, CN II, Second nerve
The cable from each eye to the brain, and really a piece of brain grown out to the eye.
The optic nerve carries everything you see from the back of your eye to your brain. It is a cable of roughly a million nerve fibers, one cable per eye. Strictly speaking, it is not an ordinary nerve at all. It grows out of the brain before birth, so it is really part of the brain itself, wrapped in the same protective layers.
Where it is
It leaves the back of each eyeball, runs through a small bony tunnel at the back of the eye socket, and meets its partner from the other eye at the optic chiasm, just above the pituitary gland.
What it does
Carries the picture
Light-sensing cells in the retina turn light into electrical signals, and the optic nerve carries them to the brain in a split second.
Sets up depth vision
By meeting the other eye's fibers at the chiasm, it lets the brain compare two slightly different views and judge distance.
Pupil reflex, sensing half
It tells the brain how bright the light is, so your pupils can shrink in sunshine and widen in the dark.
Setting the body clock
Special light-sensing cells in the retina send signals down the optic nerve to the brain's master clock, helping set your sleep and wake cycle.
Think of it as a fiber-optic cable with about a million strands, each carrying one tiny piece of the picture from a camera to a computer.
Works with
- Optic chiasmThe two optic nerves meet here, and about half of each nerve's fibers cross to the other side.
- Lateral geniculate nucleusMost of its fibers end up at this relay station in the thalamus, which passes vision on to the cortex.
- Superior colliculusSome fibers go to this midbrain center, which snaps your eyes toward sudden movements.
- HypothalamusA small group of fibers reaches the body clock in the hypothalamus to tell it whether it is day or night.
- Primary visual cortexAfter a relay in the thalamus, its signals arrive here at the back of the brain, the first cortex stop for seeing.
- OligodendrocyteIts fibers are insulated by oligodendrocytes, the same myelin makers used inside the brain, a sign that it is really brain tissue.
When it goes wrong
How doctors check it
Reading an eye chart, counting fingers at the edge of your vision, and shining a light to watch the pupils shrink all test this nerve. A doctor can also look straight at where the nerve starts, through your pupil, with a small lighted scope.
Optic neuritis
Inflammation of the nerve can cause blurry or dim vision in one eye and pain when the eye moves. It is often linked with multiple sclerosis, a disease that attacks myelin.
Glaucoma
In glaucoma, the nerve fibers slowly die, often linked to high pressure inside the eye. Side vision fades first, usually without the person noticing, which is why regular eye checks matter.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- Everyone has a blind spot in each eye, right where the optic nerve leaves, because there are no light sensors there. You never notice it, because your brain fills in the gap and the other eye covers it.
- When a doctor looks into your eye with a light, they are looking straight at part of your central nervous system, with no surgery needed.
- Because it is brain tissue, a damaged optic nerve does not grow back on its own, unlike the nerves in your arms and legs.
By the numbers
Where the name comes from
From the Greek optikos, "having to do with seeing".
Learn it in a lesson
- SeeingFrom the eye to the back of the brain. 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 optic nerve (II)?
Show the answer
Carries the picture. Light-sensing cells in the retina turn light into electrical signals, and the optic nerve carries them to the brain in a split second.
Carries the picture. Light-sensing cells in the retina turn light into electrical signals, and the optic nerve carries them to the brain in a split second.
The optic nerve (II) 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 a fiber-optic cable with about a million strands, each carrying one tiny piece of the picture from a camera to a computer."
Show the answer
Optic nerve (II). It is the optic nerve (II). The cable from each eye to the brain, and really a piece of brain grown out to the eye.
Optic nerve (II). It is the optic nerve (II). The cable from each eye to the brain, and really a piece of brain grown out to the eye.
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 2 (Optic), StatPearls, NCBI Bookshelf
- Optic neuritis, MedlinePlus, National Library of Medicine
- Glaucoma, National Eye Institute (NEI)
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Explore the optic nerve (II) 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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