The optic nerve is a thick bundle of about 1.2 million individual nerve fibers. It travels from the retina to the brain’s vision center, carrying electrical data that the brain will interpret as images.
The individual fibers come from the retina’s photosensitive cells – the rods and cones. Please see our page on The Retina for more information. These cells receive the light that travels into the eye through the cornea, the pupil, the lens and then the vitreous gel. They convert the light data to electrical data so that the nerve fibers can pick it up.
The optic nerve fibers travel across the retina and converge near its center. Their convergence forms the optic nerve. At this location there are no light-sensitive cells, since the nerve fibers are taking up the space. This is the eye’s blind spot. It is near the macula, the retinal area giving us our sharpest, bright-light vision. We use it for reading or any close work where small details are important. Glaucoma is diagnosed partly by the presence of damage to the optic nerve at a point called the cup, where the nerve leaves the retina.
Around the entire brain there is a membrane called the meninges. It also surrounds the optic nerve. When an embryo is developing, the optic nerve develops as part of the brain. But by the time the child’s binocular vision is functioning, the optic nerve acts like an envoy from the eye (outside the brain) to the brain. Therefore it can be seen both as part of the brain and part of the eye.
Since we have two eyes, there are two optic nerves at the beginning. They meet at a place called the optic chiasm, located centrally behind the eyes near the pituitary gland. Here each nerve splits:
This crossover means that:
From the chiasm, both left and right nerve bundles leave diagonally back toward the ears, loop around, and then travel diagonally to the vision center in the center-back area of the brain.
They travel in three strands and enter the vision center in six places – three from each side.
Depending on where damage might occur to an eye or its nerve pathway, different kinds of vision loss occur. Your eye doctor can locate the damage based on vision test results.
For example, if vision is lost only on one side, it will be that side’s optic nerve before the chiasm that is damaged. If the outside areas of both left and right visual fields are lost, the damage will be at the chiasm.
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