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Eye Anatomy - The Optic Nerve

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.

Two Blood Vessels Inside the Optic Nerve

  1. The central retinal artery is bringing oxygen and nutrients to the retina. Its branching capillaries spread over the entire retina, connecting with the capillaries of:
  2. The central retinal vein. This vein carries waste products such as carbon dioxide away from 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.

Optic Nerve Pathway Crossover

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:

  1. The left eye:
    • Keeps fibers from its left side on the brain’s left side
    • Sends fibers from its right side to the brain’s right side, crossing over at the chiasm
  2. The right eye:
    • Sends fibers from its left side to the brain’s left side, crossing over at the chiasm
    • Keeps fibers from its right side on the brain’s right side

This crossover means that:

  • The right side of the brain receives vision data from the left visual field of both eyes
  • The left side of the brain receives vision data from the right visual field of both eyes

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.

Patterns of Vision Loss

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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