We now explore the retina and optic nerve. The retina is a light-sensitive layer at the back of the eye that covers about 65 percent of its interior surface. Photosensitive cells called rods and cones in the retina convert light energy into signals that are carried to the brain by the optic nerve. The retina contains about 7 million cones and 75 to 150 million rods. In the middle of the retina is a small dimple called the fovea centralis. It is the center of the eye’s sharpest vision and the location of most color perception.
The retina’s function is to capture an image and communicate it to the brain. So how does this happen? Light rays enter the cornea, through the pupil and then the lens focuses this light onto a specific part of the retina. The retina has light-sensitive cells called photoreceptors, which transforms these light rays into electrical impulses. The electrical impulses travel down the optic nerve fibers and into the brain. The brain decodes the signals and we “see” an image!
The optic nerve is the connection between the eye and the brain. The optic nerve is the second cranial nerve. The cranial nerves emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge from the spinal column. This makes the optic nerve part of both the eye and the brain.