3 WHAT’S WRONG WITH COREY?
Understanding what goes wrong in Leber congenital amaurosis requires a trip through an eyeball, from the pupil, where light enters, to the back of the eye. Here, the retina is the layer of the eye’s wall that includes the photoreceptor cells— the rods and cones— that captures light energy and changes it into the electrical language of the nervous system. The rod cells provide black-and-white vision and detect motion, and the cone cells send signals for color. The retina also has cell layers that transmit the light signals to the optic nerve, which sends the information to the part of the brain that interprets the input as a visual image. The comparison of the human eye to an old-fashioned camera is apt— the back of the retina is like a sheet of photographic film.
At first Corey’s night blindness suggested a problem with his rod cells. Each eye has 100 million of these long, skinny cells, and each has about two thousand translucent discs that fold inward from the surrounding cell membrane, making the rod look a little like an electric toothbrush. The aligned discs resemble toothbrush bristles at one end, and a neural connection at the other end of the cell that goes to the brain corresponds to the part of the toothbrush that plugs into a power outlet.
Embedded in the rod’s folded discs are many molecules of a pigment called rhodopsin, which actually provides vision. Each rhodopsin molecule is built of a protein part called opsin and another, smaller part made from vitamin A, called retinal. A flash of light lasting mere trillionths of a second changes the shape of the retinal, which in turn changes the shape of the opsin. The change in opsin triggers chemical reactions that signal the nearby optic nerve, which stimulates the visual cortex in the brain. In this way, each of the 100 million rods and 3 million cones of a human eye contribute a tiny glimpse of a scene, which the brain then integrates into an image.