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How the Retina Works

Much of the construction of an image takes place in the retina itself through the use of specialized neural circuits

Helga Kolb

The retina is a filmy piece of tissue, barely half a millimeter thick, that lines the inside of the eyeball. The tissue develops from a pouch of the embryonic forebrain, and the retina is therefore considered part of the brain. This most important part of the eye has a basic structure similar to that of a three-layer cake, with the bodies of nerve cells arrayed in three rows separated by two layers packed with synaptic connections. The retina includes both the sensory neurons that respond to light and intricate neural circuits that perform the first stages of image processing; ultimately, an electrical message travels down the optic nerve into the brain for further processing and visual perception.

Figure 1. Intricately wired neurons . . .Click to Enlarge Image

Intuitively, one might expect that the surface of the retina (the layer exposed to the liquid in the eyeball's vitreous chamber) would contain the sensory cells, the photoreceptors, but actually these cells lie at the very back of the retina; light rays must pass through the entire retina before reaching pigment molecules to excite. This is because the pigment-bearing membranes of the photoreceptors have to be in contact with the eye's pigment epithelial layer, which provides a steady stream of the vital molecule, retinal or vitamin A. Retinal becomes fixed in the photoreceptors' opsin proteins, where this small molecule changes its conformation in response to photons, or packets of light. Once retinal molecules are exposed to light and undergo their conformational change, they are recycled back into the pigment epithelium. This tissue behind the retina is usually very dark because its cells are full of melanin granules. The pigment granules absorb stray photons, preventing their reflection back into the photoreceptors, which would cause images to blur. They also protect the cells from too much exposure to light radiation.

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