vision: The Role of the Retina

The Role of the Retina

The retina—the embryonic outgrowth of the brain—is a very complex tissue. Its most important elements are its many light-sensitive nerve cells, the rods and cones. The cones secrete the pigment iodopsin and are most effective in bright light; they alone provide color vision. The rods, which secrete a substance called visual purple, or rhodopsin, provide vision in dim light or semidarkness; since rods do not provide color vision, objects in such light appear in shades of gray.

Light rays brought to focus on the rods and cones produce a chemical reaction in those cells, in which the two pigments are broken down to form a protein and a vitamin A compound. This chemical process stimulates an electrical impulse that is sent to the brain. The structural change of pigment is normally balanced by the formation of new pigment through the recombination of the protein and vitamin A compound; thus vision is uninterrupted.

The division of function between rods and cones is a result of the different sensitivity of their pigments to light. The iodopsin of cone cells is less sensitive than rhodopsin, and therefore is not activated by weak light, while in bright light the highly sensitive rhodopsin of rod cells breaks down so rapidly that it soon becomes inactive. There is a depression near the center of the retina called the fovea that contains only cone cells. It provides the keenest possible vision when an object is viewed directly in bright light. In dim light objects must be viewed somewhat to one side so the light rays fall on the area of the retina that contains rod cells.

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