The visual cycle (part 2)

The Rhodopsin cycle (click on drawing to enlarge)

The ability to see in the dark depends on an adequate concentration of retinal. The ins and outs of photosenstive cis retinal and its recycling  from trans-retinal are illustrated here.

Photo-receptors e.g. the eye, have the ability to convert light’s energy in photons to electrical impulses. These signals are then carried in nerve cells to the brain to be interpreted e.g. shape or color. Although the physical structure of the “eye” is very different in different animal species, the chemical reaction that captures lights energy is highly conserved. Thus studies in the Drosophila fruit fly have answered many questions about the visual cycle of the vertebrate eye! The photosensitive visual molecule Rhodopsin consists of a protein called opsin, coupled with retinal, which is the light sensitive component. As discussed in part 1 of the visual cycle on this blog, Vitamin A (retinol) is an essential vitamin i.e. supplied in the diet, and is the precursor of retinal.

Rhodopsin comes in different variants, which respond to different quantities and qualities of light. The molecule is held in the plasma membranes of two types of cells in the retina: the rods and the cones. These names describe the shape of the cells as seen under the microscope. The rods, mainly found in the periphery of the retina respond to small amounts of light i.e. work well when the iris dilates in dim light. The cones are found centrally, and respond best to bright light. They enable us to discern colours and detail. The opsin protein has several helices traversing the plasma membranes and these form a pocket to hold the retinal . The latter, in its “cis-retinal” form is the photosensitive part of the molecule. In the dark it holds the opsin in an inactive shape. But when activated by light it converts to “trans-retinal” and this pushes the opsin into an active form. This triggers a cascade of events altering the permeability of the cell membrane and a signal of light and/or color is conducted to the brain by the optic nerves for interpretation. The photo-activated trans-retinal cannot be reused until it is converted back into an active cis-retinal form. This is known as the visual or rhodopsin cycle. It is transported from the rod or cone cell cytoplasm, and released into the sub-retinal space, where it is bound to a transporting protein and carried into the retinal pigment cell for reconversion again to active cis-retinal. The recycling and production of new supplies of retinal from beta carotene and retinol occur in the retinal pigment layer. This explains why the rods and cones lie between the nerve cells and this pigment layer. This allows more efficient recycling of cis- retinal. Nevertheless regeneration and transport of cis-retinal takes time, and explains why we are temporarily blinded when a bright flash of light depletes all the retinal bound to opsins in the rods. As discussed in “trip tips” this is the reason why flashlight photography and spot-lights may disturb nocturnal animals and birds. The duration of the resulting night blindness depends on the speed of retinal recycling, but takes up to 30 minutes for complete recovery. It is interesting to note that cone cells, used in bright light, have supplementary cis-retinal recycling from helper  Mϋeller neural glial cells in the retina

The pupil appears dark because of the black retinal pigment layer which absorbs almost all the light. Apparently only 0.1% of the light entering the human eyed gets reflected out again i.e. pupil is black. Although the opsins are not photosensitive, variation in their composition accounts for our ability to respond to and see different colors. There are 3 types of cone cells each responsive to a particular wave length of light i.e. the three primary colors red, green, and blue.

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