Using atomic-force microscopy, vision researchers have taken pictures of some of the eyes photon receptors in their natural state, and have analyzed their packing arrangement. Their findings, published in the Jan. 9 issue of Nature, offer insight on how light signaling might be controlled in the retinas outer edge.
THE WELL-ORGANIZED EYE: This close up, high-magnification image of the disc membrane on a rod from the retina shows protrusions lined up in neat, double rows like eggs in a carton. The protrusions are a paracrystal form of rhodopsin, a light absorbing chemical.
The retina receives light through rods and cones. Rods, which are most heavily concentrated on the retinas outer edge, are sensitive to dim light and to movement, but not to color. Rods, like cones, face away from incoming light. Within rods, light causes a chemical reaction with rhodopsin. This begins a chain of stimulation along the visual pathway, which sends information to the brain for interpretation. The brain can detect one photon of light, the smallest unit of energy, when it is absorbed by a photoreceptor.
The outer segment of a rod looks roughly like a stack of microscopic coins inside a wrapping. The segment is made up of discs covered by a membrane. Scientists studying the retina knew that the outer-segment disc membranes of rods are densely packed with rhodopsin molecules. This bunching together allows for optimum absorption of dim light and for subsequent amplification of the faint signal by the visual pathway. However, how rhodopsin molecules are physically arranged to increase the probability of being activated by a photon was not known.
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