by Dana Moses

Created on: May 11, 2009   Last Updated: May 12, 2009

Photoreceptor cells are specialized cells in the retina of the eye that detect light, and therefore allow the eye to see. Photoreceptors absorb photons, or packets of light, and transmit information from photons to other cells, and eventually to the brain. The information that photoreceptors transmit is the intensity of light, or how bright the light is. By combining this information over space there are millions of photoreceptors spread out over a human retina and time, a human brain stitches together a seamless image we call sight.

The two most famous types of photoreceptors are rods and cones. Cones are best suited to detecting photons in daylight and distinguishing colors. Cones are further specialized into 3 types of cells that are each adapted to detecting a specific range of colors: one is most sensitive to blue, a second type is adapted to be most sensitive to green light, and the third type is tailored to detect red. Groups of cones are needed to determine the color of an object because each cone cell responds to a range of wavelengths, or colors, of light, but the relative response of the 3 different types of cones varies depending on the wavelength of the light absorbed. In contrast, rods work better than cones under dim light because they are more sensitive, but cannot distinguish colors.

Photoreceptor cells are neurons, or nervous system cells, and transmit information using the neurotransmitter glutamate. When photoreceptor cells detect light, the amount of glutamate they release decreases, which is caused by a complex series of reactions. Rods and cones are filled with a pigment called rhodopsin. Rhodopsin is made of proteins called opsin and a form of vitamin A, retinal. When light hits rhodopsin, the structure and shape of the retinal molecule change. The changed shape of the retinal sends a signal to other proteins in the photoreceptor cell, eventually causing a specific chemical reaction. The cell responds to the chemical reaction by closing its sodium ion channels. This blocks sodium ions from entering the photoreceptor cell and causes the cell to release less glutamate.

Bipolar cells reside next to photoreceptor cells in the retina, and detect how much glutamate the photoreceptor is releasing. They in turn signal to the next cells in the series, retinal ganglion cells, and this type of signaling continues until the signal reaches the brain. There, the fact that a photoreceptor cell detects light is transmitted into useful information.

Photoreceptor cells are an amazing cell type able to transmit a photon of light into chemical information, and are truly the eyes of the eye.

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