What is a photoreceptor?

Photoreceptors are nerve cells that have been designed to be sensitive to light. These cells are found in the eye, which enables an organism to see, and the process through which they work is complex and quite fascinating. There are three types of photoreceptors: rods, cones, and light-sensitive ganglion cells, and each one plays a different role in vision.

When a photoreceptor is exposed to light, light-sensitive proteins in the neuron are stimulated, triggering a series of responses that convert the light into a signal that the brain can read. This process occurs in a fraction of a second, allowing a photoreceptor to provide constant information to the brain about the visual environment. Certain photoreceptors are sensitized to particular spectra, and the brain uses the information from these cells to distinguish colors. Instead of literally seeing color, in other words, the photoreceptor responds to specific spectra, and the brain averages the responses to determine what the eye is seeing.

Photoreceptor cells that have been sensitized to specific spectra are known as cone cells, the cells that allow people to "see" color. These cells have a cone shape when viewed under magnification, explaining the name, and require bright light to function effectively. The wands, on the other hand, work in very low light, but do not distinguish color well. Many organisms have a mix of rods and cones that is designed to strike a balance between being able to see well in the dark and distinguishing colors.

The third type of photoreceptor, photosensitive ganglion cells, appear to play a role in circadian rhythms, and may also control the size of the pupils by providing the brain with information about the amount of light available. Along with rods and cones, photosensitive ganglion cells are found in the retina of the eye. These cells were discovered in the 1990s, long after the presence of rods and cones had been established. Researchers have studied these cells by examining subjects with blocked rod and cone inputs.

For photoreceptors to function properly, they need a constant supply of the proteins they use to detect light. One of these proteins is synthesized from vitamin A, so people are encouraged to include plenty of this vitamin in their diets. Nutritional deficiencies can lead to decreased production of these proteins, causing vision problems. Some examples of proteins found in photoreceptors include: melanopsin, opsin, and retina.

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