Photoreceptors are nerve cells which have been designed to be sensitive to light. These cells are located in the eye, allowing 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 photosensitive ganglion cells, and each plays a distinct role in vision.
When a photoreceptor is exposed to light, photosensitive proteins in the neuron are stimulated, triggering a series of responses which convert the light into a signal which can be read by the brain. This process happens 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 information from these cells to distinguish colors. Rather than literally seeing color, in other words, the photoreceptor responds to specific spectra, and the brain averages responses to determine what the eye is seeing.
Photoreceptor cells which have been sensitized to specific spectra are known as cones, the cells which famously allow people to “see” color. These cells have a cone-like shape when viewed under magnification, explaining the name, and they require bright light to function effectively. Rods, on the other hand, work in very low light, but do not distinguish color well. Many organisms have a mixture of rods and cones which is designed to strike a balance between being able to see well in the dark, and being able to distinguish colors.
The third type of photoreceptor, photosensitive ganglion cells, appear to play a role in circadian rhythms, and they can control pupil size as well by providing the brain with information about how much light is available. Along with rods and cones, the 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 inputs from rods and cones.
In order for photoreceptors to work properly, they need a steady supply of the proteins they use to detect light. One of these proteins is famously synthesized from vitamin A, which is why people are encouraged to include lots of this vitamin in their diets. Nutritional deficiencies can lead to a declined production of these proteins, causing vision problems. Some examples of proteins found in photoreceptors include: melanopsin, opsin, and retinal.