Monday, March 23, 2009

A photoreceptor is a specialized nerve cell that is found in the eye's retina. A photoreceptor cell is capable of phototransduction, which means it converts light into a chain of biological processes. More specifically, the photoreceptor absorbs photons from the field of view, and through a specific and complex biochemical pathway, signals this information through a change in its membrane potential.

The two classic photoreceptors are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight. But a third photoreceptor has been discovered, the ganglion cell, which is found in the inner retina, have dendrites and long axons projecting to the midbrain, the suprachiasmatic nucleas in the hypothalamus, and the thalamus.

Cones detect colors and are adapted to bright light conditions. On the other hand, rods do not detect color well, but are more sensitive to low light. In humans there are three different types of cones, which correspond to short (blue), medium (green) and long (yellow-red) light. The human retina contains about 120 million rod cells and 6 million cone cells.