Photoreceptors are photosensitive
neurons in the outer part of the retina. In a process known as
phototransduction, photoreceptors convert light information into
electrical signals. Visual pigments in photoreceptor outer segments
absorb incident photons and initiate a series of specific and complex
biochemical reactions that change the membrane potential of the cells.
This information is used by the visual system to form a complete
representation of the visual world. In vertebrates, there are two types
of photoreceptor cells: cones and rods. Cones are adapted to detect fine
detail, central and colors vision and function well in bright light.
Rods are responsible for peripheral and dim light vision. The number and
ratio of rods to cones varies among animals, dependent on whether the
animal is primarily diurnal or nocturnal.