photoreceptor cell

A photoreceptor cell is a specialized type of neuron found in the retina that is capable of phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes.

The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight. A third class is represented by the photosensitive ganglion cells.

There are major functional differences between the rods and cones. Rods are extremely sensitive, and can be triggered by as few as 6 photons. At very low light levels, visual experience is based solely on the rod signal. This explains why colors cannot be seen at low light levels: only one type of photoreceptor cell is active.

Described here are vertebrate photoreceptors. Invertebrate photoreceptors in organisms such as insects and molluscs are different in both their morphological organization and their underlying biochemical pathways.