fototransductiecascade

The phototransduction cascade is the biological process by which light energy is converted into an electrical signal in photoreceptor cells of the retina. In the vertebrate eye, this process occurs in rod and cone cells. When a photon strikes a photopigment molecule called rhodopsin (in rods) or a similar cone opsin, it causes a conformational change in a protein called retinal, which is bound to the opsin. This change activates the opsin.

The activated opsin then binds to and activates a G protein called transducin. Activated transducin, in turn, activates an enzyme called phosphodiesterase (PDE). PDE hydrolyzes cyclic guanosine monophosphate (cGMP), reducing its concentration in the cytoplasm of the photoreceptor cell. cGMP normally keeps ion channels, specifically sodium and calcium channels, open.

When cGMP levels fall, these ion channels close. The closure of these channels causes a hyperpolarization of the photoreceptor cell membrane, meaning the inside of the cell becomes more negatively charged relative to the outside. This change in membrane potential is the electrical signal that is ultimately transmitted to other neurons in the retina. This cascade allows for the detection of light and the initiation of vision. The amplification inherent in this cascade allows for the detection of even single photons.