Photoreception
The photoreception is a process of which the light causes transformation of cis-rhodopsin to trans-rhodopsin on the laminae of the rods and cones. The rhodopsin really is a molecule of 11-cis-retinal and seven-membrane structured opsin (the all-trans-retinal, Vitamin A is a stable isomer and does not bind to opsin). Opsin is a protein with 348 amino acids residues on which the retinal attaches to tne 296th.
The decomposition of rhodopsin stimulates G protein transducin which stimulates phosphodiesterase (PDE). The PDE decomposes cGMP at the rate of 103 cGMP per second. The decreased amount of cGMP causes the Na+ and Ca+2 channels to close and deactivate neuron firing. The PDE itself is deactivated by opsin kinase, then by arrestin. Adaptation is mediated by Ca+2 which inhibits cGMP synthesis.