Fig. 8. Phototransduction cascade. Schematic representation of events involved in the control of cGMP levels in the photoreceptor cell. Activation of rhodopsin by light sets in motion a cascade of events that ultimately lead to the hydrolysis of cGMP. These biochemical events occur within few milliseconds of photon capture. (P, phosphate; PP2A, protein phosphatase 2A; M II, metarhodopsin II; RGS, regulator of G-protein signaling; PDE, phosphodiesterase; GCAP, guanylate cyclase activation protein). (Courtesy of Michael W. Bell, PhD)