Signal cascades photoreceptors

The photoreceptor complex comprises cw-retinal and opsin (R-O) for which the regulatory signal is light. This interacts with the regulatory component of the complex R, which then dissociates (in the form ciT-R) from the O component. The free O is now catalytically active and catalyses the initial reaction in the visual cascade sequence which leads to activation of the target system (i.e. produces an action potential in the optic nerve). 

Figure 29-5. The visual cycle of vitamin A is central to vision. In the retina, light stimulates the conversion of 1 l-c/.s-retinal, part of rhodopsin (Rho), to all-iraws-retinal and activates rhodopsin (Rho ).This initiates the first step of the signal transduction cascade that results in the transmission of the visual signal to the brain.The visual cycle involves biochemical and metabolic events in both the photoreceptors (rods) and the retinal pigment epithelium.

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