Visual Cascade


Vision begins in the outer segments of rod and cone photoreceptor cells when visual pigments absorb light photons and become activated. Approximately 104ā€“105 cyclic nucleotide molecules are affected by activation of one rhodopsin molecule, so that signal generation requires enormous amplification through numerous molecules and mechanisms in the visual cascade.

Keywords: photoreceptors; phototransduction; hyperpolarization; rhodopsin

Figure 1.

Model of rod phototransduction. Rhodopsin serially activates many copies of the G protein transducin (Gt) (gain 1), and the α subunit of Gt, in turn, activates a cGMP‐specific phosphodiesterase (PDE), which degrades cytoplasmic cGMP with rapid turnover (gain 2). The drop in (cGMP) causes cGMP‐gated cation (CGC) channels in the plasma membrane to close, which produces hyperpolarization of the outer segment plasma membrane. Closure of cation channels prevents entry of cations, and cytoplasmic (Ca2+) drops owing to its continued extrusion by the light‐independent exchanger, also located in the plasma membrane. In a negative feedback loop, the drop in (Ca2+) causes stimulation of a guanylate cyclase (GC) by one or several specific Ca2+‐binding proteins, termed guanylate cyclase‐activating proteins (GCAPs). Sites where regulatory proteins (phosducin, Pdc; arrestin, Arr; recoverin, Rec) interact with components of the cascade are indicated by arrows. A simplified movie version of the visual cascade can be seen at (click first on webvision, then click on photoreceptors/transduction). A computer‐generated simulation of the lateral diffusional activation of components of the visual cascade can be seen at



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Further Reading

Baehr W and Palczewski K (2002) Photoreceptors and Calcium. New York, Boston, Dordrecht, London, Moscow, Georgetown TX: Kluwer Academic/Plenum Publishers/Landes Bioscience/

Palczewski K, Polans AS, Baehr W and Ames JB (2000b) Calcium binding proteins in the retina: Structure, function, and the etiology of human visual diseases. BioEssays 22: 337–350.

Ridge KD, Abdulaev NG, Sousa M and Palczewski K (2003) Phototransduction: crystal clear. Trends in Biochemical Science 28: 479–487.

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Baehr, Wolfgang, and Liebman, Paul(Jan 2006) Visual Cascade. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0004065]