Light is able to stimulate directly the radial net of the sea urchin; but in vertebrates light has no effect on the optic nerve. In order to trigger the discharges in the optic nerve there must be a light-sensitive mediating mechanism. In the vertebrate eye this is achieved by the photostimulation of the visual pigments. Rhod-opsin when photostimulated undergoes a series of spontaneous reactions which ultimately results in stable products. These sequences of events do not require enzymatic or metabolic energy. On the other hand, the regeneration of the visual pigment from the product of reaction is energy consuming. The photoactivation and regeneration of the visual pigment, termed the visual cycle (Wald, 1935) is illustrated in Fig. 3.1.
KeywordsOuter Segment Difference Spectrum Visual Pigment Disc Membrane Photoreceptor Outer Segment
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