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Phototransduction, Scotopic and Photopic Vision.docx

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McMaster University
Gautam Ullal

October 10 , 2012 Psych 2E03: Sensory Processes Phototransduction, Scotopic and Photopic Vision The Basic Wald’s Rhodopsin Cycle - Bleaching - Matarhodopsin II - All-trans-retinal will continuously accumulate, toxic - Accumulation of retinal is harmful to the retina - *know steps of rhodopsin breakdown - retinol being stored and converted to retinal continuously Importance of Retinal-Retinol cycle - retinal (an aldehyde) toxic in large amounts retinol (an alcohol) stored in pigment layer Phototransduction - dark: receptors depolarized when there is no light, cyclic GMP keeps the ion channels open, ions enter in the rod, GMP activates, cis-retinal bond to opsin - light: moment there is light, hydrolyzes cyclic GMP, destroys GMP, closing the channel, sodium and calcium ions can not come inside, becomes hyperpolarized, more negative then when it was, rhodopsin molecule splits activating phosphodiesterase which destroys cyclic GMP Phototransduction - dark: inner segment of rods and cones, sodium and calcium continuously leak into receptor, more glutamate - light: sodium calcium is no longer permitted to enter, less glutamate Phototransduction: Amplification/Cascade - small change in the pigment leads hundreds of enzymes cascade The Phototransduction Cascade - trans-retinal (and formation of metarhodopsin II) - transduction (g-protein_ - phosphodiesterase - no cyclic GMP 2+ + - stop Ca and Na entry Rod and Cone Vision - higher convergence in the rod-system makes them more sensitive to light than the cones - sensitive to low light intensity - less-co
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