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Range of Visual Sensitivity: Scotopic, Mesopic, Photopic
Photoreceptors in primates: humans and monkeys
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Spectral Sensitivity
Visual pigments: seven membrane-spanning loops
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Visual pigments: homologies in amino acid sequences
Phototransduction – a well studied G-protein cascade
Rod opsin (rhodopsin) in the rod disc membrane can be activated by light when 11-cis retinal is bound to it
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The visual cycle: RPE provides 11-cis-retinalfor rodsChoriocapillaris
https://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Retina/VisualCycle.html0
Details of the visual cycle
RPE
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Biochemical steps in the phototransductioncascade
Phototransduction
Leads to closure of a cation channel in the plasma membrane. This interrupts the dark current, and hyperpolarizes the rod or cone photoreceptor
The opsin in the outer segments,catches light and is activated when 11-cis retinal is attached to it
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Rod photocurrents:prolonged responses
Cone photocurrents:
brief responses
Individual rods are 70-100 times more sensitive than cones
Suction electroderecordings from outer segments
Psychophysically measured sensitivity
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Rod and cone sensitivity: During dark adaptation and recovery from bleaching of photopigment, cones recover sensitivitymore quickly than rods. (Cone visual cycle can occur in Mueller cells)