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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)