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Lecture 6

Lecture 61

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Dirk De Clercq

Outline of Lecture 61 and 62 (03-25 C and 03-25 D; Yau) Photoreceptors and Retina 0. Introduction - Images from the retina are analyzed for form/movement/color, then later for depth - Development of retina: omitted from this outline, not covered in lecture I. Structure of retina - The retina is composed of three main layers 1) Outer nuclear layer: photoreceptors, no direct blood supply 2) Inner nuclear layer: bipolar, horizontal, amacrine, Mueller glial cell bodies 3) Ganglion cell layer: retinal ganglion cells - The outer and inner plexiform layers are sites of synapses between layers II. Photoreceptors - There are two types of photoreceptors 1) rods: most sensitive in dim light, only one type 2) cones: less sensitive to light, three types mediate color vision - Distribution of photoreceptors is not uniform: fovea has high density of cones and few rods, periphery has more rods - The outer segment of rods and cones have stacks of disks with high density of pigment and provide a high probability of absorption of an incident photon - Pigment epithelial cells serve to (1) absorb light not absorbed by the rods/cones using melanin, (2) phagocytose shedded fragments of rods/cones, (3) regenerate pigment III. Visual pigments - Pigment is composed of a chromophore covalently bound to an opsin 7TM protein - Humans have 4 pigments (1 rod, 3 cones) each maximally sensitive at different wavelengths - Upon photon absorption, the chromophore undergoes several configuration changes; the configuration that triggers vision takes several ms to reach; the ultimate result is the release of chromophore, and binding of fresh chromophore with opsin IV. Phototransduction - Rods/cones hyperpolarize in response to light in a graded fashion with respect to intensity, with cones requiring more light and responding more rapidly - Phototransduction mechanism is shown on p. 10, a prototypical signal transduction mechanism - In dark, cGMP keeps a nonselective cation channel open, and cell is depolarized - Rhodopsin is coupled to G-protein that activates phosphodiesterase and lowers [cGMP] -
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