human phys lec 10

human phys lec 10 - Partsoftheeye

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Parts of the eye
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Organization of vertebrate retina: Extensive circuitry among multiple cell types Note two classes of photoreceptor lls Outer retina cells Inner retina
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Outer segments of rods (yellow) and cones (green) g (y ) (g ) (The color was added to make it easier to see)
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Phototransduction takes place in the outer segment Rod Cone
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Visual pigments (opsins) are GPCR proteins that bsorb photons in photoreceptor outer segments absorb photons in photoreceptor outer segments Rhodopsin: the visual pigment of rods
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Opsins are like other GPCRs that carry out metabotropic signaling except that at rest they carry their own activating ligand covalently attached! In a dark adapted visual pigment, retinal is covalently attached to the opsin protein and is in the cis configuration Absorption of a photon of the appropriate wavelength causes isomerization of retinal to trans , which activates the opsin Photon (450 580 nm) The all rans f rm of retinal etaches from the psin. At that point, e a ta s o o et a detac es o te ops tta t po t, the opsin can no longer absorb light and is said to be bleached . Adjacent cells called pigment epithelium play a role to regenerate the resting opsins. g p
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More structurally precise representation of rhodopsin This the covalently attached all cis retinal
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Each type of photoreceptor opsin absorbs light at specific wavelengths S ML Rhod Humans have three cone opsins (S, M and L) plus rhodopsin in rods Bear in mind that opsins only absorb light if the all cis retinal is attached
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Elements of G protein signaling cascades in vertebrate rod photoreceptors Activating ligand All trans retinal GPCR Rhodopsin eterotrimeric rotein ansducin Heterotrimeric G protein Transducin (G T ) Effector enzyme cGMP phosphodiesterase (PDE) Second messenger cGMP (destroyed in this scade) cascade) All of the necessary elements of a typical G protein coupled cascade are present. owever there are some unusual features in vertebrate hototransduction However, there are some unusual features in vertebrate phototransduction.
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Vertebrate photoreceptors in the dark Guanylate cyclase MP + Na + and Ca 2+ TP cGMP cGMP gated channel (open) GTP Guanylate cyclase is active Intracellular cGMP levels are high cGMP steadily activates a cationic channel There is steady Na + and Ca 2+ influx into the cell (called the dark current)
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Responses to light: The vertebrate phototransduction cascade GMP Na + and Ca 2+ cGMP
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human phys lec 10 - Partsoftheeye

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