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Sensation of light
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Retina 39. Bipolar cell Horizontal cell amacrine cell ganglion cell
rod cone Bipolar cell Horizontal cell amacrine cell ganglion cell 5 cell types, 50 various cells
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Chemical and light signals
40. Chemical and light signals neuron photoreceptor NEUROTRANSMITTER LIGHT G protein –coupled receptor (rodopsin) G protein –coupled receptor Effector enzyme Effector enzyme opszin retinál G protein G protein (transducin) Secondary messenger (increase of amount) Secondary messenger (decrease of amount) Ion channel (open or close) Ion channel (close)
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Phototransduction 41. dark light depolarized state hyperpolarization
cGMP-gated Na+- channel Na+: INWARD K+: OUTWARD Na+: K+: OUTWARD Dark current K+- channel K+ K+ Na+ Na+ depolarized state hyperpolarization - 40 mV mV Na+/K+ pump light impulse
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Phototransduction 42. rhodopsin = opsin + retinal retinal binding site
Space between discs George Wald Nobel Prise, 1967 disc membrane opsin cytosol retinal cytoplasmic tail cis-retinal Light absorption curved cytoplazmic tail rhodopsin = opsin + retinal retinal: isomerization (G-protein-coupled metabotropic receptor) trans-retinal opsin: structural change non-curved cytoplazmic tail
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glutamate neurotransmitter
Dark current - depolarization 43. Na+ Na+ cGMP-gated ion channel Na+ Na+ Na+ cGMP phosphodiesterase rod rhodopsin R cGMP T GDP I + K+ transducin K+ Na+ I + K+ K+ + I guanilyl cyclase GTP cGMP Transducin: G-protein rodopsin: G-protein-coupled metabotropic receptor glutamate neurotransmitter (analogous release)
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+ + + + + + I I I LIGHT I I I Phototransduction 44.
- hyperpolarization 44. Na+ Na+ Na+ Na+ cGMP-gated ion channel Na+ cGMP phosphodiesterase rhodopsin R cGMP T T I + GMP I + GDP GTP GTP GDP K+ transducin I + Na+ K+ LIGHT K+ I + K+ I + I + 1 photon activates 500 transducin molecules, which in turn close several hundred ionchannel, and protects the entrance of 106 Na+ - Approximately 10% of the incoming light reaches the retina glutamate neurotransmitter
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