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Photoreceptor locations in retina Back of retina -> pigment epithelium (rhodopsin regeneration) -> outer segment (phototransduction) -> connecting cilium -> inner segment (normal cellular machinery) -> front of retina
Rod outer segment Stacked membranous discs w/ visual pigment, contains transduction cascade enzymes, new discs pinch off base and migrate to tip of outer segment (phagocytosed by pigment epithelium)
Cone outer segment Continuous folds of invaginating lamellae -> tightly packed to increase SA
Rhodopsin Rod visual pigment GPCR -> rod outer segment stacked membranous discs maximise # of rhodopsin molecules -> single photon reaches multiple rhodopsins
Inactive rhodopsin Covalently bound to 11-cis-retinal chromophore by Lys296 in 7th TMD -> blocks its own binding pocket
Free retinal absorption Usually only absorbs UV -> when covalently bound to opsins, peak absorption wavelength shifts to 500 nm (visible light)
Active rhodopsin 11-cis-retinal chromophore binds photon -> isomerises to all-trans-retinal -> rhodopsin conformational change -> all-trans-retinal fits into binding pocket and rhodopsin changes into metarhodopsin II (R*) -> activates phototransduction cascade
Photopigment transport out of rod outer segment All-trans-retinal dissociates slowly from opsin (rhodopsin is bleached) -> reduced to all-trans-retinol (NADPH) -> transported out of photoreceptor via IRBP into RPE
Photopigment regeneration RPE -> all-trans-retinol esterified to all-trans-retinyl ester -> de-esterified/isomerised into 11-cis-retinol (isomerohydrolase) -> oxidised to 11-cis-retinal (NAD+) -> transported back to rod outer segment via IRBP Takes >30 mins for full regeneration
Cone photopigment regeneration Alternative pathway to RPE via Muller cells for rapid pigment regneration
Retinal Derivative of 11-cis-retinol (Vit A) -> Vit A deficiency leads to night blindness
Retinitis pigmentosa Hereditary (5-10% cases from rhodopsin gene mutations) -> 1/3000 people, adolescence (gradual night blindness onset), adulthood (loss of all peripheral vision), extreme cases (total blindness)
What is the dark current? High [cGMP] -> cation channels open -> Na+/Ca2+ infux into outer segment and K+ efflux out of inner segment -> cell depolarisation held at -30 mV
What is the purpose of the dark current? Sets absolute lower limit for vision (signal = dark), rhodopsin thermal ismoerisation -> spontaneous outer segment cGMP channel closure (cGMP hydrolysis) -> reduction of dark current (inner K+ efflux continues) -> hyperpolarisation -> reduce Glu release
What is adaptation? Avoid saturation of photoreceptors as they must operate over wide range of intensities, as light intensity increases, photoreceptor sensitivity reduces
Why do photoreceptors saturate? cGMP hydrolysed and cation channels close -> cGMP must be resynthesised via guanylyl cyclase (inhibited by Ca2+ ions)
When do rods and cones saturate? Rods saturate at relatiely low light intensities, cones are 50x less sensitive than rods but respond more quickly -> mediate photopic vision as they continue adapting even in the brightest conditions
What are presynaptic ribbons? Photoreceptors have modified presynaptic density of synapses transmitting graded signals to BC/GC-> different density of clustered vesicles around the synaptic ribbon
What is the synaptic triad? BC/HC and photoreceptor synapses
Cone presynaptic ribbon? Pedicle -> synaptic swelling w/ <30 synapses at each pedicle diverging signal to multiple BCs -> invaginating/flat synaptic contacts
Rod presynaptic ribbon? Spherule -> many rods at each spherule converging multiple signals to single BC -> loss of spatial resolution -> at low light intensities, occasional photo is caught and signal needs to converge to strengthen detection
Magnocellular retinal GC receptive field Centre-surround -> on/off
Photoreceptor receptive fields Centre-surround -> light must hyperpolarise photoreceptor so must be off-centre (light hitting centre region will cause hyperpolarisation)
BC receptive fields Centre-surround -> on/off
Critical fusion frequency Frequency of light above which a flickering light is perceived as steady
Photoreceptor fusion frequency Slow rod responses -> <15 Hz, fast cone responses (better temporal resolution) -> <60 Hz
Created by: vykleung
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