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Photoreceptors
| Term | Definition |
|---|---|
| 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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