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Lecture 3 – Vision 2 – the Retina

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Lecture 3 – Vision 2 – the Retina Lecture 3 – Vision 2 – The Retina All lecture material from the following two links: 1) http://hubel.med.harvard.edu/book/bcontex.htm 2) http://www.ib.cnea.gov.ar/~redneu/2013/BOOKS/Principles%20of%20Neural%20Science%20- %20Kandel/gateway.ut.ovid.com/gw2/ovidweb.cgisidnjhkoalgmeho00dbookimagebookdb_7c_2fc~32.htm Raghav Rajan Bio 354 – Neurobiology 2 January 15th 2015 1 From the last class ..... ● Is there a difference in which percept is predominant? (Harini) ● What happens to percepts in split-brain patients? (Sahana) ● What happens in induced synesthesia? ● Why do you see an after-image with opposite colours much like a photo negative? (Gaurav) ● 2 Vision – the brain's BEST GUESS at what is out there ● Takes in information from the 2-d image on the retina and creates a 3- d image ● Many complex processes going on – eg: filling-in, etc. ● Constrained by architecture, wiring, properties of the nervous system ● Biased by experience ● Ultimately just one image of the world ● Achieved by distributed processing across multiple brain areas ● Visual attention ● HOW IS ALL THIS DONE – STARTING WITH LIGHT SENSING? 3 The eyeball ● Non-retinal parts – important to keep a clear, focused image on both retinas ● 3 pairs of extra-ocular muscles ● Cornea and lens ● How do we focus? Ciliary muscles ● Pupil – center of iris – controls amount of light coming in ● Self-cleaning – blinking and tear glands 4 ● A number of reflexes control – focusing, controlling amount of light, self-clearning The retina 0.25 mm ● How is light detected and how is this converted into a chemical signal? ● How do the other cells respond? 5 ● What is the output of the retina? The retina FOVEA - pit ● Retina – contains the photoreceptors – rods and cones ● Photoreceptors at the back – light passes through other layers (unmyelinated), except at the fovea ● Pigment melanin behind the retina – two functions – absorb light – help to restore light-sensitive visual pigment in the receptors after bleaching 6 Photoreceptors - Rods and cones ● about 1.25 million photoreceptors in each eye ● Rods vs. cones – numbers ● # of rods > # of cones - ~ 20:1 – distribution ● no rods in fovea – light detection capabilities ● rods are highly sensitive – can detect even single photons ● cones are less sensitive ● rods have long integration time and are slow (< 12 Hz); cones are fast (< 55 Hz) – photopigment absorption characteristics ● rods have only one photopigment ● cones are of 3 types – 3 different photopigments and so confer COLOR vision – Pattern of connections to bipolar cells ● many rods project to one bipolar cell – convergent 7 ● less convergence from cones – in the fovea, one cone projects to one bipolar cell Morphology of rods and cones ● Photopigment present in the outer segment – Each pigment molecule – 1 small light absorbing molecule + a large membrane protein ● Can be upto 108 pigment molecules in one cell ● Discs are membrane invaginations – greater surface area ● Outer segments constantly renewed – New discs formed at a rapid rate 8 – Old discs discarded at tips and phagocytosed by pigment epithelial cells Dark current in photoreceptors ● High amounts of cGMP inside cell ● cGMP-gated channels open – Na+ comes in ● K+ channels are open – K+ goes out ● Cell remains depolarised 9 Rhodopsin – the light detecting molecule ● Photopigment in rods – rhodopsin – Opsin (membrane protein) + retinal (light responsive molecule) ● Retinal (derivative of Vitamin A) can assume different isomeric configurations – 11-cis retinal – All-trans retinal 10 Light comes in ... ● Non-activated rhodopsin has 11-cis retinal which is bound to opsin ● 11-cis retinal absorbs light ● Rotation around double bond makes it into the more stable All-trans retinal ● Now retinal does not fit into binding site in opsin ● Rhodopsin becomes Metarhodopsin II ● Metarhodopsin II – unstable – splits 11 Phototransduction cascade Opsin + 11-cis retinal Opsin + All-trans retinal Activated rhodopsin Metarhodopsin II All-trans retinal Opsin Triggers decrease in Goes into pigment cGMP levels by epithelium, converted activating cGMP back into 11-cis retinal Phosphodiesterase 12 Phototransduction cascade ● opsin, which is released, activates G protein (transducin) ● Transducin (GTP-bound α subunit) activates cGMP phosphodiesterase to reduce cGMP 13 Light causes cell to hyperpolarise 14 How are rods so sensitive? ● Amplification – One activated rhodopsin activates hundreds of transducin each of which can hydrolyse 103 cGMP molecules/s 15 What stops this transduction, amplification process? ● Transducin getting inactivated through its GTPase activity – hydrolyses bound GTP to GDP ● Activated rhodopsin is a target for phosphorylation – following which it interacts with a regulatory protein arrestin causing rapid inactivation of rhodopsin 16 The retina 0.25 mm ● How is light detected and how is this converted into a chemical signal? ● How do the other cells respond? 17 ● What is the output of the retina? The circuitry of the retina ● Direct pathway – photoreceptor – bipolar cell – RGC ● Indirect pathway through horizontal cells, amacrine cells 18 ● Connectivity patterns are different in the fovea and in the periphery Receptive fields of RGCs – output of retina ● Stephen Kuffler was the first to find these responses in the cat ● Diffuse light throughout the field did not evoke any responses! (earlier research) ● Cat was a good choice because of no motion- selective cells, no color complications 19.
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