Lecture 3 Vision 2 The Retina

Transcription

1 Lecture 3 Vision 2 The Retina All lecture material from the following two links: 1) 2) Raghav Rajan Bio 354 Neurobiology 2 January 15th

2 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

3 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 3d 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

4 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 A number of reflexes control focusing, controlling amount of light, self-clearning 4

5 The retina 0.25 mm How is light detected and how is this converted into a chemical signal? How do the other cells respond? What is the output of the retina? 5

6 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

7 Photoreceptors - Rods and cones about 1.25 million photoreceptors in each eye Rods vs. cones numbers distribution 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 no rods in fovea light detection capabilities # of rods > # of cones - ~ 20:1 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 less convergence from cones in the fovea, one cone projects to one bipolar cell 7

8 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 Old discs discarded at tips and phagocytosed by pigment epithelial cells 8

9 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

10 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

11 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

12 Phototransduction cascade Opsin + 11-cis retinal Opsin + All-trans retinal Activated rhodopsin Metarhodopsin II All-trans retinal Goes into pigment epithelium, converted back into 11-cis retinal Opsin Triggers decrease in cgmp levels by activating cgmp Phosphodiesterase 12

13 Phototransduction cascade opsin, which is released, activates G protein (transducin) Transducin (GTP-bound α subunit) activates cgmp phosphodiesterase to reduce cgmp 13

14 Light causes cell to hyperpolarise 14

15 How are rods so sensitive? Amplification One activated rhodopsin activates hundreds of transducin each of which can hydrolyse 103 cgmp molecules/s 15

16 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

17 The retina 0.25 mm How is light detected and how is this converted into a chemical signal? How do the other cells respond? What is the output of the retina? 17

18 The circuitry of the retina Direct pathway photoreceptor bipolar cell RGC Indirect pathway through horizontal cells, amacrine cells Connectivity patterns are different in the fovea and in the periphery 18

19 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 motionselective cells, no color complications 19