Lecture 3 – Vision 2 – The

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- parts – important to keep a clear, focused image on both

● 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 – 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 – the light detecting molecule

● Photopigment in rods – rhodopsin – (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

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