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Lecture 4 – Vision 3 – The Retina

Raghav RajanBio 354 – Neurobiology 2

January 21st 2015

All lecture material from the following two links:1) http://hubel.med.harvard.edu/book/bcontex.htm2) 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

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From the last two classes ...!!

● 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)

● Cone opsins – how do they work?

● Why 3 pairs of extra-ocular muscles?

●

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Retinal processing

● Photoreceptors – graded potentials (no spikes)

● Photoreceptors are depolarised at rest – dark current

● Light hyperpolarizes the photoreceptors

– cGMP dependent mechanism

● Output of the retina consists of ON-center, OFF-surround RGCs and OFF-center, ON-surround RGCs

● How does this arise?

● What are the responses of other cells?

● What processing goes on in the retina?

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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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Receptive fields

● Useful for predicting responses of neurons to stimulus

● Not obvious from the wiring diagram

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Wiring diagram of retina

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Receptive fields of neighbouring RGCs

● Neighbouring RGCs have considerable overlap in their receptive fields

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Size of receptive fields

● Size measured by either size on retina or angle

● Primate retina, near fovea – ~10um or ~0.017degrees

● Further out, size of receptive field centers increase, size of receptor population connecting to RGCs (direct pathway) increases and visual acuity falls

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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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How do bipolar cells respond?

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Bipolar cells also have center-surround receptive fields

● Two types – ON-center and OFF-center

http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/bipolar-cell-pathways-in-the-vertebrate-retina/

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Direct pathway to ON and OFF center bipolar cells

● Cones/rods

– hyperpolarized by light

● ON center bipolar cell

– depolarized by light in center

– Inhibitory connection from cones/rods

● OFF center bipolar cell

– hyperpolarized by light in center

– Excitatory connection from cones/rods

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ON and OFF bipolar cells - mechanism

● Photoreceptors release only 1 neurotransmitter – glutamate

● OFF bipolars express ionotropic receptors

● ON bipolars express mGluR6, which links to TRPM1-L channels, non-selective cation channel

http://retina.anatomy.upenn.edu/~noga/noga.html

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Indirect pathway for SURROUND

● Lateral inhibition through horizontal cells

● GABA used as neurotransmitter

● Horizontal cells are thought to contribute to most of SURROUND responses

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CENTER-SURROUND interactions

● Horizontal cells mediate surround interactions

http://thebrain.mcgill.ca/flash/a/a_02/a_02_cl/a_02_cl_vis/a_02_cl_vis.html

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Human retina has 4 major types of RGCs

● Midget (M) cells – 80% - project to parvocellular pathways

– small cells

– slow responses

– colour, details

● Parasol (P) cells – 10% - project to magnocellular pathways

– big cells

– fast responses

– movement

● Bistratified cells

● Light sensitive RGCshttp://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/morphology-and-circuitry-of-ganglion-cells/

BIO 354 - Neurobiology 2 17

Dorsal and ventral streams – M cell and P cell pathways

http://www.ib.cnea.gov.ar/~redneu/2013/BOOKS/Principles%20of%20Neural%20Science%20-%20Kandel/gateway.ut.ovid.com/gw2/ovidweb.cgisidnjhkoalgmeho00dbookimagebookdb_7c_2fc~31.htm

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Actually lots more cell types in retina

● Don't know what most of these cells are doing

http://wws.weizmann.ac.il/neurobiology/labs/ulanovsky/sites/neurobiology.labs.ulanovsky/files/uploads/schneidman_compulsory_reading_mastland_2001_gollisch_meister_2010.pdf

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More complicated processing too

● Retina has direction selective ganglion cells (rabbit, frogs)

● Color comparisons are done by RGCs

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Starburst amacrine cells are direction selective – input to direction selective

RGCs

● Selective for outward motion

● Timing of excitation and inhibition determines motion selectivity

http://www.sciencedirect.com/science/article/pii/S096098220602611X

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Distribution of S, M and L cones in the human retina

● Diagram of distribution based on data

http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html

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Colour comparison computations start in the retina

● a – Foveal Midget cell that does M–L

● b – Foveal Midget cell that only connects to L cones

● c – Peripheral midget cell Some bias in the connections of this cell

● d – Small bistratified ganglion cell does S – (M+L)

http://neurosurgery.stanford.edu/research/chichilnisky/PDFs/Field2010-NV.pdf

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Retinal processing and output

● Center-surround organisation of receptive fields

– Basically codes for CONTRAST – compares illumination of center with surround

● Two different pathways for responding to light increases and light decreases

– ON-center and OFF-center RGCs

● M and P cell pathways

● Lateral inhibition, convergence, etc.

● Direct pathway – photoreceptor --> Bipolar cell --> RGC

● Indirect pathway – through horizontal and amacrine cells – SURROUND

● More complex responses – motion sensitivity, colour comparisons, etc. in retina itself