Lecture #21

Retinal wiring4/16/13

Wiki

• Wiki how to session for last 15 minutes of class

• Stop me at 1:30

Rest of semester schedule

4/18 Intro page and 3-5 refs4/25 One of main pages done5/2 Question for exam5/9 Last class: Intro page and 3 detail

pages done5/16 Final

Final exam

Thursday

May 16th

1:30-3:30

Human tetrachromats

Human red and green opsins

530 nm

560 nm

A

S

A

A164S=+2 nm

Y

F

T

F261Y=+10 nmA269T=+14 nm

554 nm

Human red opsins

552 nm

557 nm

A

S

Two variants which occur frequently in population

38%

62%

Three M/LWS cone opsins

Green MWS opsin 530 nm

Red LWS opsin 557 nm

Shorter red opsin 552 nm

At least two genes on each X chromosome. Since females are XX they have 4 M/LWS genes which can be of 3 different types

Does this matter?

• Does having 3 M/LWS genes plus an SWS gene make these women tetrachromats with superior color discrimination?

Previous studies used color matching

• Use sliders to mix green and red to match a target yellow

+ green + redJust right

Color matching

• Tetrachromatic females perform same as trichromatic females

• CaveatsSmall (2-10°) fields of viewCan only add either “red” or “green”

Human red opsins

552 nm

557 nm

A

S

Two variants which occur frequently in population = S and A

38%

62%

Opsin genotypes of 64 subjects for psychophysical tests

Sex Vision LWS versions #

Females Tetrachromats S / A 23

Trichromats S/S or A/A 15

Males Trichromats S or A 22

Dichromat S* or A* 4

Visual task - how many defined colors do you see?

Visual task - how many defined colors do you see?

“Normal” visual observers see 7 : ROYGBIV

Visual tasks• Mark edges of all

colors that you see• Count # colors

Visual tasks• Mark edges of all

colors that you see• Count # colors

• Female Tet 10±3• Female Tri 7.6 ±2• Male Tri 7.3 ±2• Male Di 5.3 ±1.5

Location of color start and stop - all 15 female trichromats

end start

Female tetrachromat color boundaries

Extra opsins

• These studies suggest that unique opsins are integrated into color opponency system and used

• This is similar to other senses where there are multiple receptors which change in # between species 100-1000 olfactory genes

• Why do most of us only have 3 visual pigments?

Kolb 2003

Wassle 2005

The retina• Complex set of cells

which preprocess visual information before it goes to the brain

Kolb 2003Light

Overall retinal responses

• Response to white vs blackTotal light received = luminance

• Response to blue vs yellow• Response to green vs red

• These three channels can describe visual perception (Young 1802)

How to understand retina

• Stain neurons Distinguish by morphologyUnravel connectivity

The retina = 3 layer cake• Three cellular

layers

Kolb2003

Outer nuclear layer

Inner nuclear layer

Ganglion cell layer

The retina = 3 layer cake• Three cellular

layers

• Two synaptic layersOuter plexiformInner plexiform

Kolb2003

6 kinds of cells

1 rod2 cone

3 horizontal cell4 bipolar cell5 amacrine cell

6 ganglion cell

Wassle 2005

Multiple cell types in each layer with specialized functions

• Outer nuclear layer3 types cones + rods

• Inner nuclear layer1-4 types horizontal cells1 rod and 10 cone bipolar cells22-30 types amacrine cells

• Ganglion cell layer20 types of ganglion cells

Neurotransmitters• Glutamate is the

signal from photoreceptors to bipolar cells to ganglion cells

• Horizontal and amacrine cellsExcitatory and inhibitory amino acids, catecholamines, peptides and NO

Rod connections – very simple

• Rod spherule connects toHorizontal cells andBipolar cells

Rod bipolarCone OFF

BP

Rod

Rod connections• Rod spherule is

junction of several cellsRod ON bipolar cells

metabotropic GluR

Horizontal cells ionotropic GluR -

AMPA

GluR=glutamate receptor

Synaptic ribbons• Only in neurons that

do not generate action potentialsPhotoreceptorsBipolar cells

• Ribbons have affinity for synaptic vessicles

• Improves / speeds neurotransmitter release

Cone connections more complex

• Cone pedicle connections

• TriadsON bipolar cells (invaginating)Horizontal cells

• Connections to OFF bipolar cells (flat)

Also have synaptic ribbons

How many connections for rods and cones?

Cone Rod

Bipolar cell

Also direct contacts between neurons• Gap junctions can

provide direct feedback between :Cone and coneCone and rod2 horizontal cells2 amacrine cells

• May help average out noise - increase receptive fieldSignal flows between many cells

Light responseLight

Cell hyperpolarizes

Glutamate output decreases

Glutamate release

Channels close

ON and OFF bipolar cells have different glutamate receptor

Light ON Turns offPhotoreceptor response Glu decrease Glu increaseBP cell ON bipolar OFF bipolarGluR type Inhibitory ExcitatoryBipolar response Glu increase Glu decrease

ON and OFF bipolar cells have different glutamate receptor

Light ON ONPhotoreceptor response Glu decrease Glu decreaseBP cell ON bipolar OFF bipolarGluR type Inhibitory ExcitatoryBipolar response Glu increase Glu decrease

Understanding retina

• Electrical response to lightCone or rod hyperpolarizes

ON bipolar depolarizes

OFF bipolar hyperpolarizes

Webvision

Bipolar cell response• ON bipolars go to

ON ganglion cells

• OFF bipolars go to OFF ganglion cells

Good to read light letters on dark background

Good to read dark letters on light background

The retina - 3 layer cake

Three cellular layers

Outer plexiform layer

Inner plexiform layer

Kolb2003

Many kinds of bipolar cells

If project to upper half of Inner Plexiform Layer (IPL) -

Off cellsIf project to lower half of IPL -On cells

Many kinds of bipolar cells

DB - diffuse bipolars contact many conesFMB - flat midget bipolars contact 1 cone - OFF responseIMB - invaginating midget bipolars contact 1 cone - ON responseBB - blue bipolars contact several S cones - ON responseRB - rod bipolars contact several rods - ON response

Cone connections more complex

• Cone pedicle connections

• TriadsON bipolar cells (invaginating)Horizontal cells

• Connections to OFF bipolar cells (flat)

Also have synaptic ribbons

Three types of horizontal cells

• HI robust dendrites but small fields

• HII finer dendrites• HIII bigger than HI so

bigger fields

Horizontal cells wired in certain ways

• HI - mostly M and L cones though to some rods or S cones in the field

• HII - more S cones as well as M and L

• HIII - just M and L cones

Action of horizontal cells fine tunes photoreceptor response

Narrow spatial response of photoreceptor

Horizontal cell has much larger receptive field - input from more cones - input from other horizontals

Horizontal cells modify cone response

They sharpen the response by indirectly or directly feeding back to conesCones can also feedback to each other or to bipolar

This makes a center surround response

Bipolar cells wired to ganglion cells in center surround fashion

ON center responds when light falls in middle and not neighboring receptors around

OFF center responds when light around them but not in middle

Smaller visual fields in foveaMidget ganglion cell wired 1:1 with midget bipolar cell

Maximizes visual acuity

Red / green cones are wired to two ganglion cells so can register ON or OFF response

Blue cones wired through separate bipolar and ganglion cells

Can send blue ON or yellow OFF responses

Types of ganglion cells• Determine these

morphologically• Also identify by which

layer of inner plexiform layer they synapse in

• Now over 24 kinds of ganglion cells

Amacrine cells also play role

Modify bipolar cell to ganglion cell response14 kindsNeurotransmitters are glycine and GABA

AII - glycine role in rodsA17 - GABA

Layered inner plexiform layer

• 5 layers with distinct types of cellular connections

OFF

ON

Center - surround color opponency

Webvision

Red ON green OFFRed OFF green ON

Green ON red OFFGreen OFF red ON

Center surround types

Center surround demo

Center surround can be used to detect stripes

Wolfe et al Sensation and Perception

Sensitive to angular spacing and locationEdge detection?

Rod pathways are connected to ON cone pathways #1

ON1 rod response Rod hyperpolarizes

Rod bipolar depolarizes

Contacts amacrine AII using GABA

AII directly couples to cone ON by gap junction

ON cone BP synapses to ON ganglion cell Wassle et al 2005

Rod pathways are connected to ON cone pathways #2

ON2 rod response Rod hyperpolarizes

Connects to cone by gap junction

Cone bipolar depolarizes

ON cone BP synapses to ON ganglion cell

Rod pathways are connected to OFF cone pathways

OFF1 : Rod depolarizes Rod BP hyperpolarizes - AII inverts through glycine emission - OFF cone BP - OFF ganglion

OFF2 : Rod - cone - OFF cone BP - OFF ganglion cell

OFF3 : Rod - OFF cone BP - OFF ganglion

Serial section Transmission Electron Microscopy

Mouse eye stained with antibodies to identify different cell types

Section retina embedded in plastic

Cut 70 nm thick sections

Use TEM to take pictures of sections

Each slide - take 1000 pictures = 3000 pictures per day

Start with low res picture of each section

Recreate section from 1000 high res pictures (5500 x)

One high res picture

Bipolar cells with amacrine cells - notice synaptic ribbon

24 panel display for viewing

Can then stain with antibodies to know which cell type it is

Antibodies to glutamate, GABA, glycine etc attached to silver particles

Another holy grail

• Retinal implantsThough photoreceptors are often destroyed by disease, the retinal ganglion cells often remainIf tie into these RGCs, can send message to brain

Retinal prostheses

Alternative strategies

Retinal implants

Optic nerve cuffs

Cortical implants

Camera sends signal to one of these three areas

Phosphenes• Firing from RGCs without light

Mechanical stimulationMagnetic stimulation - high fieldsElectrical stimulation

• Use implants which convert a visual scene into electrical stimulationRGCs - lower current and higher resolutionVisual cortex - takes higher currents

more difficult access

One other approach

• Make RGCs light sensitiveAdd light sensitive molecule which turns on RGCsThey can then be direct light detector

Possible retinal implant schema

Implant location

• Epiretinal - on top of retinaMaintains retina / RPE contactDevice is in vitreous where feels motion of fluid as eye moves

• Subretinal - behind retinaCan have light detectors which then directly stimulate RGCs - no camera needed

Retinal implants

Humayan et al at USC First used 4 x 4 array of electrodes

Up to 8 x 8 array

Goal is 32 x 32 array

Tests on 15 people by Liu and Humayan

• Electrical stimulation led to phosphenes• 14 of 15 identified location and tracked

stimulus as it moved• Spatial patterns were recognized• Macula had lower threshold (more sensitive)

Dr. Humayan

Babak Parviz, U Wash - contact lens virtual display