Lecture #21 Retinal wiring 4/16/13
Feb 24, 2016
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