Retinal Physiology: from photon
capture to spike trains, an overview
Retinal Physiology: from photon
capture to spike trains, an overview Cast of characters and
personalities, Who’s on first . . . Wiring
diagrams of the most studied of human neural circuits.
Themes, patterns and guiding principles; some views of the forest.
Cast of characters and personalities,
Who’s on first . . . Wiring diagrams of the most studied of human neural circuits.
Themes, patterns and guiding principles; some views of the forest.
Building a light sensorBuilding a light sensor
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Tartuferi 1887Tartuferi 1887
Ganglion cell layer Amacrine cells
Bipolar Cells
Horizontal Cells
Rod and Cone Photoreceptors
Key points to rememberKey points to remember
Duel photoreceptors system (rods & cones) extend the range of visual function.
The minimal direct pathway for a signal is photoreceptor to bipolar cell to ganglion cell.
Horizontal and amacrine cell form lateral connections and are critical for lateral inhibtion and center-surround organization.
Duel photoreceptors system (rods & cones) extend the range of visual function.
The minimal direct pathway for a signal is photoreceptor to bipolar cell to ganglion cell.
Horizontal and amacrine cell form lateral connections and are critical for lateral inhibtion and center-surround organization.
Key points to rememberKey points to remember
The retina is interested in contrast differences, edges, light vs. dark. The on-off pathways are critical for
making these comparisons. This division begins at the bipolar cell level.
The center-surround receptive field is a key feature of retinal output. It starts at the bipolar cells.
The retina is interested in contrast differences, edges, light vs. dark. The on-off pathways are critical for
making these comparisons. This division begins at the bipolar cell level.
The center-surround receptive field is a key feature of retinal output. It starts at the bipolar cells.
LIGHT
Local specializations Local specializations
ON head
Major blood vessels
ON head
Major blood vessels
PhotoreceptorsPhotoreceptors
The only neurons of the visual system that sense light..
Duality (rods and cones) permits specialization into two systems (1) for high sensitivity and (2) for spatial and temporal sensitivity (plus color).
Metabolically very high maintenance cells.
The only neurons of the visual system that sense light..
Duality (rods and cones) permits specialization into two systems (1) for high sensitivity and (2) for spatial and temporal sensitivity (plus color).
Metabolically very high maintenance cells.
v. Photoreceptor distribution
v. Photoreceptor distribution
Fig 15.12
Rod photoreceptor (system)
Rod photoreceptor (system)
95% of photoreceptor in human eye
Single photon sensitivity (amazing feat)
High degree of spatial summation . . . Therefore LOW spatial acuity
Slow time-to-peak, slow recovery . . . Therefore LOW temporal resolution
Dark, starlight, moonlight (2.5 log units)
95% of photoreceptor in human eye
Single photon sensitivity (amazing feat)
High degree of spatial summation . . . Therefore LOW spatial acuity
Slow time-to-peak, slow recovery . . . Therefore LOW temporal resolution
Dark, starlight, moonlight (2.5 log units)
Cone(system) functionCone(system) function
In foveate animals the overwhelming majority of visual behavior depends on this minute (<1%) patch of retina. (Tiny area of high spatial acuity).
Fast response time, but insensitive.
Wide range of adaptation (6+ Log Units)
. . . And in living color too.
In foveate animals the overwhelming majority of visual behavior depends on this minute (<1%) patch of retina. (Tiny area of high spatial acuity).
Fast response time, but insensitive.
Wide range of adaptation (6+ Log Units)
. . . And in living color too.
Light is the ligand that triggers activation of the enzyme.
Biochemistry of Phototransduction
Biochemistry of Phototransduction
Rhodopsin is the classic example of a 7-trans-membrane spanning G-protein coupled receptor. (ligand = photons)
High gain means high sensitivity (But takes time to develop).
Smaller Responses , due to lower sensitivity (low gain) are over faster resulting in higher . . .
Rhodopsin is the classic example of a 7-trans-membrane spanning G-protein coupled receptor. (ligand = photons)
High gain means high sensitivity (But takes time to develop).
Smaller Responses , due to lower sensitivity (low gain) are over faster resulting in higher . . .
Photocurrent & Photovoltages
Graded responses (no spikes)
Photocurrent & Photovoltages
Graded responses (no spikes) Photoreceptors are partially
depolarized in the dark (due to an influx of Na+ and Ca2+ ions).
Light shuts off the influx, thus the cells hyperpolarize and . . .
Neurotransmitter is released constantly in the dark and this release is attenuated by light!!!! (GLUTAMATE)
Photoreceptors are partially depolarized in the dark (due to an influx of Na+ and Ca2+ ions).
Light shuts off the influx, thus the cells hyperpolarize and . . .
Neurotransmitter is released constantly in the dark and this release is attenuated by light!!!! (GLUTAMATE)
Themes of retinal circuitry(RECEPTIVE FIELDS)
Themes of retinal circuitry(RECEPTIVE FIELDS)
Highest visual acuity and fidelity of signals carrying that message requires requires a private line (Midget system).
SPLITTING (push-pull or on-off systems) Divergent wiring also Midget.
Highest sensitivity (Greatest summation) Convergent wiring.
Highest visual acuity and fidelity of signals carrying that message requires requires a private line (Midget system).
SPLITTING (push-pull or on-off systems) Divergent wiring also Midget.
Highest sensitivity (Greatest summation) Convergent wiring.
Private Line Divergence Convergence
Private Line Divergence Convergence
Themes of retinal circuitry
Themes of retinal circuitry
Horizontal Cells and Amacrine cells provide lateral pathways in the retina.
Feedback and feed forward synaptic interactions add flexibility and complexity
Spatial filters (Lateral inhibition) Temporal filters (Directional selectivity) Network gain control (light/dark
adaptation)
Horizontal Cells and Amacrine cells provide lateral pathways in the retina.
Feedback and feed forward synaptic interactions add flexibility and complexity
Spatial filters (Lateral inhibition) Temporal filters (Directional selectivity) Network gain control (light/dark
adaptation)
On and Off pathwaysOn and Off pathways
Divergent wiring Same neurotransmitter different
responses.(receptor biochemistry)
On bipolar :sign inverting feeds onto ON ganglion cells (SPIKING INCREASES)
Off bipolar :sign conserving feeds onto Off ganglion cells (SPIKING Diminishes)
Divergent wiring Same neurotransmitter different
responses.(receptor biochemistry)
On bipolar :sign inverting feeds onto ON ganglion cells (SPIKING INCREASES)
Off bipolar :sign conserving feeds onto Off ganglion cells (SPIKING Diminishes)
The Off pathwayThe Off pathway Light hyperpolarizes
photoreceptors. Transmitter release goes
down. Off bipolar cells
hyperpolarize. Transmitter release goes
down. Ganglion cells hyperpolarize. Spike frequency (rate) goes
down.
Light hyperpolarizes photoreceptors.
Transmitter release goes down.
Off bipolar cells hyperpolarize.
Transmitter release goes down.
Ganglion cells hyperpolarize. Spike frequency (rate) goes
down.
The On pathwayThe On pathway Light hyperpolarizes
photoreceptors. Transmitter release goes
down. On bipolar cells depolarize. Transmitter release goes
up. Ganglion cells depolarize. Spike frequency (rate) goes
up.
Light hyperpolarizes photoreceptors.
Transmitter release goes down.
On bipolar cells depolarize. Transmitter release goes
up. Ganglion cells depolarize. Spike frequency (rate) goes
up.
Building Receptive FieldsBuilding Receptive Fields
Center and surround organizations
On or Off responses (anatomical correlation with sublaminae of IPL)
Transient and sustained physiology
Color coding
Center and surround organizations
On or Off responses (anatomical correlation with sublaminae of IPL)
Transient and sustained physiology
Color coding
Center Surround receptive fields require lateral interactions.
Center Surround receptive fields require lateral interactions.
`̀
Bipolar Cell morphology/physiolgy
Bipolar Cell morphology/physiolgy
Rod vs. Cone (Rod Bp output ??)
Midget vs. diffuse On vs. Off Unique Blue cone bipolar
(non-midget) Some of the anatomical
subdivisions have no physiological correlates and recently vice versa (contrast sensitivity)
Rod vs. Cone (Rod Bp output ??)
Midget vs. diffuse On vs. Off Unique Blue cone bipolar
(non-midget) Some of the anatomical
subdivisions have no physiological correlates and recently vice versa (contrast sensitivity)
Several distinct morphological types have been identified.
Some match physiological types
Others remain unclassified
Several distinct morphological types have been identified.
Some match physiological types
Others remain unclassified
Ganglion Cell morphology predicts physiology
Ganglion Cell morphology predicts physiology
The Rod piggyback-pathway
The Rod piggyback-pathway
Without a rod-specific ganglion cells, how does the brain receive rod signals?
Through the AII amacrine cells, rods piggy-back their signal through the cone pathway.
Rod Rod Bp AII Cone Bp cone ganglion cells etc. etc.
Without a rod-specific ganglion cells, how does the brain receive rod signals?
Through the AII amacrine cells, rods piggy-back their signal through the cone pathway.
Rod Rod Bp AII Cone Bp cone ganglion cells etc. etc.
Rod Pathway
Rod Pathway No Direct
Ganglion cell output.
Rod bipolar to amacrine cell
A combination of electric and chemical synapses
Output through cone G-cells
No Direct Ganglion cell output.
Rod bipolar to amacrine cell
A combination of electric and chemical synapses
Output through cone G-cells
AII amacrine cellAII amacrine cell
How to spikes encode information?
How to spikes encode information?
Spatial information (by anatomical mapping to topographic cortex)
Temporal codes Functional mapping (color
signals to color cortex, motion signals to motion cortex etc.)
Cross correlation between neighboring cells or groups of cells (new horizons).
Spatial information (by anatomical mapping to topographic cortex)
Temporal codes Functional mapping (color
signals to color cortex, motion signals to motion cortex etc.)
Cross correlation between neighboring cells or groups of cells (new horizons).
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
Center from on (+) bipolar Surround from off (-)
bipolars
Center from on (+) bipolar Surround from off (-)
bipolars
-+-
Antagonistic Center Vs. Surround
Center Surround receptive fields require lateral interactions.
Center Surround receptive fields require lateral interactions.
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
What if the small spot of light illuminates the center of the cells’ receptive field ?
What if the small spot of light illuminates the center of the cells’ receptive field ?
-
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
What if a large spot of light illuminates the center of the cells’ receptive field ?
What if a large spot of light illuminates the center of the cells’ receptive field ?
-
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
What if the spot of light hits the surround?
What if the spot of light hits the surround?
-
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
What if the surround is optimally stimulated?
What if the surround is optimally stimulated?
-
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
What uniform illumination?
What uniform illumination?
-
THE VISUAL SYSTEM CARES ABOUT CHANGE, CONTRAST, Not about uniform retinal illumination.
Ganglion cell Receptive Fields
Ganglion cell Receptive Fields
Figure 28-3Summary of ganglion cell receptive fields, showing the spike trains generated by the stimulus.
How do spikes encode information?
How do spikes encode information?
Spatial information (by anatomical mapping to topographic cortex)
Temporal codes Functional mapping (color
signals to color cortex, motion signals to motion cortex etc.)
Cross correlation between neighboring cells or groups of cells (new horizons).
Spatial information (by anatomical mapping to topographic cortex)
Temporal codes Functional mapping (color
signals to color cortex, motion signals to motion cortex etc.)
Cross correlation between neighboring cells or groups of cells (new horizons).
Building Receptive FieldsBuilding Receptive Fields
Center and surround organizations
On or Off responses (anatomical correlation with sublaminae of IPL)
Transient and sustained physiology
Color coding
Center and surround organizations
On or Off responses (anatomical correlation with sublaminae of IPL)
Transient and sustained physiology
Color coding