retinal physiology : from photon capture to spike trains, an overview cast of characters and...

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