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.
Building a light sensor
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Tartuferi 1887
Ganglion cell layer
Amacrine cells
Bipolar Cells
Horizontal Cells
Rod and Cone
Photoreceptors
Key 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.
Key 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.
LIGHT
Local specializations
 ON head
 Major
blood
vessels
Photoreceptors
 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
Fig 15.12
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)
Cone(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.
Light is the
ligand that
triggers
activation
of the
enzyme.
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 . .
.
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)
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.
 Private Line
 Divergence
 Convergence
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)
On 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)
The 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.
The 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.
Building Receptive Fields
 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.
`
Bipolar Cell
morphology/physiolgy
Rod vs. Cone (Rod Bp output ??)
Midget vs. diffuse
On vs. Off
Unique Blue cone bipolar (nonmidget)
 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
Ganglion Cell morphology
predicts physiology
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.
Rod Pathway
 No Direct
Ganglion cell
output.
 Rod bipolar to
amacrine cell
 A combination of
electric and
chemical
synapses
 Output through
cone G-cells
AII amacrine cell
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).
Ganglion cell Receptive Fields
 Center from on (+) bipolar
 Surround from off (-) bipolars
+-
Antagonistic Center Vs. Surround
 Center Surround
receptive fields
require lateral
interactions.
Ganglion cell Receptive Fields
 What if the small spot of
light illuminates the center
of the cells’ receptive field ?
-
Ganglion cell Receptive Fields
 What if a large spot of light
illuminates the center of the
cells’ receptive field ?
-
Ganglion cell Receptive Fields
 What if the spot of light hits
the surround?
-
Ganglion cell Receptive Fields
 What if the surround is
optimally stimulated?
-
Ganglion cell Receptive Fields
 What uniform illumination?
-
THE VISUAL SYSTEM CARES ABOUT CHANGE,
CONTRAST, Not about uniform retinal illumination.
Ganglion cell Receptive Fields
Figure 28-3
Summary of ganglion cell
receptive fields, showing
the spike trains generated
by the stimulus.
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).
Building Receptive Fields
 Center and surround
organizations
 On or Off responses
(anatomical correlation with
sublaminae of IPL)
 Transient and sustained
physiology
 Color coding