The
Vision: Retina
Structure of the eye
and retina
The photoreceptor
cells:
phototransduction
Information
processing in the
retina
Retinal ganglion cells
Cross section of the human eye
Choroid
1
Layered structure of the retina
Human retina as seen through an
ophthalmoscope
patients right eye
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2
Foveal pit: only photoreceptor cells.
Other cell layers are displaced laterally to
allow light to strike the photoreceptors directly
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Blind spot: light falling on the optic nerve
head cannot be detected
Demonstration: using your own fingers to detect the
blind spot
Why are we normally unaware of the blind spot?
Perceptual fill-in by the visual cortex.
Why cant we see our own blood vessels? Stabilized
image fades rapidly with time.
Corollary: no blood vessels in the fovea!
You actually can sometimes see white blood cells
traversing thin retinal vessels. Because they are
moving and dont contain hemoglobin, you can see a
white spot moving along the vessel.
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Retinal disease: macular degeneration
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Retinal disease: macular degeneration
with geographic atrophy
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Retinal disease: macular degeneration
nei.nih.gov
Retinal disease: retinitis pigmentosa
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Retinal disease: retinitis pigmentosa
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Retinal disease: glaucoma
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Retinal disease: diabetic retinopathy
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Phototransduction: converting light
into an electrical signal
First, something in the photoreceptor must
detect the presence of light. That is, there
must be a molecule that can absorb photons
in the visible part of the electromagnetic
spectrum.
Second, absorption of light by the
photosensor molecule must initiate some sort
of cellular signal.
Ultimately, to produce an electrical signal, the
ionic permeability of the plasma membrane
must be altered by opening or closing ion
channels
7
Primate rods and cones: scanning EM
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8
9
Light response of rod photoreceptor
light
ash
-40
Em
(mV)
-60
10
Rod photoreceptor
Close-up of OS plasma membrane
The photosensor molecule of rods: rhodopsin
Rhodopsin is the combination of a protein molecule, called
opsin, and a light-absorbing chromophore molecule, retinal
(short for retinaldehyde). Retinal is covalently attached to opsin
at a particular lysine residue (residue 296).
11
Photoisomerization of retinal
Retinal is a derivative of
vitamin A1, which is in turn
derived from -carotene
(the orange pigment from
carrots)
In the dark state, retinal
attached to opsin is in the
bent form (11-cis retinal)
When retinal absorbs a
photon, it photoisomerizes
to the straight form (alltrans retinal)
This simple change is the
only light-dependent event
in all of vision
attached here to
lysine 296 of
opsin
Light
Rhodopsin
psec
Lumirhodopsin
~ 1 msec
Metarhodopsin I
~ 1 msec
Metarhodopsin II
minutes
Pararhodopsin
minutes
Opsin + free all-transretinal
(bleaching)
12
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Rhodopsin is a G-protein-coupled receptor
Opsin is related to neurotransmitter-gated receptors
that act indirectly through activation of G proteins
(recall neurotransmitter lectures).
Rhodopsin can be considered a ligand-activated
receptor, except that the ligand (retinal) is covalently
attached to the receptor site.
In the dark, the ligand is in an inactive state (11-cis
retinal).
Upon photoisomerization, the ligand converts to state the
active (all-trans retinal) and the G-protein-coupled
receptor is activated.
The G protein that rhodopsin activates is called
transducin.
Transducin is a heterotrimeric G protein, like those
involved in many neurotransmitter effects that act via
G-protein-coupled receptors.
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Heterotrimeric G-protein, Transducin
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The phototransduction cycle
depolarization
Na+ and Ca2+
influx
channel opens
with cGMP bound
cGMP-gated channel
14
Phosphodiesterase
hydrolyzes cGMP to GMP
Structure of cGMP-gated channel
cGMP
15
Turning off
rhodopsin:
interaction
between arrestin
& rhodopsin
Transduction summary
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Adaptation
Just like some of the skin sensory receptors,
photoreceptors also show adaptation during
continued presence of light.
In darkness, photoreceptors are maximally
sensitive to light, and their responses are large
and slow.
In the presence of background light, however,
photoreceptors become less sensitive to light
and their responses become faster.
This adaptation process is controlled by the
calcium concentration inside the photoreceptor
outer segment.
In darkness, Ca2+ enters through open cGMP-gated
channels and binds to two target proteins: recoverin and
GCAP (guanylate cyclase activating protein).
17
With Ca2+ bound, recoverin inhibits rhodopsin kinase,
which prolongs and strengthens the light response
(good in darkness).
Without Ca2+ bound, GCAP binds to and activates guanylate
cyclase, which then synthesizes cGMP at a higher rate. This
promotes fast recovery and makes responses smaller (good in
background light).
18
Bipolar cells and
horizontal cells
receive synaptic
inputs from
photoreceptors
Two types of bipolar cells: On and Off
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Horizontal cells are depolarized by glutamate
released from photoreceptor synapses
ike photoreceptors, horizontal cells are relatively
L
depolarized in darkness and hyperpolarize in response to
light falling on the photoreceptors from which they receive
synaptic contacts.
eurotransmitter release from horizontal cells is therefore
N
highest in darkness and decreases during illumination.
orizontal cells produce a synaptic effect in bipolar cells
H
that opposes the effect of illuminating the photoreceptors
that are directly connected to the bipolar cell.
o, an Off-type bipolar cell depolarizes when the
S
horizontal cells hyperpolarize, and an On-type bipolar cell
hyperpolarizes when the horizontal cells hyperpolarize.
Receptive field of a
bipolar cell
Example: Off-type bipolar cell.
orizontal cells produce negative
H
feedback at the synapse between
photoreceptors and bipolar cells
(i.e., the action of the horizontal
cell opposes the direct action of the
photoreceptor on the bipolar cell).
n the case of Off-type bipolar
I
cells, horizontal cells cause the
bipolar cell to depolarize.
orizontal cells receive inputs
H
from many more photoreceptors
than bipolar cells typically do. So,
the spatial extent of the response
mediated by horizontal cell
feedback is larger.
20
Center-surround
receptive field:
bipolar cell
responds best to
spatial contrast
of illumination.
Diffuse
illumination is
not very
effective.
Receptive field of retinal ganglion cells also has centersurround organization. This is an ON-center cell.
Now, however, instead of graded potentials like bipolar cells,
information about illumination is encoded by the frequency of
action potentials.
21
This is an OFF-center cell.
Movie of ON-center cell
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Movie of OFF-center cell
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