Transcript PPT

The Visual System
General plan for visual system material:
• How the visual input is received and transduced at
the retina by photoreceptors (rods and cones)
• How that input is modified by processing in the
retina, concept of receptive fields
• How that input is projected to the central nervous
system (to the lateral geniculate nucleus (LGN)
and cortex), the anatomy and processing within
that pathway
The Retina
Images focused by
the lens go through
the liquid within the
eyeball and fall on
the retina. In most
of the retina, layers
of neurons lie over
the photoreceptors,
and light must go
through them.
Fig 15-1
The Retina
At the fovea, the overlying cells are displaced, so the visual image
is clearest there. There are more photoreceptors at the fovea.
The Retina –
classes of neurons
• Light goes through other
layers of neurons before being
transduced at the rod and cone
photoreceptors
• Photoreceptors send the light
information to the bipolar
cells, which send it to the
ganglion cells, and the
information exits the retina to
the brain
• Horizontal cells and amacrine
cells modulate the information
Fig 15-2
Photoreceptors of the Retina
• Rods and cones
Fig 15-3
Photoreceptors of the Retina
• Rods and cones are specialized
Rods are highly sensitive to light, and thus are good for
dim light or night vision. They are able to capture more
light, but they do not respond well to moving stimuli
because their response time is slow.
Cones are not as sensitive to dim light, and are able to
respond quickly, and therefore transduce moving stimuli.
They are used more in daytime vision, and also are able to
detect color because three classes of cones express three
different photopigments. Cones are more concentrated in
the fovea of the retina.
Photoreceptors of the Retina
The outer
segments
contain
stacks of
membranes,
where the
lightabsorbing
molecules
are found
Fig 15-7
Light response
• Light causes a
hyperpolarization
In darkness, the rod
membrane potential is
relatively depolarized.
Light causes a
hyperpolarization by
reducing a Na
permeability; the current
is through a cGMPdependent cation
channel.
Light response
• Light causes a
hyperpolarization
Light causes closure
of a cation-permeable
channel, via a process
using cGMP as a second
messenger. If more
cGMP-dependent cation
channels are closed, the
membrane potential is
more hyperpolarized.
Light transduction
• Light is detected by
retinal
Rhodopsin (from rods)
is a combination of
retinal, the lightabsorbing molecule, and
a large protein called
opsin. The opsins are
closely related in
structure to the Gprotein-coupled
receptors.
Fig 15-9
Light transduction
• Light changes
retinal conformation
When retinal absorbs
light, it undergoes a
conformational change.
This change moves the
position of the opsin
molecule, which
activates a G protein
(transducin) and begins
a second messenger
pathway.
Fig 15-8
Phototransduction - cGMP changes
• Transducin activates a phosphodiesterase
In the dark, phosphodiesterase activity is quite low, thus levels of
cGMP are high; the cGMP-dependent cation channels are gated
open. When light activates rhodopsin, the levels of cGMP are
reduced by the phosphodiesterase, and the channels are closed.
Phototransduction - dark current
• In the dark, Vm is approximately -40 mV
In the rod, the cGMP channels
are located in the outer segment,
and a K channel is present in the
inner segment. In the dark, when
the cGMP concentrations are
high, current flows in through the
cGMP-gated channels and out
through the K channels.
Phototransduction - dark current
• Light hyperpolarizes the photoreceptor
With light stimulation, the cGMP
channels close.
Pathway: light
rhodopsin
rhodopsin
transducin
transducin
phosphodiesterase
phosphodiesterase
cGMP
cGMP
cGMP-gated channel
Phototransduction - dark current
• Light hyperpolarizes the photoreceptor
With light stimulation, the cGMP
channels are closed.
Pathway: light
rhodopsin
rhodopsin
transducin
transducin
phosphodiesterase
phosphodiesterase
cGMP
cGMP
cGMP-gated channel
Fig 15-11
Phototransduction - dark current
• AMPLIFICATION!
One rhodopsin molecule absorbs one photon
500 transducin molecules are activated
500 phosphodiesterase molecules are activated
105 cyclic GMP molecules are hydrolyzed
250 cation channels close
106-107 Na+ ions per second are prevented from entering the
cell for a period of ~1 second
rod cell membrane is hyperpolarized by 1 mV
Retinal Processing
• Horizontal cells mediate
lateral interactions
Horizontal cells are
post-synaptic to many
photoreceptors, and in
the dark, receive a large
glutamate stimulus; thus
they are depolarized in
the dark. Light causes
hyperpolarization. Via
electrical synapses (gap
junctions) they also then
hyperpolarize their
neighboring cells.
Fig 15-13
Processing in the retina
• Cones send information in parallel pathways
Bipolar cells are a direct
pathway from the
photoreceptors to ganglion
cells. In the dark, cones are
depolarized and release
glutamate, which has opposite
effects on two types of bipolar
cells that mediate the parallel
direct pathways to ganglion
cells. When a cone is
stimulated by light, it reduces
transmitter release.
Excitatory
Inhibitory
Retinal Processing
• Off-type bipolar cells
hyperpolarize with light
In the dark, cones release
glutamate onto off-center
bipolar cells and open a
ionotropic cation channel.
This depolarizes the bipolar
cells until a light stimulus.
The bipolar cell then
hyperpolarizes, and reduces
the firing of the off-center
ganglion cell that is next in
the sequence.
Offtype
Retinal Processing
• On-type bipolar cells
depolarize with light
The on-center bipolar cells
are inhibited by glutamate
(by two different
mechanisms). With light
stimulation, the glutamate
concentration from the cone
is decreased and the bipolar
cells are depolarized. This
increases the firing of the
on-center ganglion cell next
in line.
Ontype
Retinal Processing
• Horizontal cells influence a
wide distribution of cones.
At the photoreceptor-horizontal
cell synapse, in the dark, glutamate
will depolarize horizontal cells, and
horizontal neuron GABA will
hyperpolarize cones. With light, the
horizontal cell is hyperpolarized and
less GABA is released.
Light stimulus at one point of the
horizontal cell receptive field causes
a depolarization in other
photoreceptors in the receptive field.
Fig 15-17
+
Retinal Processing
• Synaptic interactions increase
the receptive field of bipolar cells
Off-center bipolar cells respond to
a spot of light by hyperpolarizing,
because of reduced cone glutamate.
Horizontal cells hyperpolarize with
light, reducing their inhibitory
transmitter output onto cones; with
less hyperpolarization, the cones
secrete more transmitter and bipolar
cell is depolarized. With the
inverted influence of the horizontal
cell, the bipolar cell response is a
combination of the two.
Fig 15-16