Download --The image of that apple is formed on your retina -

The LGN
•
Retina overview
--The image of that apple is formed on your retina
--Light from this image is going to excite and inhibit the rods & cones.
--This induces a chemical reaction, which turns light into an electrical
signal. This signal either excites or inhibits the retinal ganglion cells
(RGC).
• The RGC send these signals along the optic nerve.
Some of these signals go to the Superior Colliculus to
control eye movements, but the majority goes to the
Lateral Geniculate Nucleus of the Thalamus.
What’s the thalamus, you ask?
• Major relay of info to the cerebral cortex
while also processing signals from the
cortex.
• Divided into separate nuclei that process
information from the periphery & also
other parts of the brain.
The LGN is a bean shaped nucleus.
internal medullary
medullary lamina
lamina
internal
intralaminar nuclei
nuclei
intralaminar
anterior nuclei
nuclei
anterior
other medial
medial nuclei
nuclei
other
midline (medial)
(medial) nuclei
nuclei
midline
MD
MD
interthalamic adhesion
adhesion
interthalamic
LD
LD
VA
VA
LP
LP
pulvinar
pulvinar
VL
VL
CM
CM
VI
VI
VPL VPM
VPL
VPM
medial geniculate
geniculate
medial
nucleus
nucleus
lateral geniculate
geniculate
lateral
nucleus
nucleus
thalamic reticular
reticular nucleus
nucleus
thalamic
(pulled away)
away)
(pulled
visual
cortex
lateral
geniculate
nucleus
Thalamic nuclei
nuclei
Thalamic
CM centromedian
centromedian
CM
LD lateral
lateral dorsal
dorsal
LD
LP
lateral posterior
posterior
LP
lateral
MD medial
medial dorsal
dorsal
MD
VA ventral
ventral anterior
anterior
VA
VI
ventral intermedial
intermedial
VI
ventral
VL ventral
ventral lateral
lateral
VL
VPL ventral
ventral posterolateral
posterolateral
VPL
VPM ventral
ventral posteromedial
posteromedial
VPM
retina
The LGN does not ONLY relay
information from the retina to the
cortex!!!!!!!!
• It regulates neural information from
the retina & other parts of the brain as
it flows to & from the cortex
layer 4
layer 6
Visual
Cortex
Glu
GABA
TRN
ACh
excitatory
inhibitory
relay
cells
Input to
beRetina
Relayed
interneurons
Thalamic
LGN
Relay
PBR
midbrain
The LGN’s function is not only
dependent on information sent
from the retina, but also:
• Other neurons in the LGN
• Neurons from the cortex
• Neurons in the brain stem
• **Signals that come down from the visual
cortex to the LGN actually outnumber the
signals that travel from the retina to the
LGN.
Most impressive aspect of the
LGN is how it organizes the
information that flows into it.
For instance, signals
from the retina are
routed to different
layers of the LGN
based on the eye that
the signals come from
& the type of RGC are
propagating that
signal.
C
[on/off]
6
Parvo
5
Parvo
4
Parvo
3
Parvo
2
Magno
C [off/on] 1
Magno
Konio
I
[off/on]
Konio
C
[on/off]
Konio
I
[off/on]
Konio
I
[on/off]
Konio
The LGN is comprised of multiple
layers.
• Each layer receives input from only one
eye.
• Some get Ipsilateral input (from the
eye on the same side of the LGN) to
LGN layers 2,3 & 5.
• Others get Contralateral input (from
the eye on the opposite side of the
LGN) into LGN layers 1,4 & 6.
Inputs to the LGN from the
retina will be from “similar”
cells.
In other words, retinal
ganglion cells that have redon/green-off center
surround receptive fields
will project onto LGN cells
that also have redon/green-off center
surround receptive fields.
There are 4 types of Retinal
Ganglion cells.
1)Parasol cells, aka M-cells synapse onto layers 1& 2 of the LGN. These
layers are called the magnocellular layers.
2) midget cells, aka P-cells, synapse onto layers 3-6 of the LGN. These
layers are called the parvocellular layers.
3) S-cells synapse onto the interlaminar layers of the LGN.
The cells that populate these layers are called koniocellular cells.
The Primate Lateral Geniculate Nucleus
Parvo-cells
•
•
•
•
•
•
small receptive fields
medium conduction velocity
high spatial resolution
slow temporal resolution
project to brain regions
responsible for color and
form perception
Excited by red/green
stimuli
Magno-cells
•
•
•
•
•
•
large receptive fields
high conduction
velocity
low spatial resolution
fast temporal
resolution
project to brain
regions responsible for
motion perception
Excited by contrast
luminant stimuli
Konio-cells
•
•
•
•
•
•
Very large receptive
fields
Snail-like conduction
velocity
low spatial resolution
slow temporal resolution
project to brain regions
responsible for motion
perception & the primary
visual cortex…
Excited by blue/yellow
stimuli
Two types of neurons exist in
the dLGN: relay cells and
interneurons.
• The relay cells' axons go the visual
cortex.
• Interneurons' axons do not leave
the dLGN
Interneurons
• have small cell bodies (somas)
• represent about 20-25 % of the total
cell population
• have a complex branching pattern of
the dendrites
• have center-surround receptive fields
• receive feedback excitation from visual
cortex
• interneurons act inhibitorily (on cells
within dLGN) using the
neurotransmitter GABA
Relay cells
• have center-surround receptive
fields
• Relay cells emit the
neurotransmitter glutamate
(and are thus glutamatergic).
• Glutamate generally acts in an
excitatory fashion on the
receiving cell.
1st Order Nuclei
• The LGN is a nucleus of the Thalamus
that is considered a 1st order nucleus.
• it relays subcortical (i.e., retinal)
information to cortex for the first
time.
Higher Order Nuclei
• pulvinar complex, seems largely to be a
higher-order relay, since much of it
seems to relay information from one
cortical area to another
Area “A” (FO)
Area “B” (HO)
layer 5
layer 6
TRN
glomerulus
FO
(e.g., LGN)
HO
(e.g., Pulvinar)
Why should Higher Order Nuclei
concern us?
• Much more cortico-cortical processing
may involve these "re-entry" routes
than previously thought.
• If so, the thalamus sits at indispensable
position for cortical processing.
Cortico-cortical Information Flow is Relayed through Thalamus?
Cortical area 1 (FO)
1-3
Cortical area 2 (HO)
Cortical area 3 (HO)
4
5
6
“first order”
thalamic relay
(LGN, MGNv,
VP, etc.)
driver
modulator
modulator?
“higher order”
thalamic relay
(Pul, MGNmagno,
POm, etc.)
from
brainstem
2 pathways of information flow
• Driving pathway: Drives principal
information into a thalamic nucleus
• Modulating pathway: Modulates the way
the information is processed.
• It turns out that these pathways differ
both morphologically and functionally.
On the way to V1
The center/surround
receptive fields of 3
geniculate cells are
aligned so that when
output axons of these
cells converge onto a
cortical cell in layer 4,
the receptive field of
the cortical cell has an
elongated shape with
orientation selectivity