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The LGN
•
--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).
Retina overview
• 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.
anterior nuclei
internal medullary laminaintralaminar nuclei
other medial nucleimidline (medial) nuclei
interthalamic adhesion
pulvinar
medial geniculatenucleus
lateral geniculatenucleus
MD
LDLP
VPL VPMCM
VAVL
VI
Thalamic nuclei
CM centromedianLD lateral dorsalLP lateral posteriorMD medial dorsalVA ventral anteriorVI ventral intermedialVL ventral lateralVPL ventral posterolateralVPM ventral posteromedial
thalamic reticular nucleus (pulled away)
anterior nuclei
internal medullary laminaintralaminar nuclei
other medial nucleimidline (medial) nuclei
interthalamic adhesion
pulvinar
medial geniculatenucleus
lateral geniculatenucleus
MD
LDLP
VPL VPMCM
VAVL
VI
Thalamic nuclei
CM centromedianLD lateral dorsalLP lateral posteriorMD medial dorsalVA ventral anteriorVI ventral intermedialVL ventral lateralVPL ventral posterolateralVPM ventral posteromedial
thalamic reticular nucleus (pulled away)
lateralgeniculate
nucleus
visualcortex
retina
The LGN is a bean shaped nucleus.
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
Glu
GABA
ACh
Retina
layer 4
relaycells
VisualCortex
LGN
excitatoryinhibitory
Glu
GABA
ACh
layer 4
relaycells
Input tobe Relayed
ThalamicRelay
excitatoryinhibitory
interneurons
TRN
layer 6
PBRmid-brain
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
Konio
I [off/on] 5 Parvo
Konio
C [on/off] 4 Parvo
Konio
I [off/on] 3 Parvo
Konio
I [on/off] 2 Magno
Konio
C [off/on] 1 Magno
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 red-on/green-off center surround receptive fields will project onto LGN cells that also have red-on/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
TH
AL
AM
US
(e.g., LGN)
layer 5
TRN
CO
RT
EX
glomerulus
(e.g., Pulvinar)
layer 6
FO HO
Area “A” (FO) Area “B” (HO)
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.
Cortical area 1 (FO)1-3
4
5
6
“first order” thalamic relay(LGN, MGNv,
VP, etc.) frombrainste
m
drivermodulator
Cortico-cortical Information Flow is Relayed through Thalamus?
Cortical area 2 (HO)
“higher order” thalamic relay
(Pul, MGNmagno, POm, etc.)
Cortical area 3 (HO)
modulator?
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