--The image of that apple is formed on your retina --Light from this image is going to excite and inhibit the rods

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The LGN
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--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
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• 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.

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Whats 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.

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The LGN is a bean shaped nucleus.
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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

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layer 4
layer 4
Visual Cortex
Glu
Glu
GABA
GABA
ACh
ACh
excitatory
excitatory
inhibitory
inhibitory
relay
relay
cells
cells
Retina
LGN
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The LGNs 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.

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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
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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.

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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.
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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.
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The Primate Lateral Geniculate Nucleus

• 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

• 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

• 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

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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

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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

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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.

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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.

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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

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CORTEX
layer 5
layer 6


TRN
THALAMUS
glomerulus
(e.g., LGN)
(e.g., Pulvinar)
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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.

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Cortico-cortical Information Flow is Relayed
through Thalamus?
Cortical area 1 (FO)
1-3
4
5
6
driver
modulator
first order thalamic relay (LGN, MGNv, VP, etc.)
from brainstem
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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.

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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