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