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 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. anterior nuclei internal medullary lamina intralaminar nuclei other medial nuclei midline (medial) nuclei interthalamic adhesion pulvinar medial geniculate nucleus lateral geniculate MD LD LP VPL VPM CM VA VL VI Thalamic nuclei CM centromedian LD lateral dorsal LP lateral posterior MD medial dorsal VA ventral anterior VI ventral intermedial VL ventral lateral VPL ventral posterolateral VPM ventral posteromedial thalamic reticular nucleus (pulled away) lateral geniculate visual cortex retina anterior nuclei internal medullary lamina intralaminar nuclei other medial nuclei midline (medial) nuclei interthalamic adhesion pulvinar medial geniculate nucleus lateral geniculate MD LD LP VPL VPM CM VA VL VI Thalamic nuclei CM centromedian LD lateral dorsal LP lateral posterior MD medial dorsal VA ventral anterior VI ventral intermedial VL ventral lateral VPL ventral posterolateral VPM ventral posteromedial thalamic reticular nucleus (pulled away)

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

Visual Cortex TRN Input to be Relayed Retina PBR Thalamic LGN Relay layer 4 layer 4 Visual Cortex layer 6 Glu Glu GABA GABA interneurons TRN ACh ACh PBR mid- brain excitatory excitatory inhibitory inhibitory relay relay cells cells Input to be Relayed Thalamic Relay Retina LGN

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 C [on/off] 4 Parvo I [off/on] 3 Parvo I [on/off] 2 Magno 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 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

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) FO HO CORTEX layer 5 layer 6 TRN THALAMUS glomerulus (e.g., LGN) (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) Cortical area 2 (HO) “higher order” thalamic relay (Pul, MGNmagno, POm, etc.) Cortical area 3 (HO) 1-3 modulator? 4 5 6 driver modulator “first order” thalamic relay (LGN, MGNv, VP, 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