THE LATERAL GENICULATE NUCLEUSMANOJ ARYALIOM,MMC

These are the oval shaped structures situated at termination of optic tract.

provides a relay station for retinal axons synapsing with neurons of the geniculocalcarine pathway, transferring information from the optic tract to optic radiation and thence to visual cortex

there is a roughly 1:1 relationship between retinal axons entering the lateral geniculate nucleus and geniculocalcarine neurons leaving it.

Eighty per cent of the synaptic connections of the lateral geniculate nucleus are with retinofugal axons.

The nucleus is one of the nuclei of the thalamus

It lies anterolateral to the medial geniculate nucleus

The lateral geniculate nucleus consists of a dorsal nucleus, and a phylogenetically older ventral nucleus.

The dorsal, or principal, nucleus makes up the major portion of the lateral geniculate nucleus

Much of the lateral geniculate nucleus is hidden, being overlapped by the pulvinar and visible only in sections.

In coronal section: it is like a peaked

cap, the peak projecting laterally.

Schematic representation of a coronal sectionthrough the lateral geniculate body viewed from its posterioraspect.

In horizontal section, related anteriorly to the optic tract

which end in it, laterally with the retrolenticular part of

the internal capsule, medially with the medial geniculate

body, posteriorly with the hippocampal gyrus

and posterolaterally with the inferior cornu of the lateral ventricle.

LAYERING OF THE LATERAL GENICULATE

there are six laminae of 'grey matter' and intervening 'white' strata composed of axons and dendrites

The grey lammae are like six irregularly stacked cones, numbering from one ventrally, at the hilum, to six dorsally.

The two inner layers consist of loosely arranged large cells (the magnocellular layers 1 and 2) and

the four outer layers consist of polar staining small and medium-sized cells (the parvocellular layer 3-6)

Crossed fibres of the optic tract end in laminae 1, 4 and 6,

uncrossed in 2, 3 and 5

so that fibres from corresponding parts of the two hemiretmae (e.g. right temporal and left nasal retina) end in neighbouring laminae.

Impulses from equivalent spots (a, b)in the two retinae pass back m the optictract to the same region of the lateralgeniculate body .Crossed impulses (b)terminate in laminae 1, 4 and 6 anduncrossed Impulses (a) terminate in laminae 2, 3 and5.

Retinotopic projection

Fibres from each retina pass to both magnocellular (1 and 2) and parvocellular (3-6) laminae.

This segregation is achieved within the nucleus itself, since the crossed and uncrossed fibres are still intermingled as they enter the lateral geniculate nucleus

POSITION OF VISUAL FIBRES

Macular fibres coming In optic tract occupy two third of LGB

Upper retinal fibres occupy the medial half of the anterior one third of LGB.

Lower retinal fibres occupy the lateral half of anterior one third of the LGB.

FUNCTIONS

RELAY FUNCTIONo Serves as relay station to relay visual

information from optic tract to visual cortex by the way of geniculocalcarine tract.

To gate the transmission of signalo i.e. to control how much of signal be

allowed to pass to the cortex.

LGB receives GATING control signals from two major sources Cortigofugal fibres from the primary visual

cortex. Reticular area of mesencephalon.

Both of these are inhibitory and thus controls the visual information that is allowed to pass.

BLOOD SUPPLY OF THE LATERALGENICULATE NUCLEUS

Posterior cerebral and posterior choroidal arteries.

Anterior choroidal artery.