Lateral geniculate body

Lateral geniculate body(lateral geniculate body, LCT) is an easily recognizable structure of the brain, which is located on the lower lateral side of the thalamic cushion in the form of a fairly large flat tubercle. In the LCT of primates and humans, six layers are morphologically defined: 1 and 2 - layers of large cells (magnocellular), 3-6 - layers of small cells (parvocellular). Layers 1, 4, and 6 receive afferents from the contralateral (located in the opposite hemisphere of the LCT) eye, and layers 2, 3 and 5 from the ipsilateral (located in the same hemisphere as LCT).

Schematic diagram of primate LCT. Layers 1 and 2 are located more ventrally, closer to the incoming fibers of the optical path.

The number of LCT layers involved in processing the signal coming from the retinal ganglion cells is different depending on the eccentricity of the retina:

• - at eccentricity less than 1º, two parvocellular layers are involved in the treatment;
• - from 1º to 12º (blind spot eccentricity) - all six layers;
• - from 12º to 50º - four layers;
• - from 50º - two layers associated with the contralateral eye

There are no binocular neurons in the LBT of primates. They appear only in the primary visual cortex.

Literature

1. Hubel D. Eye, brain, vision / D. Hubel; Per. from English O. V. Levashova and G. A. Sharaeva. - M .: "Mir", 1990. - 239 p.
2. Morphology of the nervous system: Textbook. allowance / D. K. Obukhov, N. G. Andreeva, G. P. Demyanenko and others; Resp. ed. V.P. Babmindra. - L .: Nauka, 1985. - 161 p.
3. Erwin E. Relationship between laminar topology and retinotopy in the rhesus lateral geniculate nucleus: results from a functional atlas / E. Erwin, F.H. Baker, W.F. Busen et al. // Journal of Comparative Neurology. 1999. Vol.407, No. 1. P.92-102.

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See what the "Lateral geniculate body" is in other dictionaries:

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lateral geniculate body- (c. g. laterale, PNA, BNA, JNA) K. t., lying on the lower surface of the thalamus laterally from the handle of the upper mound of the quadruple: the location of the subcortical center of vision ... Comprehensive Medical Dictionary

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Lateral geniculate body. Functional organization of the lateral geniculate body. Receptive fields of the lateral geniculate body.

Ganglion cell axons form topographically organized connections with neurons of the lateral geniculate body, which are represented by six layers of cells. The first two layers, located ventrally, consist of magnocellular cells that have synapses with the M-cells of the retina, the first layer receiving signals from the nasal half of the retina of the contralateral eye, and the second from the temporal half of the ipsilateral eye. The remaining four layers of cells located dorsally receive signals from P-cells of the retina: the fourth and sixth from the nasal half of the retina of the contralateral eye, and the third and fifth from the temporal half of the retina of the ipsilateral eye. As a result of this organization of afferent inputs in each lateral geniculate body, that is, left and right, are formed six located exactly one above the other neural maps of the opposite half of the visual field. Neural maps are organized retinotopically, in each of them about 25% of cells receive information from photoreceptors of the central fossa.

Receptive fields of neurons of the lateral geniculate body have a rounded shape with on- or off-type centers and a periphery antagonistic to the center. A small number of ganglion cell axons converge to each neuron, and therefore the nature of the information transmitted to the visual cortex remains almost unchanged. Signals from the retinal parvocellular and magnocellular cells are processed independently of each other and transmitted to the visual cortex in parallel paths. Neurons lateral geniculate body receive from the retina no more than 20% of afferent inputs, and the rest of the afferents are formed mainly by neurons of the reticular formation and cortex. These entrances to lateral geniculate body regulate the transmission of signals from the retina to the cortex.

External geniculate body represents a small oblong eminence at the postero-inferior end of the optic hillock, lateral to the pulvinar. In the ganglion cells of the lateral geniculate body, the fibers of the optic tract end and the fibers of the Graziole bundle originate from them. Thus, the peripheral neuron ends here and the central neuron of the visual path begins.

It was found that although majority fibers of the optic tract and ends in the lateral geniculate body, yet a small part of them goes to the pulvinar and the anterior quadruple. These anatomical data served as the basis for the opinion widespread for a long time, according to which both the lateral geniculate and the pulvinar and anterior quadruple were considered the primary visual centers.
Currently a lot of data has accumulated that do not allow us to consider the pulvinar and the anterior quadruple to be the primary visual centers.

Comparison clinical and pathological data, as well as the data of embryology and comparative anatomy, does not allow assigning pulvinar the role of the primary visual center. So, according to Genshen's observations, in the presence of pathological changes in the pulvinar, the field of vision remains normal. Browver notes that with an altered lateral geniculate body and an unchanged pulvinar, homonymous hemianopsia is observed; with changes in the pulvinar and unchanged external geniculate body, the field of vision remains normal.

Likewise the situation is also the case with the anterior quadruple. The fibers of the optic tract form the visual layer in it and end in the cell groups located near this layer. However, Pribytkov's experiments showed that enucleation of one eye in animals is not accompanied by degeneration of these fibers.
Based on all of the above above at present, there is reason to believe that only the lateral geniculate body is the primary visual center.

Moving on to the question of projection of the retina in the lateral geniculate body, the following should be noted. Monakov generally denied the presence of any projection of the retina in the lateral geniculate body. He believed that all fibers coming from different parts of the retina, including papillo-macular, are evenly distributed throughout the external geniculate body. Genshen, back in the 90s of the last century, proved the fallacy of this view. In 2 patients with homonic lower quadrant hemianopsia, during postmortem examination, he found limited changes in the dorsal part of the lateral geniculate body.

Ronne with atrophy of the optic nerves with central scotomas on the basis of alcohol intoxication, he found limited changes in ganglion cells in the lateral geniculate body, indicating that the macular region is projected onto the dorsal part of the geniculate body.

Observations from certainty prove the presence of a certain projection of the retina in the lateral geniculate body. But the clinical and anatomical observations available in this respect are too few in number and do not yet give an accurate idea of ​​the nature of this projection. The experimental studies of Brower and Zeman on monkeys mentioned by us made it possible to study to some extent the projection of the retina in the lateral geniculate body.

Optic nerve fibers begin in each eye and end on the cells of the right and left lateral geniculate body (LCT) (Fig. 1), which has a clearly distinguishable layered structure (“geniculate” - means “bent like a knee”). In the cat's LCT, you can see three distinct, well-distinguishable layers of cells (A, A1, C), one of which (A1) has a complex structure and is further subdivided. In monkeys and other primates, including

Rice. 1. Lateral geniculate body (LCT). (A) The cat's LCT has three layers of cells: A, A, and C. (B) The monkey's LCT has 6 main layers, including small-cell (pagvocellular), or C (3, 4, 5, 6), large-cell (magnocellular ), or M (1, 2), separated by koniocellular layers (K). In both animals, each layer receives signals from only one eye and contains cells with specialized physiological properties.

human, LCT has six layers of cells. The cells in deeper layers 1 and 2 are larger in size than in layers 3, 4, 5 and 6, which is why these layers are called large-cell (M, magnocellular) and small-cell (P, parvocellular), respectively. The classification also correlates with large (M) and small (P) retinal ganglion cells, which send their processes to the LCT. Between each M and P layers lies a zone of very small cells: the intralaminar, or koniocellular (K, koniocellular) layer. Cells of the K layer differ from M and P cells in their functional and neurochemical properties, forming a third channel of information into the visual cortex.

In both the cat and the monkey, each layer of LCT receives signals from either one or the other eye. In monkeys, layers 6, 4, and 1 receive information from the contralateral eye, and layers 5, 3, and 2 from the ipsilateral eye. The separation of the course of nerve endings from each eye into different layers has been shown using electrophysiological and a variety of anatomical methods. Particularly surprising is the type of branching of an individual optic nerve fiber when the enzyme horseradish peroxidase is injected into it (Fig. 2).

Terminal formation is limited to the LCT layers for this eye, without going beyond the boundaries of these layers. Due to the systematic and definite separation of the optic nerve fibers in the chiasm region, all the receptive fields of the LCT cells are located in the visual field of the opposite side.

Rice. 2. The endings of the optic nerve fibers in the cat's LCT. Horseradish peroxidase was injected into one of the axons from the "on" zone in the center of the contralateral eye. Axon branches end on cells in layers A and C, but not A1.

Rice. 3. Receptive fields of cells of PC. The concentric receptive fields of the LBT cells resemble those of the ganglion cells in the retina, dividing into fields with an "on" and "off" "center. The responses of a cell with an" on "center of the Feline LBT are shown. The bar above the signal indicates the duration of illumination. zones neutralize the effects of each other, therefore, diffuse illumination of the entire receptive field gives only weak responses (lower record), even less pronounced than in the ganglion cells of the retina.

Anatomically, the LCT belongs to the metathalamus, its dimensions are 8.5 x 5 mm. The cytoarchitectonics of the LCT is determined by its six-layer structure, which is found only in higher mammals, primates and humans.
Each LCT contains two main nuclei: dorsal (superior) and ventral (inferior). The LCT has six layers of nerve cells, four layers in the dorsal nucleus and two in the ventral. In the ventral part of the LCT, nerve cells are larger and react in a special way to visual stimuli. The nerve cells of the dorsal nucleus of the LCT are smaller and similar to each other histologically and in electrophysiological properties. In this regard, the ventral layers of the LCT are called large-celled (magnocellular), and the dorsal - small-celled (parvocellular).
Parvocellular structures of LCT are represented by layers 3, 4, 5, 6 (P-cells); magnocellular layers - 1 and 2 (M-cells). The endings of the axons of the retinal magnocellular and parvocellular ganglion cells are morphologically different, and therefore in different layers of the nerve cells of the LCT there are synapses that differ from each other. Magnoaxon terminals are radially symmetric, with thick dendrites and large ovoid ends. Parvoaxon terminals are elongated, have thin dendrites and medium-sized rounded ends.
In the LCT, there are also axonal endings with a different morphology, belonging to other classes of retinal ganglion cells, in particular, the system of blue-sensitive cones. These axonal endings create synapses in a heterogeneous group of LCT layers collectively called "coniocellular" or K-layers.
Due to the intersection in the chiasm of the optic nerve fibers from the right and left eyes, nerve fibers from the retinas of both eyes enter the LCT on each side. The endings of the nerve fibers in each of the layers of the LCT are distributed in accordance with the principle of retinotopic projection and form a projection of the retina onto the layers of the nerve cells of the LCT. This is facilitated by the fact that 1.5 million LKT neurons with their dendrites provide a very reliable connection between synaptic transmission of impulses from 1 million axons of retinal ganglion cells.
In the geniculate body, the projection of the central fossa of the macula is most developed. The projection of the visual path in the LCT promotes the recognition of objects, their color, movement and stereoscopic depth perception (primary center of vision).

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Functionally, the receptive fields of LBT neurons have a concentric shape and are similar to similar fields of retinal ganglion cells, for example, the central zone is excitatory, and the peripheral, annular zone is inhibitory. LKT neurons are divided into two classes: on-center and off-center (darkening the center activates the neuron). LBT neurons perform different functions.
Symmetrical binocular loss of the visual field is characteristic of pathological processes localized in the area of ​​chiasm, optic tract and LCT.

These are true hemianopsias, which, depending on the localization of the lesion, can be:

• homonymous (of the same name) right and left,
• heteronymous (dissimilar) - bitemporal or binasal,
• altitude - upper or lower.

Visual acuity in such neurological patients decreases depending on the degree of damage to the papillomacular bundle of the visual pathway. Even with unilateral damage to the visual pathway in the LCT area (right or left), the central vision of both eyes suffers. At the same time, one feature is noted that has an important differential diagnostic value. Pathological foci located peripheral to the LCT give positive scotomas in the field of vision and are felt by patients as darkening of vision or vision of a gray spot. In contrast to these lesions, foci located above the LCT, including foci in the cortex of the occipital lobe of the brain, usually give negative scotomas, that is, they are not felt by patients as visual impairment.