Metathalamus

As metathalamus ( gr μετά meta ' about, after, behind '; ' Nachthalamus ') is a part of the brain of mammals, more particularly referred to the diencephalon ( diencephalon ), namely, an area of ​​the thalamus to the dorsal ( posterior ) section. The metathalamus consists of the two, each pair, lateral and medial geniculate bodies ( lateral geniculate bodies et media ).

Lateral geniculate body

The lateral geniculate nucleus ( lateral geniculate body, CGL ) is a core area where about 90 % of the axons of the optic pathway ( optic tract ) end. However, the neurons of the CGL not only receive input from the retina, but also from other thalamic nuclei of neurons and the CGL itself, and from the cerebral cortex and the upper hills of the midbrain. All this information is integrated here, modified and distributed to a large extent on the optic radiation ( optic radiation ) to the visual cortex. There are also efferent fibers to the core areas in the superior colliculus, pretectales to the nuclei and the suprachiasmatic nucleus of the hypothalamus.

( The remaining 10 % of optic nerve fibers terminate directly on nuclei in the superior colliculus, the pretectal area or in the hypothalamus. Their information is used, among others, the reflex head and eye movements, the pupillary reflex and the day -night rhythm. )

The core areas of the lateral geniculate body show each hand, a structure of six cap-like curved, arranged in approximately concentric layers, for which a large cell ( magnozelluläres ) Area ( nucleus ventralis corporis geniculati lateralis) and a small cell ( parvozelluläres ) area (nucleus dorsalis corporis geniculati lateralis can be distinguished ).

Division into layers

Assignment of the visual field half

The CGL is made up of six layers, which are referred to from ventral to dorsal layer with 1 to 6. The layers 1, 4 and 6 receive input from the contralateral eye, the layers 2, 3 and 5 of the ipsilateral eye, in each case on the CGL opposite half of the field. So the right CGL reach information on the left half of the visual field, which originate from the left eye ( layers 1, 4 and 6), and information on the left half of the visual field of the right eye ( layers 2, 3 and 5). Thus, each half of the brain receives information from both eyes well, but for now only on the mutual field half. This separation remains in the projections of the CGL to the primary visual cortex (V1, striate area, Area 17 ) obtained, thus achieving the neocortikale layer 4 of the visual cortex. Only in the other cortical processing, the information on both halves of the field are then processed together to varying degrees.

The two ventral layers 1 and 2 are referred to as the CGL magnocellular layers (or ventral core ), the layers 3-6 as parvocellular layers (or posterior nucleus). They receive input from different types of retinal ganglion cells. Between the six layers are the layers koniozellulären very small cells whose function is not yet fully understood. They receive input from retinal ganglion cells of the bistratified type.

Magnocellular layers ( nucleus ventralis CGL)

The neurons of the ventral layers of the CGL are characterized by larger cell bodies, extensive dendritic trees and axons from stronger compared to the neurons of the other layers. The larger diameter of their axons causes a very fast line speed to downstream units. The input cells of this layer - the magnocellular retinal ganglion cells from the parasol - type - characterized by a large receptive field. However, they are completely insensitive to color perception, as their receptive fields in both the center and the periphery from the same thong made ​​( color perception is only possible if the center and periphery of the receptive field input from different types of cones preserved). The function of the magnocellular system one has examined with lesion studies in monkeys: The magno - and parvocellular layers was selectively disabled by pharmaceutical methods. Monkeys, in which the magnocellular layers of the CGL were turned off, showed significant deficits in movement, position and velocity perception. Color vision and visual accuracy, however, were not affected. Therefore, one sees in the magnocellular layers major input to the " dorsal route" of visual processing in the brain, which processes esp. movement, place and action perception. The dorsal route includes starting from different areas V1 toward parietal cortex, such as area MT, MST, MIT.

Parvocellular layers ( dorsal nucleus CGL)

In the third to sixth layer of the CGL are parvocellular neurons with small cell bodies, dendritic trees and thinner axons more limited ( due to slower conduction velocity ). They receive input from the parvocellular retinal ganglion cells of the midget - type, which have a smaller receptive field and offer a better visual resolution. Color perception is possible only with the parvocellular system. The center of a receptive field of a parvocellular ganglion cell consists of a thong- other than their periphery. By comparing the excitation strength of the types of cones, a color may be perceived. There are three types of cones, which (blue, green or red) show, respectively, for short-, medium - or long-wavelength light, the greatest sensitivity.

The parvocellular system is necessary for a perception with visual resolution finer accuracy (size, shape, texture) as well as for the color perception. It is the most important input for the " ventral route" of visual processing in the brain, which runs from V1 to the inferior temporal lobe ventral direction (eg V4, fusiform gyrus ). It is responsible for shape, sharpness, color perception and object recognition.

Medial geniculate body

The medial geniculate body ( medial geniculate body ) is switching station of the auditory pathway. Their information access via the lateral lemniscus and the inferior colliculus to the geniculate and from there via the radiotherapy acustica the auditory cortex in the temporal lobe.

• Diencephalon