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Scheme showing the course of the fibers of the Lemniscus Medial Lemniscus in blue, Lateral in red
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Thalamus
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The (from
Greek ''θαλαμος'' = ''bedroom, chamber''), also known as the "dorsal thalamus", is part of the
Diencephalon , a structure located in the center of the
Brain . The other parts of the
Diencephalon include the
Ventral Thalamus ,
Epithalamus , and
Hypothalamus . In the caudal (tail) to oral (mouth) sequence of neuromeres, the diencephalon is located between the mesencephalon (cerebral peduncule, belonging to the brain stem) and the telencephalon. Due to different ontogenetic origins, two diencephalic elements must be distinguished from the thalamus proper: the epithalamus (comprising mainly the habenula) and the perithalamus (past ventral thalamus and "reticulate nucleus") separated from the thalamus by the lamella lateralis. The two thalami are prominent bulb-shaped masses, about 5.7 in length in normal humans, located obliquely and symmetrically on each side of the third ventricle. The two can adhere on a variable extent in 30% of humans through the adhesio interthalmica (or massa intermedia, with no interthalamic connection in our species). The thalamus is in majority but not completely homogeneous.
The thalamus comprises a system of inner lamellae (made up of myelinated fibers) separating, more or less tighly, different thalamic subparts. This is used since Burdach's (1822) for the description of the inside of the thalamus. There are other "limiting elements", cellular this time, such as the periventricular gray, the "nucleus limitans"... These elements do not have the same neuronal constitution than that of the main part of the thalamus. Following the subdivision proposed by the Vogts for the cortex, it has been proposed to put them together into the "allothalamus" as opposed to the "isothalamus" (Percheron, 2003). The allothalamus is an inhomogeneous set that comprises the Formatio Paramediana (periventricular), the Formatio Intraminaris-limitans and the Regio centralis corresponding to the "centre-median-parascicular complex", which has three divisions in upper primates. The allothalamus will be analysed later.
The remaining, constituting the bulk of the thalamus (more than 90%), the isothalamus, in spite of the variety of the afferent systems is built up with only few elements according to a relatively simple arrangement, using only few mediators. One may describe first the two constituting neuronal genera implying the outputsystem, then the afferent systems decomposed into subcortical and cortical afferents.The constituting neurons belong to two highly contrasted genera. The first correspond to the thalamocortical neurons (or principal) that are highly characteristic among neurons. They are said to be radiate (or "tufted") as their dendritic arborisation is made up of straight dendritic distal branches starting from short and thick stems. The number of branches and the diameter of the arborisation are linked to the specific system of which they are a part and to the animal species. They have the rather rare particularity of having no inital axonal collaterals, which imply that one thalamocortical neuron do not send information to its neighbour. They have a thick myelinated axon directed to particular cortical place(s). Its mediator is glutamate. The other genus is made up of "microneurons". These have short and thin dendrites and short axon(s) and thus belong to local circuitry neurons. Their percentage in comparison to thalamocortical neurons depends on the considered system and deeply from the animal genera, highly increasing in evolution. Their short axonal parts contact thalamocortical or other local circuitry neurons. Their mediator is GABA. The dendrites of the two constituting genera receive synapses from the afferent axons. The connection back to the thalamocortical neurons create "triads" modulating the thalamocortical output. One subcortical afference comes from the perithalamus ("reticulate nucleus"). This receives axonal branches from thalamocortical neurons. Its afferences are GABAergic. The number of perithalamic neurons strongly decreases in evolution in opposition to the large increase in microneurons. To some extent the perithalamus play a role in the local circuitry. The circuitous connection with corticothalamic neurons participates in the elaboration of thalamic rythms. The other subcortical afferent systems of axons end in various ways, more or less dense or homogeneous. Their ending form the center of the synaptic glomerule. The set of all afferent axons from one source constitute a territory which is the main differentiator of thalamic "functions". A series of main territories do not overlap spatially and allow the description of functionally differentiated subparts. Along with main afferences coafferences from other sources may exist. There are more corticothalamic than thalamocortical axons. Corticothalamic endings are of two kinds. One, classical, emanating from layer VI of the cortex is thin and has a long almost straight trajectory through the thalamus, not respecting intrathalamic borders. Its terminal synapses are essentially located distally on the thalamic dendritic arborizations. Its mediator is glutamate. The second kind of corticothamic axon is the Rockland II type (1994). This emanating from large pyramids is much thicker. Its ending is small, dense , globular. Its synapses are located close to the soma of the thalamic neuron, often forming the center of glomerular complexes. The isothalamic device made up of the arrangement of these few elements in various but close ways probably serves as a translator of various input information into information readable by the cortex. The second function is the selecive distribution to one or several part of the cortex.
Isothalamic subparts
The thalamic parts delineated by the lamellar and cellular "limiting" elements constituted the classic thalamic nuclei. These have been further subdivided later. The Louvain symposium made the recommendation to call the classical subdivisions "region". One region may be made by one or several nuclei. These may have one (or several) pars if there is a particular coafference for instance.
The regio separated by the superior lamella is the Anterior region (A). The regio separated medially by the medial lamina is the Lateral region (L). Close to it are the Geniculate bodies (G). The remaining isothalamus is made up of the medial region (M, medial to the medial lamina) and posteriorly, with no complete separation in man, of the posterior regio or pulvinar (Pu). The last two represent a huge medioposterior ensemble. The classical separation into ''relay nuclei'', receiving "specific" subcortical afferences or ''association nuclei'', which would not, cannot be retained as absolute. The lateral region and the geniculate bodies indeed receive strong lower "specific" afferences and can be seen as the "sensorimotor" part of the thalamus. The medioposterior ensemble, in most of its volume does not receive subcortical afferents and abundant afferences from the "associative" cortex but in some, essentially ventral parts, in fact receives subcortical afferences, such as tectal, spinothalamic or amygdalar. The anterior region receives a particular afference that is not entirely subcortical (directly or indirectly from the subiculum). The elements of the lateral region have been frequently separated into ventral and dorsal (in fact named lateral) nuclei. Recent analyses of the three-dimensional geometry of the main afferent terrirories in macaques have shown that this subdivision does not hold true: the dorsal element on transverse sections being simply the posterior part of the preceding territory. This is one reason why the nomenclature selected by the Nomina anatomica and the Terminologia anatomica () is hardly applicable. The evolution of the thalamus follows that of the cortex and there are differences including between primates (new world monkeys and old world; old world and humans).
The thalamus is largely made of
Nuclear groups that relate to specific functions in the brain (see
List Of Thalamic Nuclei ). Thalamic nuclei have
Subcortical Projections , and can be classified as either (see below). Thalamic nuclei also have strong reciprocal connections with the cerebral cortex, forming cortico-thalamo-cortical recurrent loops that are believed may be involved with
Consciousness .
Traditionally, the thalamus has been thought of as a "relay" that simply forwards signals from
Auditory ,
Somatic , visceral and
Visual System s (though not olfactory) to the cerebral cortex. However, newer research suggests that thalamic function may be more complicated. It is currently believed that the thalamus also modulates sensory signals to and from cortex. The thalamus also plays an important role in regulating states of sleep and wakefulness.
''Thalamic nuclei can be classified as either "relay nuclei" or "association nuclei" on the basis of the source of their driving inputs, whether they are subcortical or cortical.'' Relay nuclei receive their driving inputs from subcortical sources including ascending sensory
Afferent s (
Medial Lemniscus for somatosensory information,
Optic Tract for visual information, etc...) and project predominantly to primary sensory cortical areas. On the other hand, association nuclei receive their driving inputs from other cortical areas. (See Sherman and Guillery's "Exploring the Thalamus", 2002)
''Alternative ways for subdividing thalamus are also coming into vogue. For example,
Ted Jones has recently proposed a Matrix-Core model for the thalamus, which is not subdivided based on nuclei, but rather on chemically-defined populations of
Neuron s. Specifically, Jones proposes the
Calbindin immunopositive neurons constitute a "matrix", whereas
Parvalbumin immunopositive neurons form the "core". In Jones' scheme, the "matrix", which includes much of the intralaminar nuclei, is involved in state functions (arousal, level of attention, mood), whereas the "core" is involved in discriminative sensory-motor functions. '
'''
The thalamus plays a major role in regulating arousal, the level of awareness and activity. An animal with a severely damaged or severed thalamus suffers permanent
Coma .
Cerebrovascular accidents (
Stroke s) can cause ''thalamic syndrome'' (Dejerine and Roussy, 1906), which results in a controlateral hemianaesthesia, burning or aching sensation on one half of a body (painful anaesthesia), often accompanied by
Mood Swings . Ischaemia of the territory of the paramedian artery, if bilateral, causes serious troubles including akinetic mutism accompanied or not by oculomotor troubles.
Korsakoff's Syndrome , a memory disorder induced by alcoholism, stems from mammillary bodies, mammilothalamic, or thalamic lesions.
