This article is about the portion of thebrain. For the British video game developer, seeThalamus Ltd. For the botanical structure, seeReceptacle (botany).
Anatomically, the thalami are paramedian symmetrical structures (left and right), within thevertebrate brain, situated between the cerebral cortex and themidbrain. It forms duringembryonic development as the main product of the diencephalon, as first recognized by the Swissembryologist and anatomistWilhelm His Sr. in 1893.[5]
The thalami are paired structures of gray matter about four centimetres long and ovoid in appearance,[6] located in theforebrain which is superior to themidbrain, near the center of the brain with nerve fibers projecting out to the cerebral cortex in all directions. In fact, almost all thalamic neurons (with the notable exception of thethalamic reticular nucleus[7]) project to the cerebral cortex, and every region of the cortex so far studied has been found to innervate the thalamus.[8]
Each of the thalami may be subdivided into at least 30nuclei, giving a total of at least 60 for the 'whole thalamus'.[4][9]
Estimates of the volume of the 'whole thalamus' vary. A post-mortem study of 10 people with average age 71 years found average volume 13.68 cm.[10] In an MRI study of 12 healthy males with average age 17 years, scans showed mean 'whole thalamus' volume 8.68cm.[11]
The medial surface of the thalamus constitutes the upper part of the lateral wall of thethird ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, theinterthalamic adhesion.
The surface of the thalamus is covered by two layers ofwhite matter, thestratum zonale covers the dorsal surface, and theexternal medullary lamina covers the lateral surface. (This stratum zonale should not be confused with thestratum zonale of thesuperior colliculus.) The grey matter of thalamus is partitioned by a Y shapedinternal medullary lamina which divides the nuclei into anterior, medial, and lateral groups.[14][15][16]
Derivatives of thediencephalon include the dorsally locatedepithalamus (essentially thehabenula and annexes) and the peri-thalamus (pre-thalamus) containing thezona incerta and the thalamic reticular nucleus. Due to their differentontogenetic origins, the epithalamus and the peri-thalamus are formally distinguished from the thalamus proper. Themetathalamus is made up of the lateral geniculate and medial geniculate nuclei.[17][18]
The thalamus comprises a system oflamellae (made up ofmyelinatedfibers) that separate different thalamic subparts. Other areas are defined by distinct clusters ofneurons, such as theperiventricular nucleus, the intralaminar elements, the "nucleus limitans", and others.[19] These latter structures, different in structure from the major part of the thalamus, have been grouped together into theallothalamus as opposed to theisothalamus.[20] This distinction simplifies the global description of the thalamus.
Thalamic nuclei.Metathalamus labelled MTh. (Left thalamus viewed from left.)Nuclei of right thalamus (viewed from above right)
The interior medullary lamina is subdivided intointralaminar nuclei. Additional structures are the reticular nucleus (which envelops the lateral thalamus), the stratum zonale,[21] and theinterthalamic adhesion.[22]
Combining these division principles yields the following hierarchy, which is subject to many further subdivisions.[23]
anterior group
medial group
medial dorsal nucleus
midline group
lateral group
ventral group
ventral anterior group
ventral lateral group
ventral posterior group
pulvinar group
lateral dorsal nucleus
lateral posterior nucleus
metathalamus
lateral geniculate nucleus
medial geniculate nucleus
intralaminar group
reticular nucleus
stratum zonale
interthalamic adhesion
The term "lateral nuclear group" is used with two meanings. It can mean either the complete set of nuclei in the lateral "third" of the trisection by the lamina, or the subset which excludes the ventral group and the geniculate nuclei.[24][25]
Some people have theartery of Percheron, which is a rare anatomic variation in which a single arterial trunk arises from the posterior cerebral artery to supply both parts of the thalamus.
The thalamus has multiple functions, and is generally believed to act as a relay station, orhub, relaying information between different subcortical areas and the cerebral cortex.[31] In particular, every sensory system (with the exception of theolfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area.[32][33]
The thalamus is believed to both process sensory information as well as relay it—each of the primary sensory relay areas receives strong feedback connections from the cerebral cortex.[36]
The thalamus also plays an important role in regulating states ofsleep, andwakefulness.[37] Thalamic nuclei have strong reciprocal connections with the cerebral cortex, formingthalamo-cortico-thalamic circuits that are believed to be involved withconsciousness.[38][39] The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanentcoma.[40]
The role of the thalamus in the more anteriorpallidal andnigral territories in thebasal ganglia system disturbances is recognized but still poorly understood. The contribution of the thalamus to vestibular or totectal functions is almost ignored. The thalamus has been thought of as a "relay" that simply forwards signals to the cerebral cortex. Newer research suggests that thalamic function is more selective.[41] Many different functions are linked to various regions of the thalamus. This is the case for many of the sensory systems (except for the olfactory system), such as theauditory,somatic,visceral,gustatory andvisual systems where localized lesions provoke specific sensory deficits. A major role of the thalamus is support of motor and language systems, and much of the circuitry implicated for these systems is shared.
The thalamus isfunctionally connected to thehippocampus[42] as part of the extended hippocampal system at the thalamic anterior nuclei.[43] With respect to spatial memory and spatial sensory datum they are crucial for human episodic event memory.[44][45] The thalamic region's connection to themedial temporal lobe provides differentiation of the functioning of recollective and familiarity memory.[29]
The neuronal information processes necessary for motor control were proposed as a network involving the thalamus as a subcortical motor center.[46] Through investigations of the anatomy of the brains of primates[47] the nature of the interconnected tissues of thecerebellum to the multiple motor cortices suggested that the thalamus fulfills a key function in providing the specific channels from the basal ganglia and cerebellum to the cortical motor areas.[48][49] In an investigation of thesaccade andantisaccade[50] motor response in three monkeys, the thalamic regions were found to be involved in the generation of antisaccade eye-movement (that is, the ability to inhibit the reflexive ballistic movement of the eyes in the direction of a presented stimulus).[51]
Recent research suggests that the mediodorsal thalamus (MD) may play a broader role in cognition. Specifically, the mediodorsal thalamus may "amplify the connectivity (signaling strength) of just the circuits in the cortex appropriate for the current context and thereby contribute to the flexibility (of the mammalian brain) to make complex decisions by wiring the many associations on which decisions depend into weakly connected cortical circuits."[52] Researchers found that "enhancing MD activity magnified the ability of mice to "think,"[52] driving down by more than 25 percent their error rate in deciding which conflicting sensory stimuli to follow to find the reward."[53]
The thalamic complex is composed of the perithalamus (or prethalamus, previously also known as ventral thalamus), the mid-diencephalic organiser (which forms later thezona limitans intrathalamica (ZLI) ) and the thalamus (dorsal thalamus).[54][55] The development of the thalamus can be subdivided into three steps.[56]The thalamus is the largest structure deriving from the embryonicdiencephalon, the posterior part of the forebrain situated between the midbrain and the cerebrum.
Afterneurulation, the early developmental stage (primordium) of theprethalamus and the thalamus is induced within theneural tube. Data from different vertebrate model organisms support a model in which the interaction between twotranscription factors, Fez and Otx, is of decisive importance. Fez is expressed in the prethalamus, and functional experiments show that Fez is required for prethalamus formation.[57][58] Posteriorly,OTX1 andOTX2 abut the expression domain of Fez and are required for proper development of the thalamus.[59][60]
Early in thalamic development two progenitor domains form, a caudal domain, and a rostral domain. The caudal domain gives rise to all of the glutamatergic neurons in the adult thalamus while the rostral domain gives rise to all of the GABAergic neurons in the adult thalamus.[61]
The formation of the mid-diencephalic organiser (MDO)
At the interface between the expression domains of Fez and Otx, the mid-diencephalic organizer (MDO, also called the ZLI organiser) is induced within the thalamicanlage. The MDO is the central signalling organizer in the thalamus. A lack of the organizer leads to the absence of the thalamus. The MDO matures from ventral to dorsal during development. Members of thesonic hedgehog (SHH) family and of theWnt family are the main principal signals emitted by the MDO.
Besides its importance as signalling center, the organizer matures into the morphological structure of thezona limitans intrathalamica (ZLI).
After its induction, the MDO starts to orchestrate the development of the thalamic anlage by release of signalling molecules such as SHH.[62] In mice, the function of signaling at the MDO has not been addressed directly due to a complete absence of thediencephalon in SHH mutants.[63]
Studies in chicks have shown that SHH is both necessary and sufficient for thalamic gene induction.[64] Inzebrafish, it was shown that the expression of two SHH genes, SHH-a and SHH-b (formerly described as twhh) mark the MDO territory, and that SHH signaling is sufficient for the molecular differentiation of both the prethalamus and the thalamus but is not required for their maintenance and SHH signaling from the MDO/alar plate is sufficient for the maturation of prethalamic and thalamic territory while ventral Shh signals are dispensable.[65]
The exposure to SHH leads to differentiation of thalamic neurons. SHH signaling from the MDO induces a posterior-to-anterior wave of expression the proneural geneNeurogenin1 in the major (caudal) part of the thalamus, and Ascl1 (formerly Mash1) in the remaining narrow stripe of rostral thalamic cells immediately adjacent to the MDO, and in the prethalamus.[66][67]
This zonation of proneural gene expression leads to the differentiation of glutamatergic relay neurons from the Neurogenin1+ precursors and of GABAergic inhibitory neurons from the Ascl1+ precursors. In fish, selection of these alternative neurotransmitter fates is controlled by the dynamic expression of Her6 the homolog ofHES1. Expression of this hairy-like bHLHtranscription factor, which represses Neurogenin but is required for Ascl1, is progressively lost from the caudal thalamus but maintained in the prethalamus and in the stripe of rostral thalamic cells. In addition, studies on chick and mice have shown that blocking the Shh pathway leads to absence of the rostral thalamus and substantial decrease of the caudal thalamus. The rostral thalamus will give rise to the reticular nucleus mainly whereby the caudal thalamus will form the relay thalamus and will be further subdivided in thethalamic nuclei.[56]
In humans, a common genetic variation in the promoter region of theserotonin transporter (the SERT-long and -short allele:5-HTTLPR) has been shown to affect the development of several regions of the thalamus in adults. People who inherit two shortalleles (SERT-ss) have more neurons and a larger volume in thepulvinar and possibly the limbic regions of the thalamus. Enlargement of the thalamus provides an anatomical basis for why people who inherit two SERT-ss alleles are more vulnerable tomajor depression,post-traumatic stress disorder, and suicide.[68]
Fatal familial insomnia is a hereditaryprion disease in which degeneration of the thalamus occurs, causing the patient to gradually lose their ability to sleep and progressing to a state of totalinsomnia, which invariably leads to death. In contrast, damage to the thalamus can result in coma.
Microstimulation of the posterior portion of theventral medial thalamic nucleus can be used to evoke pain, temperature and visceral sensations.[75]
Krista and Tatiana Hogan have a unique thalamic connection may provide insight into the philosophical and neurological foundations of consciousness. It has been argued that there's no empirical test that can conclusively establish that for some sensations, the twins share one token experience rather than two exactly matching token experiences. Yet background considerations about the way the brain has specific locations for conscious contents, combined with the evident overlapping pathways in the twins' brains, arguably implies that the twins share some conscious experiences. If this is true, then the twins may offer a proof of concept for how experiences in general could be shared between brains.[76][77][78]
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