Thalamus

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This article is about the portion of the human brain. For the British video game developer, see Thalamus Ltd. For the botanical structure, see Receptacle (botany).
Thalamus
MRI cross-section)
The thalamus in a 360° rotation
Details
Part ofDiencephalon
PartsSee List of thalamic nuclei
ArteryPosterior cerebral artery and branches
Identifiers
Latinthalamus dorsalis
MeSHD013788
NeuroNames300
NeuroLex IDbirnlex_954
TA98A14.1.08.101
A14.1.08.601
TA25678
TEE5.14.3.4.2.1.8
FMA62007
Anatomical terms of neuroanatomy]

The thalamus (pl.: thalami; from

Nerve fibers project out of the thalamus to the cerebral cortex in all directions, known as the thalamocortical radiations, allowing hub-like exchanges of information. It has several functions, such as the relaying of sensory and motor signals to the cerebral cortex[2][3] and the regulation of consciousness, sleep, and alertness.[4]

Anatomically, it is a paramedian symmetrical structure of two halves (left and right), within the

embryologist and anatomist Wilhelm His Sr. in 1893.[5]

Anatomy

The thalamus is a paired structure of gray matter about four centimetres long, located in the forebrain which is superior to the midbrain, near the center of the brain with nerve fibers projecting out to the cerebral cortex in all directions. The medial surface of the thalamus constitutes the upper part of the lateral wall of the third ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, the interthalamic adhesion. The lateral part of the thalamus is the phylogenetically newest part of the thalamus (neothalamus), and includes the lateral nuclei, the pulvinar and the medial and lateral geniculate nuclei.[6][7] There are areas of white matter in the thalamus including the stratum zonale that covers the dorsal surface and the external and internal medullary laminae. The external lamina covers the lateral surface and the internal lamina divides the nuclei into anterior, medial, and lateral groups.[8]

Blood supply

The thalamus derives its blood supply from a number of arteries: the polar artery (posterior communicating artery), paramedian thalamic-subthalamic arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries.[9] These are all branches of the posterior cerebral artery.[10]

Some people have the artery 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.

Thalamic nuclei

See also: List of thalamic nuclei
Thalamic nuclei. Metathalamus labelled MTh
Nuclei of the thalamus
Dorsal view
Coronal section of lateral and third ventricles

Derivatives of the

ontogenetic
origins, the epithalamus and the perithalamus are formally distinguished from the thalamus proper. The metathalamus is made up of the lateral geniculate and medial geniculate nuclei.

The thalamus comprises a system of

lamellae (made up of myelinated fibers) separating different thalamic subparts. Other areas are defined by distinct clusters of neurons, such as the periventricular nucleus, the intralaminar elements, the "nucleus limitans", and others.[11] These latter structures, different in structure from the major part of the thalamus, have been grouped together into the allothalamus as opposed to the isothalamus.[12]
This distinction simplifies the global description of the thalamus.

Connections

The thalamus is connected to the spinal cord via the spinothalamic tract

The thalamus has many connections to the hippocampus via the

mammillary bodies and fornix.[13]

The thalamus is connected to the cerebral cortex via the thalamocortical radiations.[14]

The

anterior (or ventral) spinothalamic tract
, which transmits crude touch and pressure.

Function

The thalamus has multiple functions, and is generally believed to act as a relay station, or

proprioceptive information to the primary somatosensory cortex.[citation needed
]

The thalamus also plays an important role in regulating states of sleep and wakefulness.[18] Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness.[19] The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma.[20]

The role of the thalamus in the more anterior

gustatory and visual 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 is functionally connected to the hippocampus[22] as part of the extended hippocampal system at the thalamic anterior nuclei[23] with respect to spatial memory and spatial sensory datum they are crucial for human episodic event memory.[24][25] The thalamic region's connection to the mesio-temporal lobe provide differentiation of the functioning of recollective and familiarity memory.[13]

The neuronal information processes necessary for motor control were proposed as a network involving the thalamus as a subcortical motor center.[26] Through investigations of the anatomy of the brains of primates[27] the nature of the interconnected tissues of the cerebellum 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.[28][29] In an investigation of the saccade and antisaccade[30] 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 jerking movement of the eyes in the direction of a presented stimulus).[31]

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."[32] Researchers found that "enhancing MD activity magnified the ability of mice to "think,"[32] driving down by more than 25 percent their error rate in deciding which conflicting sensory stimuli to follow to find the reward."[33]

Development

The thalamic complex is composed of the perithalamus (or prethalamus, previously also known as ventral thalamus), the mid-diencephalic organiser (which forms later the zona limitans intrathalamica (ZLI) ) and the thalamus (dorsal thalamus).[34][35] The development of the thalamus can be subdivided into three steps.[36] The thalamus is the largest structure deriving from the embryonic diencephalon, the posterior part of the forebrain situated between the midbrain and the cerebrum.

Early brain development

After

prethalamus and the thalamus is induced within the neural tube. Data from different vertebrate model organisms support a model in which the interaction between two transcription factors, Fez and Otx, are of decisive importance. Fez is expressed in the prethalamus, and functional experiments show that Fez is required for prethalamus formation.[37][38] Posteriorly, Otx1 and Otx2 abut the expression domain of Fez and are required for proper development of the thalamus.[39][40]

Formation of progenitor domains

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.[41]

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 thalamic

sonic hedgehog (SHH) family and of the Wnt
family are the main principal signals emitted by the MDO.

Besides its importance as signalling center, the organizer matures into the morphological structure of the zona limitans intrathalamica (ZLI).

Maturation and parcellation of the thalamus

After its induction, the MDO starts to orchestrate the development of the thalamic anlage by release of signalling molecules such as SHH.[42] In mice, the function of signaling at the MDO has not been addressed directly due to a complete absence of the diencephalon in SHH mutants.[43]

Studies in chicks have shown that SHH is both necessary and sufficient for thalamic gene induction.[44] In zebrafish, 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.[45]

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 gene Neurogenin1 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.[46][47]

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 of

thalamic nuclei.[36]

In humans, a common genetic variation in the promoter region of the

pulvinar 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 to major depression, post-traumatic stress disorder, and suicide.[48]

Clinical significance

A thalamus damaged by a

oculomotor problems. A related concept is thalamocortical dysrhythmia. The occlusion of the artery of Percheron
can lead to a bilateral thalamus infarction.

mammillothalamic fasciculus or the thalamus.[50][51]

Fatal familial insomnia is a hereditary prion disease in which degeneration of the thalamus occurs, causing the patient to gradually lose their ability to sleep and progressing to a state of total insomnia
, which invariably leads to death. In contrast, damage to the thalamus can result in coma.

Atrophy of the thalamus is an indicator of MS.[52][53]

Additional images

  • Human brain dissection, showing the thalamus.
    Human brain dissection, showing the thalamus.
  • Human thalamus along with other subcortical structures, in glass brain.
    Human thalamus along with other subcortical structures, in glass brain.
  • Lateral group of the thalamic nuclei.
    Lateral group of the thalamic nuclei.
  • Medial group of the thalamic nuclei.
    Medial group of the thalamic nuclei.

See also

References

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