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Thalamus

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Thalamus

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Brain: Thalamus
MRI cross-section of human brain, with thalamus marked.
Scheme showing the course of the fibers of the lemniscus; medial lemniscus in blue, lateral in red.
Latin thalamus dorsalis
Gray's subject #189 808
NeuroNames hier-283
MeSH Thalamus


The thalamus (from Greek θάλαμος = bedroom, chamber, IPA= /ˈθæləməs/) is a pair and symmetric part of the brain. It constitutes the main part of the diencephalon.

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[edit] Location and topography

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. The diencephalon includes also the dorsally located epithalamus (essentially the habenula and annexes) and the perithalamus (prethalamus formerly described as ventral thalamus) containing the zona incerta and the "reticulate nucleus" (not the reticular, term of confusion). Due to their different ontogenetic origins, the epithalamus and the perithalamus are formally distinguished from the thalamus proper.

Phylogenetic modifications are such that this article essentially deals with the human thalamus and may differ in comparison with accounts in non-upper primate species. In normal humans, the two thalami are prominent bulb-shaped masses, about 5.7 cm in length, located obliquely (about 30°) and symmetrically on each side of the third ventricle. The two can adhere on a variable extent in 30% of humans. This adhesio interthalamica (interthalamic adhesion, or massa intermedia) does not contain interthalamic neural connection in our species.

[edit] Anatomy

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 gray, the intralaminar elements, the "nucleus limitans", and others. 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 (Percheron, 2003).[1] This distinction simplifies the global description of the thalamus.

Please see also List of thalamic nuclei.

[edit] Isothalamus

The isothalamus constitutes 90% or more of the thalamus, and despite the variety of functions it serves, follows a simple organizational scheme. The constituting neurons belong to two different neuronal genera. The first correspond to the thalamocortical neurons (or principal). They have a "tufted"(or radiate) morphology, 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 of, and to the animal species. They have the rather rare property of having no initial axonal collaterals, which implies that one emitting thalamocortical neuron does not send information to its neighbor. They send long-range glutamatergic projections to the cerebral cortex where they end electively at the layer IV (or around) level. 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 varies across species, highly increasing with evolution. Their short axonal parts contact thalamocortical or other local circuitry neurons. Their neurotransmitter is GABA. The dendrites of the two constituting genera receive synapses from a variety of 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 also GABAergic. The number of perithalamic neurons strongly decreases in evolution in opposition to the large increase in microneurons (Arcelli et al. 1997).[2] To some extent the perithalamus plays a role in the local circuitry. The circuitous connection with corticothalamic neurons participates in the elaboration of thalamic rhythms. The different functional modalities represented in the thalamus are segregated in specific anatomical regions, differentiated by the cerebral systems from where they receive their afferent projections. There are more corticothalamic than thalamocortical axons. Corticothalamic endings are of two kinds. The "classical" projection emanates from layer VI of the cortex,the axons are thin and have a long, almost straight, trajectory through the thalamus, not respecting intrathalamic borders. They emit only short perpendicular collaterals (the arborization formin a thin cylinder (Globus and Scheibel). Their terminal synapses are glutamatergic. The second kind of corticothamic axons is the Rockland type II (1994).[3] This emanates from larger pyramidal cells and is much thicker. Its ending is small, dense and globular. Its synapses are located close to the soma of the thalamic neuron, often forming the center of glomerular complexes. The isothalamus serves the function of transforming and distributing "prethalamic" information to the cortex.


[edit] Arterial supply

The thalamus derives its blood supply from a number of arteries including polar and paramedian arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries.[4] These are all branches of the posterior cerebral artery.

[edit] Function

The thalamus is known to have multiple functions. Deduced from the design of the isothalamus, it is generally believed to act as a translator for which various "prethalamic" inputs are processed into a form readable by the cortex. The thalamus is believed to relay information selectively to various parts of the cortex, as one thalamic point may reach one or several regions in the cortex.

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

Many different functions are linked to the system to which thalamic parts belong. This is at first the case for sensory systems (which excepts the olfactory function) auditory, somatic, visceral, gustatory and visual systems where localised lesions provoke particular sensory deficits. A major role of the thalamus is devoted to "motor" systems. This has been and continues to be a subject of interest for investigators. VIm, the relay of cerebellar afferences, is the target of stereotactians particularly for the improvement of tremor. The role of the thalamus in the more anterior pallidal and nigral territories in the basal ganglia system disturbances is recognized but still poorly known. The contribution of the thalamus to vestibular or to tectal 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 complicated.[5]

[edit] Pathology

Cerebrovascular accidents (strokes) can cause thalamic syndrome (Dejerine and Roussy, 1906),[6] which results in a contralateral 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, stems from mammillary bodies, mammilothalamic, or thalamic lesions.

[edit] Development

The thalamic complex is composed of the perithalamus (or prethalamus, previously also known as ventral thalamus), the zona limitans intrathalamica (ZLI) and the thalamus (dorsal thalamus).[7][8]

The ZLI is a transverse boundary located between the perithalamus and the functional distinct thalamus. Besides its morphological characteristics, it bears the hallmarks of a signalling centre. Fate mapping experiments in chick have shown that the ZLI is cell lineage restricted at its boundaries and therefore can be termed a true developmental compartment in the forebrain.[9]

Besides morphological characteristics, the ZLI is the only structure in the alar plate of the neural tube that expresses signaling molecules.[10]

In mouse, the function of Shh (Sonic Hedgehog) signaling at the ZLI has not been addressed directly due to a complete absence of the diencephalon in Shh mutants.[11]

Studies in chicks have shown that Shh is both necessary and sufficient for thalamic gene induction.[12]

In zebrafish, it was shown that the expression of two Shh genes, shh-a and shh-b (formerly described as twhh) mark the ZLI 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 ZLI/alar plate is sufficient for the maturation of prethalamic and thalamic territory while ventral Shh signals are dispensable.[13]

[edit] References

  1. ^ Percheron, G. (2003) "Thalamus". In Paxinos, G. and May, J.(eds). The human nervous system. 2d Ed. Elsevier. Amsterdam. pp.592-675
  2. ^ Arcelli P, Frassoni C, Regondi M, De Biasi S, Spreafico R (1997). "GABAergic neurons in mammalian thalamus: a marker of thalamic complexity?". Brain Res Bull 42 (1): 27-37. PMID.
  3. ^ Rockland K (1994). "Further evidence for two types of corticopulvinar neurons". Neuroreport 5 (15): 1865-8. PMID.
  4. ^ Percheron, G. (1982) The arterial supply of the thalamus. In Schaltenbrand and Walker, A.E.(eds) Stereotaxy of the human brain. Thieme . Stuttgart. pp.218-232
  5. ^ http://www.livescience.com/humanbiology/060817_brain_boot.html
  6. ^ Dejerine, J. and Roussy. G.(1906) Le syndrome thalamique. Rev. Neurol. 14: 521-532
  7. ^ Kuhlenbeck, H. (1937). The ontogenetic development of diencephalic centres in the bird's brain (chick) and comparison with the reptilian and mammalian diencephalon. J. Comp. Neurol. 66
  8. ^ Shimamura, K., Hartigan, D. J., Martinez, S., Puelles, L. and Rubenstein, J. L. (1995). Longitudinal organization of the anterior neural plate and neural tube. Development 121,3923 -3933.
  9. ^ Zeltser, L. M., Larsen, C. W. and Lumsden, A. (2001). A new developmental compartment in the forebrain regulated by Lunatic fringe. Nat. Neurosci. 4, 683-684.
  10. ^ Puelles, L. and Rubenstein, J. L. (2003). Forebrain gene expression domains and the evolving prosomeric model. Trends Neurosci. 26,469 -476.
  11. ^ Ishibashi, M. and McMahon, A. P. (2002). A sonic hedgehog-dependent signalling relay regulates growth of diencephalic and mesencephalic primordia in the early mouse embryo. Development 129,4807 -4819.
  12. ^ Kiecker, C. and Lumsden, A. (2004). Hedgehog signalling from the ZLI regulates diencephalic regional identity. Nat. Neurosci. 7,1242 -1249.
  13. ^ Scholpp S, Wolf O, Brand M, Lumsden A. Hedgehog signalling from the zona limitans intrathalamica orchestrates patterning of the zebrafish diencephalon'. Development. 2006 Mar;133(5):855-64[1]

[edit] See also

[edit] Additional images

Schematic representation of the chief ganglionic categories (I to V).

Deep dissection of brain-stem. Lateral view.

Deep dissection of brain-stem. Ventral view.

Mesal aspect of a brain sectioned in the median sagittal plane.

Dissection showing the ventricles of the brain.

Coronal section of brain immediately in front of pons.

Coronal section of brain through intermediate mass of third ventricle.

Coronal section of lateral and third ventricles.

Section of brain showing upper surface of temporal lobe.

Horizontal section of right cerebral hemisphere.

The left optic nerve and the optic tracts.

Parts of the thalamus

Projections of the thalamus

Parts of the thalamus

Thalamus

[edit] External links


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