The Full Wiki

Thalamus: Wikis

Advertisements
  

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

Brain: Thalamus
Brain chrischan thalamus.jpg
MRI cross-section of human brain, with thalamus marked.
Thalmus.png
Scheme showing the parts of the thalmus
Latin thalamus dorsalis
Gray's subject #189 808
Part of Diencephalon
Components See List of thalamic nuclei
NeuroNames hier-283
MeSH Thalamus
NeuroLex ID birnlex_954

The thalamus (from Greek θάλαμος = room, chamber) is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections. Its function includes relaying sensation, special sense and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep and alertness. The thalamus surrounds the third ventricle. It is the main product of the embryonic diencephalon.

Contents

Location and topography

Thalamus (Animation)

The thalamus is the largest structure in the diencephalon, the part of the brain situated between the midbrain (mesencephalon) and forebrain (telencephalon). Anatomically, the thalamus is perched on top of the brainstem, near the center of the brain, in a position to send nerve fibers out to the cerebral cortex in all directions. 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.

In humans, the two halves of the thalamus are prominent bulb-shaped masses, about 5.7 cm in length, located obliquely (about 30°) and symmetrically on each side of the third ventricle.

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.[1] 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.[2] This distinction simplifies the global description of the thalamus.

See also List of thalamic nuclei.

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.[3] These are all branches of the posterior cerebral artery.

Function

The thalamus has multiple functions. It is generally believed to act as a relay between a variety of subcortical areas and the cerebral cortex. In particular, every sensory system (with the exception of the olfactory system) includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area. For the visual system, for example, inputs from the retina are sent to the lateral geniculate nucleus of the thalamus, which in turn projects to the primary visual cortex (area V1) in the occipital lobe. The thalamus is believed to both process sensory information as well as relaying it—each of the primary sensory relay areas receives strong "back projections" from the cerebral cortex. Similarly the medial geniculate nucleus acts as a key auditory relay between the inferior colliculus of the midbrain and the primary auditory cortex, and the ventral posterior nucleus is a key somatosensory relay, which sends touch and proprioceptive information to the primary somatosensory cortex.

The thalamus also plays an important role in regulating states of sleep and wakefulness.[4] 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. Damage to the thalamus can lead to permanent coma.

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 the auditory, somatic, visceral, gustatory and visual systems where localized lesions provoke specific 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 understood. 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 selective.[5]

Pathology

Cerebrovascular accidents (strokes) can cause thalamic syndrome,[6] which results in a contralateral hemianaesthesia, burning or aching sensation on one half of a body (painful anaesthesia) often accompanied by mood swings. Ischemia of the territory of the paramedian artery, if bilateral, causes serious troubles including akinetic mutism accompanied or not by oculomotor troubles. It is also related to Thalamocortical Dysrhythmia.

Korsakoff's Syndrome stems from mammillary bodies, mammilothalamic, or thalamic lesions.

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 chicks 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 mice, the function of 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]

In humans, a common genetic variation in the promotor region of the serotonin 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 short alleles (SERT-ss) have more neurons and a larger volume in 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, posttraumatic stress disorder, and suicide.[14]

References

  1. ^ Jones Edward G.(2007) "The Thalamus" Cambridge Uni. Press
  2. ^ Percheron, G. (2003) "Thalamus". In Paxinos, G. and May, J.(eds). The human nervous system. 2d Ed. Elsevier. Amsterdam. pp.592-675
  3. ^ 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
  4. ^ Steriade, M. and Llinas, R. (1988) "The functional states of the thalamus and the associated neuronal interplay". Physiological Reviews 68: 699-742 PMID 2839857
  5. ^ Your Brain Boots Up Like a Computer | LiveScience
  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]
  14. ^ Young KA, Holcomb LA, Yazdani U, Bonkale W, Hicks PB and German DC (2007). "5HTTLPR polymorphism and enlargement of the pulvinar: Unlocking the backdoor to the limbic system". Biol Psychiatry 61 (6): 813–8. doi:10.1016/j.biopsych.2006.08.047. PMID 17083920. 

See also

Additional images

External links

Advertisements

Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message