The thalamus (from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the

diencephalon The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is asso ...

(a division of the

forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hindbrain (rhombencephalon) are the three prima ...

). Nerve fibers project out of the thalamus to the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting o ...

in all directions, allowing hub-like exchanges of information. It has several functions, such as the relaying of

sensory Sensory may refer to: Biology * Sensory ecology, how organisms obtain information about their environment * Sensory neuron, nerve cell responsible for transmitting information about external stimuli * Sensory perception, the process of acquiri ...

signals, including motor signals to the cerebral cortex and the regulation of

consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...

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

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

brain, situated between the cerebral cortex and the

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...

. It forms during

embryonic development An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

as the main product of the

, as first recognized by the Swiss embryologist and anatomist

Wilhelm His Sr. __NOTOC__ Wilhelm His Sr. (9 July 1831 – 1 May 1904) was a Swiss anatomist and professor who invented the microtome. By treating animal flesh with acids and salts to harden it and then slicing it very thinly with the microtome, scientists were ...

in 1893.

Anatomy

The thalamus is a paired structure of gray matter located in the

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 Medial may refer to: Mathematics * Medial magma, a mathematical identity in algebra Geometry * Medial axis, in geometry the set of all points having more than one closest point on an object's boundary * Medial graph, another graph that re ...

and lateral geniculate nuclei. 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.

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. These are all branches of the posterior cerebral artery. 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

Derivatives of the

include the dorsally-located epithalamus (essentially the habenula and annexes) and the perithalamus (prethalamus) containing the

zona incerta The zona incerta (ZI) is a horizontally elongated region of gray matter in the subthalamus below the thalamus. Its connections project extensively over the brain from the cerebral cortex down into the spinal cord. Its function is unknown, thoug ...

and the thalamic reticular nucleus. Due to their different 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

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

s, such as the periventricular nucleus, 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''. This distinction simplifies the global description of the thalamus.

Connections

The thalamus has many connections to the hippocampus via the mammillothalamic tract, this tract comprises the mammillary bodies and

fornix Fornix (plural ''fornices'', Latin for "arch") can refer to: *''Fornix'' or ''throat scale'', a bowed prolongation of the corolla Corolla may refer to: *Corolla (botany), the petals of a flower, considered as a unit *Toyota Corolla, an automobile ...

. The thalamus is connected to the

via the thalamocortical radiations. The

spinothalamic tract The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory ...

is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch. There are two main parts: the lateral spinothalamic tract, which transmits pain and temperature, and the anterior (or ventral) spinothalamic tract, which transmits crude touch and pressure.

Function

The thalamus has multiple functions, generally believed to act as a relay station, or hub, relaying information between different 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 visual cortex in the occipital lobe. 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. Similarly the medial geniculate nucleus acts as a key auditory relay between the inferior colliculus of the

and the primary auditory cortex. 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. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with

. The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma. 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. 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 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 as part of the extended hippocampal system at the thalamic anterior nuclei with respect to spatial memory and spatial sensory datum they are crucial for human episodic event memory. The thalamic region's connection to the mesio-temporal lobe provide differentiation of the functioning of recollective and familiarity memory. The neuronal information processes necessary for motor control were proposed as a network involving the thalamus as a subcortical motor center. Through investigations of the anatomy of the brains of primates the nature of the interconnected tissues of the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cere ...

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. In an investigation of the saccade and antisaccade 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). 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." Researchers found that "enhancing MD activity magnified the ability of mice to "think," driving down by more than 25 percent their error rate in deciding which conflicting sensory stimuli to follow to find the reward."

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). The development of the thalamus can be subdivided into three steps. The thalamus is the largest structure deriving from the embryonic

, the posterior part of the forebrain situated between the midbrain and the cerebrum.

Early brain development

After neurulation the anlage of the 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 factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...

s, Fez and Otx, are of decisive importance. Fez is expressed in the prethalamus, and functional experiments show that Fez is required for prethalamus formation. Posteriorly, Otx1 and Otx2 abut the expression domain of Fez and are required for proper development of the thalamus.

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.

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 anlage. 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 the

sonic hedgehog Sonic hedgehog protein (SHH) is encoded for by the ''SHH'' gene. The protein is named after the character ''Sonic the Hedgehog (character), Sonic the Hedgehog''. This signaling molecule is key in regulating embryonic morphogenesis in all animals ...

(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. In mice, the function of signaling at the MDO has not been addressed directly due to a complete absence of the

in SHH mutants. Studies in chicks have shown that SHH is both necessary and sufficient for thalamic gene induction. 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. 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. 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 HES1. Expression of this hairy-like bHLH

, 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 the thalamic nuclei. In humans, a common genetic variation in the promoter 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

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

s (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, post-traumatic stress disorder, and suicide.

Clinical significance

A thalamus damaged by a stroke can lead to thalamic pain syndrome, which involves a one-sided burning or aching sensation often accompanied by mood swings. Bilateral ischemia of the area supplied by the paramedian artery can cause serious problems including akinetic mutism, and be accompanied by oculomotor problems. A related concept is thalamocortical dysrhythmia. The occlusion of the artery of Percheron can lead to a bilateral thalamus infarction. Alcoholic Korsakoff syndrome stems from damage to the mammillary body, the mammillothalamic fasciculus or the thalamus. Fatal familial insomnia is a hereditary

prion Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...

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.

Additional images

File:Slide2GRE.JPG, Human brain dissection, showing the thalamus. File:Thalamus_-_DK_ATLAS.png , Human thalamus along with other subcortical structures, in glass brain. File:Thalamus_lateralgroup.jpg , Lateral group of the thalamic nuclei. File:Thalamus_medialgroup.jpg , Medial group of the thalamic nuclei.

References

External links

* {{Authority control Motor system Sensory systems