Thalamus
Encyclopedia
The thalamus is a midline paired symmetrical structure within the brain
s of vertebrate
s, including humans. It is situated between the cerebral cortex
and midbrain, both in terms of location and neurological connections. Its function includes relaying sensation, spatial 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
.
, the posterior part of the forebrain situated between the midbrain and the cerebrum. 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. Derivatives of the diencephalon
also include 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.
ated fibers
) separating different thalamic subparts. Other areas are defined by distinct clusters of neuron
s, 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
. This distinction simplifies the global description of the thalamus.
See also List of thalamic nuclei.
), paramedian thalamic-subthalamic arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries. These are all derived from the vertebrobasilar arterial system except the polar artery.
. 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 relay 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. 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 system
s 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.
) can lead to the thalamic 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
.
Korsakoff's syndrome
stems from damage to the mammillary body
, the mammillothalamic fasciculus or the thalamus.
Fatal familial insomnia
is a hereditary prion
disease in which degeneration of the thalamus occurs, causing the patient to gradually lose his ability to sleep and progressing to a state of total insomnia, which invariably leads to death.
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 subdivide into three steps
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 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. Posteriorly, Otx1 and Otx2 abut the expression domain of Fez and are required for proper development of the 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 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 diencephalon
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 transcription 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 the thalamic nuclei.
Further reading: Building a bridal chamber: development of the thalamus. from Steffen Scholpp and Andrew Lumsden in Trends of Neuroscience 2010
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 allele
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, posttraumatic stress disorder, and suicide.
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
s of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, including humans. It is situated between the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
and midbrain, both in terms of location and neurological connections. Its function includes relaying sensation, spatial sense, and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness. The thalamus surrounds the third ventricle
Third ventricle
The third ventricle is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid ....
. It is the main product of the embryonic diencephalon
Diencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
.
Location and topography
The thalamus is the largest structure deriving from the embryonic diencephalonDiencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
, the posterior part of the forebrain situated between the midbrain and the cerebrum. 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. Derivatives of the diencephalon
Diencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...
also include also the dorsally-located epithalamus
Epithalamus
The epithalamus is a dorsal posterior segment of the diencephalon which includes the habenula, the stria medullaris and the pineal body...
(essentially the habenula
Habenula
In neuroanatomy, habenula originally denoted the stalk of the pineal gland , but gradually came to refer to a neighboring group of nerve cells with which the pineal gland was believed to be associated, the habenular nucleus...
and annexes) and the perithalamus (prethalamus
Prethalamus
The prethalamus or subthalamus is part of the diencephalon and therefore part of the brain.Developmental biologists prefer the term prethalamus, as it can be genetically defined , whereas anatomists often use the expression subthalamus.The prethalamus is part of the mid-diencephalic territory...
formerly described as ventral thalamus) containing the zona incerta
Zona incerta
The zona incerta is a horizontally elongated region of gray matter cells in the subthalamus below the thalamus. Its connections project extensively over the brain from the cerebral cortex down into the spinal cord....
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 myelinMyelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
ated fibers
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
) separating different thalamic subparts. Other areas are defined by distinct clusters of neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s, 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
Allothalamus
- Paramedian formation :This or periventricular formation lies along the wall of the third ventricle. In man where there is no or almost no adhesio interthalamica this is reduced to a thin layer medial to the medial nucleus. This lamina has no clear subdivisions and the formation may be seen as one...
as opposed to the isothalamus
Isothalamus
The isothalamus is a division used by some researchers in describing the thalamus.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...
. This distinction simplifies the global description of the thalamus.
See also List of thalamic nuclei.
Arterial supply
The thalamus derives its blood supply from four arteries including the polar artery(posterior communicating arteryPosterior communicating artery
In human anatomy, the posterior communicating artery is one of a pair of right-sided and left-sided blood vessels in the circle of Willis. It connects the three cerebral arteries of the same side. Anteriorly, it connects to the internal carotid artery prior the terminal bifurcation of the ICA...
), paramedian thalamic-subthalamic arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries. These are all derived from the vertebrobasilar arterial system except the polar artery.
Function
The thalamus has multiple functions. A helpful way to remember is to think of the thalamus as a switchboard of information. It is generally believed to act as a relay between a variety of subcortical areas and the cerebral cortexCerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. In particular, every sensory system (with the exception of the olfactory system
Olfactory system
The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...
) 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
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
are sent to the lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
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 relay it—each of the primary sensory relay areas receives strong "back projections" from the cerebral cortex.
Similarly the medial geniculate nucleus
Medial geniculate nucleus
The Medial Geniculate Nucleus or Medial Geniculate Body is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus and the auditory cortex...
acts as a key auditory
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
relay between the inferior colliculus
Inferior colliculus
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex...
of the midbrain and the primary auditory cortex
Primary auditory cortex
The primary auditory cortex is the region of the brain that is responsible for the processing of auditory information. Corresponding roughly with Brodmann areas 41 and 42, it is located on the temporal lobe, and performs the basics of hearing—pitch and volume...
, and the ventral posterior nucleus
Ventral posterior nucleus
The ventral posterior nucleus is the somato-sensory relay nucleus in thalamus of the brain.-Input and output:The ventral posterior nucleus receives neuronal input from the medial lemniscus, spinal lemniscus, spinothalamic tracts, and trigeminothalamic tract...
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
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
and wakefulness. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits
Thalamo-cortico-thalamic circuits
Thalamo-cortico-thalamic circuits consist of looped neural pathways that connect the thalamus to the cerebral cortex, and connect the cerebral cortex back to the thalamus. Some researchers propose that such circuits allow the brain to obtain data on its own activity, making self-awareness possible....
that are believed to be involved with consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
. The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...
.
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
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
, somatic, visceral, gustatory and visual system
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
s 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
Tremor
A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving to-and-fro movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, eyes, face, head, vocal folds, trunk, and legs. Most tremors occur in the...
. The role of the thalamus in the more anterior pallidal and nigral territories in the basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...
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.
Pathology
A cerebrovascular accident (strokeStroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
) can lead to the thalamic syndrome
Thalamic syndrome
Dejerine–Roussy syndrome or thalamic pain syndrome is a condition developed after a thalamic stroke, a stroke causing damage to the thalamus. This condition is not to be confused with "Roussy-Levy Syndrome", a genetic disorder. Ischemic strokes and Hemorrhagic strokes can cause lesioning in the...
, which involves a one-sided burning or aching sensation often accompanied by mood swings
Mood disorder
Mood disorder is the term designating a group of diagnoses in the Diagnostic and Statistical Manual of Mental Disorders classification system where a disturbance in the person's mood is hypothesized to be the main underlying feature...
. Bilateral ischemia
Ischemia
In medicine, ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia...
of the area supplied by the paramedian artery can cause serious problems including akinetic mutism
Akinetic mutism
Akinetic mutism is a medical term describing patients who tend neither to speak nor move . It is the result of severe frontal lobe injury in which the pattern of inhibitory control is one of increasing passivity and gradually decreasing speech and motion.An example of a cause of this disorder...
, and be accompanied by oculomotor problems. A related concept is thalamocortical dysrhythmia
Thalamocortical Dysrhythmia
Thalamocortical Dysrhythmia is a theoretical framework in which neuroscientists try to explain the positive and negative symptoms induced by neurological disorders like Parkinson's Disease, neurogenic pain, Tinnitus, Epilepsy as well as neuropsychiatric disorders like depression.In TCD the...
.
Korsakoff's syndrome
Korsakoff's syndrome
Korsakoff's syndrome is a neurological disorder caused by the lack of thiamine in the brain. Its onset is linked to chronic alcohol abuse and/or severe malnutrition...
stems from damage to the mammillary body
Mammillary body
The mammillary bodies are a pair of small round bodies, located on the undersurface of the brain, that, as part of the diencephalon form part of the limbic system. They are located at the ends of the anterior arches of the fornix...
, the mammillothalamic fasciculus or the thalamus.
Fatal familial insomnia
Fatal familial insomnia
Fatal familial insomnia is a very rare autosomal dominant inherited prion disease of the brain. It is almost always caused by a mutation to the protein PrPC, but can also develop spontaneously in patients with a non-inherited mutation variant called sporadic fatal insomnia...
is a hereditary prion
Prion
A prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...
disease in which degeneration of the thalamus occurs, causing the patient to gradually lose his ability to sleep and progressing to a state of total insomnia, which invariably leads to death.
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
Zona limitans intrathalamica
The zona limitans intrathalamica is a lineage-restriction compartment and primary developmental boundary in the vertebrate forebrain that serves as a signaling center and a restrictive border between the thalamus and the prethalamus...
(ZLI) ) and the thalamus (dorsal thalamus). The development of the thalamus can be subdivide into three steps
Early brain development
After neurulation
Neurulation
Neurulation is the stage of organogenesis in vertebrate embryos, during which the neural tube is transformed into the primitive structures that will later develop into the central nervous system....
the anlage of the prethalamus
Prethalamus
The prethalamus or subthalamus is part of the diencephalon and therefore part of the brain.Developmental biologists prefer the term prethalamus, as it can be genetically defined , whereas anatomists often use the expression subthalamus.The prethalamus is part of the mid-diencephalic territory...
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. Posteriorly, Otx1 and Otx2 abut the expression domain of Fez and are required for proper development of the 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 SHH
SHH
SHH can refer to:* Summer Heights High, an Australian TV series* Sonic hedgehog, an important protein during vertebrate organogenesis* Students Helping Honduras, an international non-governmental organization operating in the U.S. and Honduras...
family and of the Wnt
Wnt signaling pathway
The Wnt signaling pathway is a network of proteins best known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.-Discovery:...
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)
Zona limitans intrathalamica
The zona limitans intrathalamica is a lineage-restriction compartment and primary developmental boundary in the vertebrate forebrain that serves as a signaling center and a restrictive border between the thalamus and the prethalamus...
.
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
SHH
SHH can refer to:* Summer Heights High, an Australian TV series* Sonic hedgehog, an important protein during vertebrate organogenesis* Students Helping Honduras, an international non-governmental organization operating in the U.S. and Honduras...
. In mice, the function of signaling at the MDO has not been addressed directly due to a complete absence of the diencephalon
Diencephalon
The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and 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
SHH
SHH can refer to:* Summer Heights High, an Australian TV series* Sonic hedgehog, an important protein during vertebrate organogenesis* Students Helping Honduras, an international non-governmental organization operating in the U.S. and Honduras...
leads to differentiation of thalamic neurons. SHH
SHH
SHH can refer to:* Summer Heights High, an Australian TV series* Sonic hedgehog, an important protein during vertebrate organogenesis* Students Helping Honduras, an international non-governmental organization operating in the U.S. and Honduras...
signaling from the MDO induces a posterior-to-anterior wave of expression the proneural gene Neurogenin1
Neurogenins
Neurogenins are a family of bHLH transcription factors involved in specifying neuronal differentiation. They are related to Drosophila atonal.The neurogenins make up one of these atonal-related gene families...
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
HES1
Transcription factor HES-1 is a protein that in humans is encoded by the HES1 gene.-Interactions:HES1 has been shown to interact with TLE2 and Sirtuin 1.- External links :...
. Expression of this hairy-like bHLH transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
, 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.
Further reading: Building a bridal chamber: development of the thalamus. from Steffen Scholpp and Andrew Lumsden in Trends of Neuroscience 2010
In humans, a common genetic variation in the promotor region of the serotonin transporter
Serotonin transporter
The serotonin transporter is a monoamine transporter protein.This protein is an integral membrane protein that transports the neurotransmitter serotonin from synaptic spaces into presynaptic neurons. This transport of serotonin by the SERT protein terminates the action of serotonin and recycles it...
(the SERT-long and -short allele: 5-HTTLPR
5-HTTLPR
5-HTTLPR is a degenerate repeat polymorphic region in SLC6A4, the gene that codes for the serotonin transporter.Since the polymorphism was identified in the middle of the 1990s,...
) has been shown to affect the development of several regions of the thalamus in adults.
People who inherit two short allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
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, posttraumatic stress disorder, and suicide.
See also
- Primate basal ganglia systemPrimate basal ganglia systemThe basal ganglia form a major brain system in all species of vertebrates, but the basal ganglia of primates have special features that justify a separate consideration. As in other vertebrates, the primate basal ganglia can be divided into striatal, pallidal, nigral, and subthalamic components...
- List of regions in the human brain
- Thalamus (non primate)
- List of thalamic nuclei
- Thalamic stimulatorThalamic stimulatorA thalamic stimulator is a relatively new medical device that can suppress tremors, such as those caused by Parkinson's Disease or essential tremor. It was approved for use by the Food and Drug Administration on August 4, 1997. Installation is invasive, so it is typically only used when the...
- ThalamotomyThalamotomyFirst introduced in the 1950s, thalamotomy is an invasive procedure, primarily effective for tremors such as those associated with Parkinson's Disease , where a selected portion of the thalamus is surgically destroyed...
External links
- http://www.scholarpedia.org/article/Thalamus