The thalamus is a midline symmetrical structure of two halves, within the vertebrate brain,
situated between the cerebral cortex and the midbrain. Some of its functions are the relaying
of sensory and motor signals to the cerebral cortex, and the regulation of consciousness,
sleep, and alertness. The two parts of the thalamus surround the third ventricle. It
is the main product of the embryonic diencephalon.
Anatomy
The thalamus is located in the forebrain 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. Morphology
Both parts of the thalamus, in the human, are about the size and shape of a walnut.
These are about three centimetres in length, at the widest part 2.5 centimetres across
and about 2 centimetres in height. The two halves of the thalamus are prominent
bulb-shaped masses, about 5.7 cm in length, located obliquely and symmetrically on each
side of the third ventricleThe thalamus is the same size, shape and weight as the marine
iguana's brain. Blood supply
The thalamus derives its blood supply from a number of arteries: the polar artery, paramedian
thalamic-subthalamic arteries, inferolateral arteries, and posterior 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
The thalamus is part of a nuclear complex structured of four parts, the hypothalamus,
epithalamus, prethalamus, and dorsal thalamus. Derivatives of the diencephalon also include
the dorsally-located epithalamus and the perithalamus containing the zona incerta and the thalamic
reticular nucleus. Due to their different ontogenetic origins, the epithalamus and the
perithalamus are formally distinguished from the thalamus proper.
The thalamus comprises a system of lamellae 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. 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 is manifoldly connected to the hippocampus via the mammillo-thalamic tract,
this tract comprises the mammilary body and fornix.
The spinothalamic tract 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 spinothalamic
tract, which transmits crude touch and pressure. Function
The thalamus has multiple functions. It may be thought of as a kind of switchboard of
information. It is generally believed to act as a relay between different subcortical areas
and the cerebral cortex. In particular, every sensory 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 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.
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,
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. 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 memory and rodent event memory.
There is support for the hypothesis that thalamic regions connection to particular parts of
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
centre. Through investigations of the anatomy of the brains of primates 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. 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. Development
The thalamic complex is composed of the perithalamus, the mid-diencephalic organiser ) and the thalamus.
The development of the thalamus can be subdivide into three steps 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 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
At the interface between the expression domains of Fez and Otx, the mid-diencephalic organizer
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. 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 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 part of the thalamus, and Ascl1
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.
In humans, a common genetic variation in the promotor region of the serotonin transporter
has been shown to affect the development of several regions of the thalamus in adults.
People who inherit two short alleles 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. Clinical significance
A cerebrovascular accident 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.
The occlusion of the artery of Percheron can lead to a bilateral thalamus infarction.
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. In contrast, damage to the thalamus can result in coma.
Gallery Images are circa 1858.
See also List of regions in the human brain
List of thalamic nuclei Neothalamus
Primate basal ganglia system Thalamic stimulator
Thalamotomy 5-HT7 receptor
References
External links Stained brain slice images which include the
"thalamus" at the BrainMaps project