In
neuroanatomy
Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defin ...
, thalamocortical radiations are the
fibers
Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
between the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
and 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 of ...
.
Structure
Thalamocortical (TC) fibers have been referred to as one of the two constituents of the
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 ...
, the other being micro neurons. Thalamocortical fibers have a bush or tree-like appearance as they extend into the
internal capsule
The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the ...
and project to the layers of the cortex. The main thalamocortical fibers extend from different nuclei of the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
and project to the visual cortex, somatosensory (and associated sensori-motor) cortex, and the auditory cortex in the brain. Thalamocortical radiations also innervate gustatory and olfactory pathways, as well as pre-frontal motor areas. Visual input from the optic tract is processed by the
lateral geniculate nucleus
In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projec ...
of the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
, auditory input in the
medial geniculate nucleus
The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei that ...
, and somatosensory input in the
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, spinothalamic tracts, and trigeminothalamic tract. It p ...
of the thalamus. Thalamic nuclei project to cortical areas of distinct architectural organization and relay the processed information back to the area of original activity in the thalamus via corticothalamic (CT) fibers.
The
thalamic reticular nucleus
The thalamic reticular nucleus is part of the ventral thalamus that forms a capsule around the thalamus laterally. However, recent evidence from mice and fish question this statement and define it as a dorsal thalamic structure. It is separated fro ...
(TRN) receives incoming signals via corticothalamic pathways and regulates activity within the thalamus accordingly.
Cortico-thalamic feedback neurons are mostly found in layer VI of the cortex.
Reciprocal CT projections to the thalamus are of a higher order than, and synapse with, the TRN in much greater number than do thalamocortical projections to cortex.
This suggests that the cortex has a much bigger role in top down processing and regulation of thalamic activity than do the processes originating in thalamic interneurons. Large-scale frequency
oscillations
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
and electrical rhythms have also been shown to regulate TC activity for long periods of time, as is evident during the
sleep cycle
The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sle ...
.
Other evidence suggests CT modulation of TC rhythms can occur over different time scales, adding even more complexity to their function.
Relay cells
Thalamic interneurons process sensory information and signal different regions of the thalamic nuclei. These nuclei extend to relay cells, which in turn innervate distinct areas of the cortex via thalamocortical fibers. Either specifically or nonspecifically, TC relay cells project specifically to organized areas of the cortex directly and nonspecifically project to large areas of cortex through the innervation of many interconnected collateral axons.
According to Jones (2001) there are two primary types of relay neurons in the thalamus of primates–core cells and matrix cells–each creating distinct pathways to various parts and layers throughout 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 of ...
.
Matrix cells of the thalamus, or calbindin-immuno-reactive neurons (CIR neurons), are widely distributed and diffusely dispersed in each of the nuclei of the dorsal thalamus. In comparison, parvalbumin immuno-reactive neurons (PIR neurons) can be found only in principal sensory and motor relay nuclei, and in the
pulvinar nuclei
The pulvinar nuclei or nuclei of the pulvinar (nuclei pulvinares) are the nuclei ( cell bodies of neurons) located in the thalamus (a part of the vertebrate brain). As a group they make up the collection called the pulvinar of the thalamus (pulvin ...
as well as the
intralaminar nuclei. The PIR neurons cluster together creating "densely terminating afferent fibers…forming a core imposed on a diffuse background matrix of PIR cells" (Jones 2001). PIR cells tend to project upon the cerebral cortex and terminate in an organized topographic manner in specifically localized zones (in deep layer III and in the middle layer IV). In contrast, CIR cells have dispersed projections wherein various adjacent cells connect to non-specific different cortical areas. CIR axons seem to terminate primarily in the superficial layers of the cortex: layers I, II, and upper III.
Function
Thalamocortical signaling is primarily excitatory, causing the activation of corresponding areas of the cortex, but is mainly regulated by inhibitory mechanisms. The specific excitatory signaling is based upon
glutamate
Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...
rgic signaling, and is dependent on the nature of the sensory information being processed. Recurrent oscillations in
thalamocortical circuits also provide large-scale regulatory feedback inputs to the thalamus via
GABAergic neurons that synapse in the TRN.
In a study done by Gibbs, Zhang, Shumate, and Coulter (1998) it was found that endogenously released zinc blocked GABA responses within the TC system specifically by interrupting communication between the thalamus and the connected TRN.
Computational neuroscientists are particularly interested in thalamocortical circuits because they represent a structure that is disproportionally larger and more complex in humans than other mammals (when body size is taken into account), which may contribute to humans' special cognitive abilities.
Evidence from one study (Arcelli et al. 1996) offers partial support to this claim by suggesting that thalamic GABAergic local circuit neurons in mammalian brains relate more to processing ability compared to sensorimotor ability, as they reflect an increasing complexity of local information processing in the thalamus.
It is proposed that core relay cells and matrix cells projecting from the dorsal thalamus allow for synchronization of cortical and thalamic cells during "
high-frequency oscillations
High-frequency oscillations (HFO) are brain waves of the frequency faster than ~80 Hz, generated by neuronal cell population. High-frequency oscillations can be recorded during an electroencephalagram (EEG), local field potential (LFP) or electr ...
that underlie discrete conscious events",
though this is a heavily debated area of research.
Projections
The majority of thalamocortical fibers project to layer IV of the cortex, wherein sensory information is directed to other layers where they either terminate or connect with axons collaterally depending on type of projection and type of initial activation.
Activation of the thalamocortical neurons relies heavily on the direct and indirect effects of Glutamate, which causes EPSP's at terminal branches in the primary sensory cortices.
Somatosensory areas
Primarily, thalamocortical somatosensory radiation from the
VPL,
VPM and
LP nuclei extends to the primary and secondary somatosensory areas, terminating in cortical layers of the lateral
postcentral gyrus
In neuroanatomy, the postcentral gyrus is a prominent gyrus in the lateral parietal lobe of the human brain. It is the location of the primary somatosensory cortex, the main sensory receptive area for the somatosensory system, sense of touch. Lik ...
. S1 receives parallel thalamocortical radiations from the posterior medial nucleus and the VPN.
Projections from the VPN to the postcentral gyrus account for the transfer of sensory information concerning touch and pain. Several studies indicate that parallel innervations to S1 and also S2 via thalamocortical pathways result in the processing of
nociceptive
Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an org ...
and non-nociceptive information. Non-specific projections to sensori-motor areas of the cortex may in part have to do with the relationship between non-noci-receptive processing and motor functions.
Past research shows a link between S1 and M1, creating a thalamocortical sensori-motor circuit. When this circuit becomes disrupted symptoms are produced similar to those that accompany
Multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
, suggesting thalamocortical rhythms are involved in regulating sensori-motor pathways in a highly specialized manner.
TC-CT rhythms evident during sleep act to inhibit these thalamocortical fibers so as to maintain the tonic cycling of low frequency waves and the subsequent suppression of motor activity.
Visual areas
The Lateral geniculate and pulvinar nuclei project to and terminate in V1, and carry motor information from the brain stem as well as other sensory input from the optic tract. The visual cortex connects with other sensory areas which allows for the integration of cognitive tasks such as selective and directed attention, and pre-motor planning, in relation to the processing of incoming visual stimuli.
Models of the pulvinar projections to the visual cortex have been proposed by several imaging studies, though the mapping of pulvinar projections has been a difficult task due to the fact that pulvinar subdivisions are not conventionally organized and have been difficult to visualize using structural MRI.
Evidence from several studies supports the idea that the Pulvinar nuclei and
superior colliculus
In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form ''tectal'' is commonly ...
receive descending projections from CT fibers while TC fibers extending from the LGN carry visual information to the various areas of the visual cortex near the
calcarine fissure
The calcarine sulcus (or calcarine fissure) is an anatomical landmark located at the caudal end of the medial surface of the brain of humans and other primates. Its name comes from the Latin "calcar" meaning "spur". It is very deep, and known as ...
.
Auditory areas
Thalamocortical axons project primarily from the
medial geniculate nucleus
The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei that ...
via the sublenticular region of the
internal capsule
The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the ...
, and terminate in an organized topographic manner in the
transverse temporal gyri
The transverse temporal gyri, also called Heschl's gyri () or Heschl's convolutions, are gyri found in the area of primary auditory cortex buried within the lateral sulcus of the human brain, occupying Brodmann areas 41 and 42. Transverse tempor ...
.
MMGN radiations terminate in specific locations while thalamocortical fibers from the
VMGN terminate in nonspecific clusters of cells and form collateral connections to neighboring cells.
Research done by staining the brains of macaque monkeys reveals projections from the ventral nucleus mainly terminating in layers IV and IIIB, with some nonspecific clusters of PIR cells terminating in layers I, II, IIIA, and VI. Fibers from the dorsal nuclei were found to project more directly to the primary auditory area, with most axons terminating in layer IIIB. The magnocellular nucleus projected a small amount of PIR cells with axons mainly terminating in layer 1, though large regions of the middle cortical layers were innervated through collaterally connected CIR neurons.
Past research suggests that the thalamocortical-auditory pathway may be the only neural correlate that can explain a direct translation of frequency information to the cortex via specific pathways.
Motor areas
The primary motor cortex receives terminating thalamocortical fibers from the VL nucleus of the thalamus. This is the primary pathway involved in the transference of cerebellar input to the primary motor cortex. The VA projects widely across the inferior parietal and premotor cortex. Other Non-specific thalamocortical projections, those that originate in the dorsal-medial nuclei of the thalamus, terminate in the prefrontal cortex and have subsequent projections to associative premotor areas via collateral connections.
The Cortico-basal ganglia-thalamocortical loop has been traditionally associated with reward-learning and though has also been noted by some researchers to have a modulatory effect on thalamocortical network functioning–this is due to inherent activation of the premotor areas connecting the VA nucleus with the cortex.
Clinical significance
Absence seizures
Thalamocortical radiations have been researched extensively in the past due to their relationship with attention, wakefulness, and arousal. Past research has shown how an increase in
spike-and-wave
Spike-and-wave is a pattern of the electroencephalogram (EEG) typically observed during epileptic seizures. A spike-and-wave discharge is a regular, symmetrical, generalized EEG pattern seen particularly during absence epilepsy, also known as ‘p ...
activity within the TC network can disrupt normal rhythms involved with the sleep-wakefulness cycle, ultimately causing
absence seizures
Absence seizures are one of several kinds of generalized seizures. These seizures are sometimes referred to as petit mal seizures (from the French for "little illness", a term dated in the late 18th century). Absence seizures are characterized b ...
and other forms of epileptic behavior. Burst firing within a part of the TC network stimulates
GABA receptors
The GABA receptors are a class of receptors that respond to the neurotransmitter gamma-aminobutyric acid (GABA), the chief inhibitory compound in the mature vertebrate central nervous system. There are two classes of GABA receptors: GABAA and ...
within the thalamus causing moments of increased inhibition, leading to frequency spikes, which offset oscillation patterns.
Another study done on rats suggests during spike-and-wave seizures, thalamic rhythms are mediated by local thalamic connections, while the cortex controls the synchronization of these rhythms over extended periods of time.
Thalamocortical dysrhythmia
Thalamocortical dysrhythmia (TCD) is a theoretical framework in which neuroscientists try to explain the positive and negative symptoms induced by neuropsychiatric disorders like Parkinson's Disease, neurogenic pain, tinnitus, visual snow syndrom ...
is a term associated with spontaneously reoccurring low frequency spike-and-wave activity in the thalamus, which causes symptoms normally associated with impulse control disorders such as
obsessive compulsive disorder
Obsession may refer to:
Psychology
* Celebrity worship syndrome, obsessive addictive disorder to a celebrity's personal and professional life
* Fixation (psychology), a persistent attachment to an object or idea
* Idée fixe (psychology), a preo ...
,
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
,
attention deficit hyperactivity disorder
Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inap ...
, and other forms of chronic psychosis.
Other evidence has shown how reductions in the distribution of connections of nonspecific thalamocortical systems is heavily associated with loss of consciousness, as can be seen with individuals in a
vegetative state
A persistent vegetative state (PVS) or post-coma unresponsiveness (PCU) is a disorder of consciousness in which patients with severe brain damage are in a state of partial arousal rather than true awareness. After four weeks in a vegetative stat ...
, or
coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
.
Prefrontal lobotomy
The bilateral interruption or severing of the connection between thalamocortical radiations the medial and anterior thalamic nuclei results in a prefrontal
lobotomy
A lobotomy, or leucotomy, is a form of neurosurgical treatment for psychiatric disorder or neurological disorder (e.g. epilepsy) that involves severing connections in the brain's prefrontal cortex. The surgery causes most of the connections t ...
, which causes a drastic personality change and a subdued behavioral disposition without cortical injury.
Research
Evolutionary theories of consciousness
Theories of consciousness have been linked to thalamocortical rhythm oscillations in TC-CT pathway activity. One such theory, the dynamic core theory of conscious experience, proposes four main pillars in support of conscious awareness as a consequence of dorsal thalamic activity:
# the results of cortical computations underlay consciousness
# vegetative states and general anesthetics work primarily to disrupt normal thalamic functioning
# the anatomy and physiology of the thalamus implies consciousness
#
neural synchronization
Neural oscillations, or brainwaves, are rhythmic or repetitive patterns of neural activity in the central nervous system. Neural tissue can generate oscillation, oscillatory activity in many ways, driven either by mechanisms within individual ne ...
accounts for the neural basis of consciousness.
This area of research is still developing, and most current theories are either partial or incomplete.
References
{{Neural tracts
Thalamus
Cerebral cortex