Basal ganglia

The basal ganglia (or basal nuclei) are a group of subcortical nuclei, of varied origin, in the brains of vertebrates, including humans, which are situated at the base of the forebrain and top of the midbrain. There are some differences in the basal ganglia of primates. Basal ganglia are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia are associated with a variety of functions, including control of voluntary motor movements, procedural learning, habit learning, eye movements, cognition,[1] and emotion.[2]

The main components of the basal ganglia – as defined functionally – are the striatum; both dorsal striatum (caudate nucleus and putamen) and ventral striatum (nucleus accumbens and olfactory tubercle), globus pallidus, ventral pallidum, substantia nigra, and subthalamic nucleus.[3] Each of these components has a complex internal anatomical and neurochemical organization. The largest component, the striatum (dorsal and ventral), receives input from many brain areas beyond the basal ganglia, but only sends output to other components of the basal ganglia. The pallidum receives input from the striatum, and sends inhibitory output to a number of motor-related areas. The substantia nigra is the source of the striatal input of the neurotransmitter dopamine, which plays an important role in basal ganglia function. The subthalamic nucleus receives input mainly from the striatum and cerebral cortex, and projects to the globus pallidus.

Popular theories implicate the basal ganglia primarily in action selection – in helping to decide which of several possible behaviors to execute at any given time. In more specific terms, the basal ganglia's primary function is likely to control and regulate activities of the motor and premotor cortical areas so that voluntary movements can be performed smoothly.[1][4] Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.[2][5]

The basal ganglia are of major importance for normal brain function and behaviour. Their dysfunction results in a wide range of neurological conditions including disorders of behaviour control and movement. Those of behaviour include Tourette syndrome, obsessive–compulsive disorder, and addiction. Movement disorders include, most notably Parkinson's disease, which involves degeneration of the dopamine-producing cells in the substantia nigra, Huntington's disease, which primarily involves damage to the striatum,[1][3]dystonia, and more rarely hemiballismus. The basal ganglia have a limbic sector whose components are assigned distinct names: the nucleus accumbens, ventral pallidum, and ventral tegmental area (VTA). There is considerable evidence that this limbic part plays a central role in reward learning, particularly the mesolimbic pathway from the VTA to the nucleus accumbens that uses the neurotransmitter dopamine. A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal. There is also evidence implicating overactivity of the VTA dopaminergic projection in schizophrenia.[6]

Basal ganglia
Basal Ganglia and Related Structures
Basal ganglia labeled at top right.
Constudoverbrain - 2
Basal ganglia on underneath view of brain
Details
Part ofCerebrum
Identifiers
Latinnuclei basales
MeSHD001479
NeuroNames224, 2677
NeuroLex IDbirnlex_826
TAA14.1.09.501
FMA84013
Anatomical terms of neuroanatomy

Structure

In terms of development, the human central nervous system is often classified based on the original three primitive vesicles from which it develops: These primary vesicles form in the normal development of the neural tube of the embryo and initially include the prosencephalon, mesencephalon, and rhombencephalon, in rostral to caudal (from head to tail) orientation. Later in development of the nervous system each section itself turns into smaller components. During development, the cells that migrate tangentially to form the basal ganglia are directed by the lateral and medial ganglionic eminences.[7] The following table demonstrates this developmental classification and traces it to the anatomic structures found in the basal ganglia.[1][3][8] The structures relevant to the basal ganglia are shown in bold.

Primary division of the neural tube Secondary subdivision Final segments in a human adult
Prosencephalon
  1. Telencephalon
  2. Diencephalon
  1. On each side of the brain: the cerebral cortices, caudate, putamen, hypothalamus
  2. Globus pallidus, ventral pallidum, thalamus, subthalamus, epithalamus, subthalamic nucleus
Mesencephalon
  1. Mesencephalon
  1. Mesencephalon (midbrain): substantia nigra pars compacta (SNc), substantia nigra pars reticulata (SNr)
Rhombencephalon
  1. Metencephalon
  2. Myelencephalon
  1. Pons and cerebellum
  2. Medulla
Video of relevant anatomy
Basal-ganglia-coronal-sections-large
Coronal slices of human brain showing the basal ganglia. White matter is shown in dark gray, gray matter is shown in light gray.
Anterior: striatum, globus pallidus (GPe and GPi)
Posterior: subthalamic nucleus (STN), substantia nigra (SN)

The basal ganglia form a fundamental component of the cerebrum. In contrast to the cortical layer that lines the surface of the forebrain, the basal ganglia are a collection of distinct masses of gray matter lying deep in the brain not far from the junction of the thalamus. They lie to the side of and surround the thalamus.[9] Like most parts of the brain, the basal ganglia consist of left and right sides that are virtual mirror images of each other.

In terms of anatomy, the basal ganglia are divided into four distinct structures, depending on how superior or rostral they are (in other words depending on how close to the top of the head they are): Two of them, the striatum and the pallidum, are relatively large; the other two, the substantia nigra and the subthalamic nucleus, are smaller. In the illustration to the right, two coronal sections of the human brain show the location of the basal ganglia components. Of note, and not seen in this section, the subthalamic nucleus and substantia nigra lie farther back (posteriorly) in the brain than the striatum and pallidum.

Striatum

Anatomy of the basal ganglia
Basal ganglia

The striatum is a subcortical structure generally divided into the dorsal striatum and ventral striatum, although a medial lateral classification has been suggested to be more relevant behaviorally[10] and is being more widely used.[11]

The striatum is composed mostly of medium spiny neurons. These GABAergic neurons project to the external (lateral) globus pallidus and internal (medial) globus pallidus as well as the substantia nigra pars reticulata. The projections into the globus pallidus and substantia nigra are primarily dopaminergic, although enkephalin, dynorphin and substance P are expressed. The striatum also contains interneurons that are classified into nitrergic neurons (due to use of nitric oxide as a neurotransmitter), tonically active cholinergic interneurons, parvalbumin-expressing neurons and calretinin-expressing neurons.[12] The dorsal striatum receives significant glutamatergic inputs from the cortex, as well as dopaminergic inputs from the substantia nigra pars compacta. The dorsal striatum is generally considered to be involved in sensorimotor activities. The ventral striatum receives glutamatergic inputs from the limbic areas as well as dopaminergic inputs from the VTA, via the mesolimbic pathway. The ventral striatum is believed to play a role in reward and other limbic functions.[13] The dorsal striatum is divided into the caudate and putamen by the internal capsule while the ventral striatum is composed of the nucleus accumbens and olfactory tubercle.[14][15] The caudate has three primary regions of connectivity, with the head of the caudate demonstrating connectivity to the prefrontal cortex, cingulate cortex and amygdala. The body and tail show differentiation between the dorsolateral rim and ventral caudate, projecting to the sensorimotor and limbic regions of the striatum respectively.[16] Striatopallidal fibres connect the striatum to the pallidus.

Pallidum

The pallidum consists of a large structure called the globus pallidus ("pale globe") together with a smaller ventral extension called the ventral pallidum. The globus pallidus appears as a single neural mass, but can be divided into two functionally distinct parts, called the internal (or medial) and external (lateral) segments, abbreviated GPi and GPe.[1] Both segments contain primarily GABAergic neurons, which therefore have inhibitory effects on their targets. The two segments participate in distinct neural circuits. The GPe receives input mainly from the striatum, and projects to the subthalamic nucleus. The GPi receives signals from the striatum via the "direct" and "indirect" pathways. Pallidal neurons operate using a disinhibition principle. These neurons fire at steady high rates in the absence of input, and signals from the striatum cause them to pause or reduce their rate of firing. Because pallidal neurons themselves have inhibitory effects on their targets, the net effect of striatal input to the pallidum is a reduction of the tonic inhibition exerted by pallidal cells on their targets (disinhibition) with an increased rate of firing in the targets.

Substantia nigra

Blausen 0076 BasalGanglia
Location of the substantia nigra within the basal ganglia

The substantia nigra is a midbrain gray matter portion of the basal ganglia that has two parts – the pars compacta (SNc) and the pars reticulata (SNr). SNr often works in unison with GPi, and the SNr-GPi complex inhibits the thalamus. Substantia nigra pars compacta (SNc) however, produces the neurotransmitter dopamine, which is very significant in maintaining balance in the striatal pathway. The circuit portion below explains the role and circuit connections of each of the components of the basal ganglia.

Subthalamic nucleus

The subthalamic nucleus is a diencephalic gray matter portion of the basal ganglia, and the only portion of the ganglia that produces an excitatory neurotransmitter, glutamate. The role of the subthalamic nucleus is to stimulate the SNr-GPi complex and it is part of the indirect pathway. The subthalamic nucleus receives inhibitory input from the external part of the globus pallidus and sends excitatory input to the GPi.

Circuit connections

Basal-ganglia-classic
Connectivity diagram showing excitatory glutamatergic pathways as red, inhibitory GABAergic pathways as blue, and modulatory dopaminergic pathways as magenta. (Abbreviations: GPe: globus pallidus external; GPi: globus pallidus internal; STN: subthalamic nucleus; SNc: substantia nigra pars compacta; SNr: substantia nigra pars reticulata)
Motor loop
Connectivity of the basal ganglia as revealed by diffusion spectrum imaging based on thirty subjects from the Human Connectome Project. Direct, indirect and hyperdirect pathways are visualized in different colors (see legend). Subcortical structures are rendered based on the Harvard-Oxford subcortical thalamus as well as the Basal Ganglia atlas (other structures). Rendering was generated using TrackVis software.
The conditional routing model
The left side of Fig.1 shows a region of the prefrontal cortex receiving multiple inputs from other regions, as cortico-cortical activity. The input from B is the strongest of these. The right side of Fig. 1 shows the input signals also being fed to the basal ganglia circuitry. The output from here, back to the same region, is shown to modify the strength of the input from B, by adding strength to the input from C thereby modifying the strongest signal from B to C. (Thalamic involvement is implicit but not shown).

Multiple models of basal ganglia circuits and function have been proposed, however there have been questions raised about the strict divisions of the direct and indirect pathways, their possible overlap and regulation.[17] The circuitry model has evolved since the first proposed model in the 1990s by DeLong in the parallel processing model, in which the cortex and substantia nigra pars compacta project into the dorsal striatum giving rise to an inhibitory indirect and excitatory direct pathway.

  • The inhibitory indirect pathway involved the inhibition of the globus pallidus externus, allowing for the disinhibition of the globus pallidus internus (through STN) allowing it to inhibit the thalamus.
  • The direct or excitatory pathway involved the disinhibition of the thalamus through the inhibition of the GPi/SNr. However the speed of the direct pathway would not be concordant with the indirect pathway in this model leading to problems with it. To get over this, a hyperdirect pathway where the cortex sends glutamatergic projections through the subthalamic nucleus exciting the inhibitory GPe under the center surround model, as well as a shorter indirect pathway have been proposed.

Generally, the basal ganglia circuitry is divided into five pathways: one limbic, two associative (prefrontal), one oculomotor, and one motor pathway. (The motor and oculomotor pathways are sometimes grouped into one motor pathway.) The five general pathways are organized as follows:[18]

  • The motor loop involving projections from the supplementary motor area, arcuate premotor area, motor cortex and somatosensory cortex into the putamen, which projects into the ventrolateral GPi and caudolateral SNr which projects into the cortex through the ventralis lateralis pars medialis and ventralis lateralis pars orialis.
  • The oculomotor loop involved projections from the frontal eye fields, the dorsolateral prefrontal cortex (DLPFC), and the posterior parietal cortex into the caudate, into the caudal dorsomedial GPi and ventrolateral SNr, finally looping back into the cortex through the lateral ventralis anterior pars magnocellularis(VAmc).
  • The first cognitive/associative pathway proposes a pathway from the DLPFC, into the dorsolateral caudate, followed by a projection into the lateral dorsomedial GPi, and rostral SNr before projecting into the lateral VAmc and medial pars magnocellularis.
  • The second cognitive/associative pathway proposed is a circuit projecting from the lateral orbitofrontal cortex, the temporal gyrus, and anterior cingulate cortex into the ventromedial caudate, followed by a projection into the lateromedial GPi, and rostrolateral SNr before looping into the cortex via the medial VAmc and medial magnocellularis.
  • The limbic circuit involving the projections from the ACC, hippocampus, entorhinal cortex, and insula into the ventral striatum, then into the rostrodorsal GPi, ventral palladium and rostrodorsal SNr, followed by a loop back into the cortex through the posteromedial part of the medial dorsal nucleus.[19] However, more subdivisions of loops have been proposed, up to 20,000.[20]

The direct pathway, originating in the dorsal striatum inhibits the GPi and SNr, resulting in a net disinhibition or excitation of the thalamus. This pathway consist of medium spiny neurons (MSNs) that express dopamine receptor D1, muscarinic acetylcholine receptor M4, and adenosine receptor A1.[21] The direct pathway has been proposed to facilitate motor actions, timing of motor actions, gating of working memory, and motor responses to specific stimuli.[20]

The (long) indirect pathway originates in the dorsal striatum and inhibits the GPe, resulting in disinhibition of the GPi which is then free to inhibit the thalamus. This pathway consists of MSNs that express dopamine receptor D2, muscarinic acetylcholine receptor M1, and adenosine receptor A2a.[21] This pathway has been proposed to result in global motor inhibition(inhibition of all motor activity), and termination of responses. Another shorter indirect pathway has been proposed, which involves cortical excitation of the subthalamic nucleus resulting in direct excitation of the GPe, and inhibition of the thalamus. This pathway is proposed to result in inhibition of specific motor programs based on associative learning.[20]

A combination of these indirect pathways resulting in a hyperdirect pathway that results in inhibition of basal ganglia inputs besides one specific focus has been proposed as part of the center surround theory.[22][23] This hyperdirect pathway is proposed to inhibit premature responses, or globally inhibit the basal ganglia to allow for more specific top down control by the cortex.[20]

The interactions of these pathways are currently under debate. Some say that all pathways directly antagonize each other in a "push pull" fashion, while others support the center surround theory, in which one focused input into the cortex is protected by inhibition of competing inputs by the rest of the indirect pathways.[20]

Basal ganglia circuits
Diagram shows two coronal slices that have been superimposed to include the involved basal ganglia structures. Green arrows (+) refer to excitatory glutamatergic pathways, red arrows (–) refer to inhibitory GABAergic pathways and turquoise arrows refer to dopaminergic pathways that are excitatory on the direct pathway and inhibitory on the indirect pathway.

Neurotransmitters

The basal ganglia contains many afferent glutamatergic inputs, with predominantly GABAergic efferent fibers, modulatory cholinergic pathways, significant dopamine in the pathways originating in the ventral tegmental area and substantia nigra, as well as various neuropeptides. Neuropeptides found in the basal ganglia include substance P, neurokinin A, cholecystokinin, neurotensin, neurokinin B, neuropeptide Y, somatostatin, dynorphin, enkephaline. Other neuromodulators found in the basal ganglia include nitric oxide, carbon monoxide, and phenylethylamine.[24]

Functional connectivity

The functional connectivity, measured by regional co-activation during functional neuroimaging studies, is broadly consistent with the parallel processing models of basal ganglia function. The putamen was generally coactivated with motor areas such as the supplementary motor area, caudal anterior cingulate cortex and primary motor cortex, while the caudate and rostral putamen were more frequently coactivated with the rostral ACC and DLPFC. The ventral striatum was significantly associated with the amygdala and hippocampus, which although was not included in the first formulations of basal ganglia models, has been an addition to more recent models.[25]

Function

Eye movements

One intensively studied function of the basal ganglia is its role in controlling eye movements.[26] Eye movement is influenced by an extensive network of brain regions that converges on a midbrain area called the superior colliculus (SC). The SC is a layered structure whose layers form two-dimensional retinotopic maps of visual space. A "bump" of neural activity in the deep layers of the SC drives an eye movement directed toward the corresponding point in space.

The SC receives a strong inhibitory projection from the basal ganglia, originating in the substantia nigra pars reticulata (SNr).[26] Neurons in the SNr usually fire continuously at high rates, but at the onset of an eye movement they "pause", thereby releasing the SC from inhibition. Eye movements of all types are associated with "pausing" in the SNr; however, individual SNr neurons may be more strongly associated with some types of movements than others. Neurons in some parts of the caudate nucleus also show activity related to eye movements. Since the great majority of caudate cells fire at very low rates, this activity almost always shows up as an increase in firing rate. Thus, eye movements begin with activation in the caudate nucleus, which inhibits the SNr via the direct GABAergic projections, which in turn disinhibits the SC.

Role in motivation

Extracellular dopamine in the basal ganglia has been linked to motivational states in rodents, with high levels being linked to satiated "euphoria", medium levels with seeking, and low with aversion. The limbic basal ganglia circuits are influenced heavily by extracellular dopamine. Increased dopamine results in inhibition of the Ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata, resulting in disinhibition of the thalamus. This model of direct D1, and indirect D2 pathways explain why selective agonists of each receptor are not rewarding, as activity at both pathways is required for disinhibition. The disinhibition of the thalamus leads to activation of the prefrontal cortex and ventral striatum, selective for increased D1 activity leading to reward.[19] There is also evidence from non-human primate and human electrophysiology studies that other basal ganglia structures including the globus pallidus internus and subthalamic nucleus are involved in reward processing.[27][28]

Decision making

Two models have been proposed for the basal ganglia, one being that actions are generated by a "critic" in the ventral striatum and estimates value, and the actions are carried out by an "actor" in the dorsal striatum. Another model proposes the basal ganglia acts as a selection mechanism, where actions are generated in the cortex and are selected based on context by the basal ganglia.[29] The CBGTC loop is also involved in reward discounting, with firing increasing with an unexpected or greater than expected reward.[30] One review supported the idea that the cortex was involved in learning actions regardless of their outcome, while the basal ganglia was involved in selecting appropriate actions based on associative reward based trial and error learning.[31]

Working memory

The basal ganglia has been proposed to gate what enters and what doesn't enter working memory. One hypothesis proposes that the direct pathway (Go, or excitatory) allows information into the PFC, where it stays independent of the pathway, however another theory proposes that in order for information to stay in the PFC the direct pathway needs to continue reverberating. The short indirect pathway has been proposed to, in a direct push pull antagonism with the direct pathway, close the gate to the PFC. Together these mechanisms regulate working memory focus.[20]

Clinical significance

Basal ganglia disease is a group of movement disorders that result from either excessive output from the basal ganglia to the thalamus – hypokinetic disorders, or from insufficient output – hyperkinetic disorders. Hypokinetic disorders arise from an excessive output from the basal ganglia, which inhibits the output from the thalamus to the cortex, and thus limits voluntary movement. Hyperkinetic disorders result from a low output from the basal ganglia to the thalamus which gives not enough inhibition to the thalamic projections to the cortex and thus gives uncontrolled/involuntary movements. Dysfunction of the basal ganglia circuitry can also lead to other disorders.[32]

The following is a list of disorders that have been linked to the basal ganglia:

History

The acceptance that the basal ganglia system constitutes one major cerebral system took time to arise. The first anatomical identification of distinct subcortical structures was published by Thomas Willis in 1664.[37] For many years, the term corpus striatum[38] was used to describe a large group of subcortical elements, some of which were later discovered to be functionally unrelated.[39] For many years, the putamen and the caudate nucleus were not associated with each other. Instead, the putamen was associated with the pallidum in what was called the nucleus lenticularis or nucleus lentiformis.

A thorough reconsideration by Cécile and Oskar Vogt (1941) simplified the description of the basal ganglia by proposing the term striatum to describe the group of structures consisting of the caudate nucleus, the putamen, and the mass linking them ventrally, the nucleus accumbens. The striatum was named on the basis of the striated (striped) appearance created by radiating dense bundles of striato-pallido-nigral axons, described by anatomist Samuel Alexander Kinnier Wilson (1912) as "pencil-like".

The anatomical link of the striatum with its primary targets, the pallidum and the substantia nigra, was discovered later. The name globus pallidus was attributed by Déjerine to Burdach (1822). For this, the Vogts proposed the simpler "pallidum". The term "locus niger" was introduced by Félix Vicq-d'Azyr as tache noire in (1786), though that structure has since become known as the substantia nigra, due to contributions by Von Sömmering in 1788. The structural similarity between the substantia nigra and globus pallidus was noted by Mirto in 1896. Together, the two are known as the pallidonigral ensemble, which represents the core of the basal ganglia. Altogether, the main structures of the basal ganglia are linked to each other by the striato-pallido-nigral bundle, which passes through the pallidum, crosses the internal capsule as the "comb bundle of Edinger", and finally reaches the substantia nigra.

Additional structures that later became associated with the basal ganglia are the "body of Luys" (1865) (nucleus of Luys on the figure) or subthalamic nucleus, whose lesion was known to produce movement disorders. More recently, other areas such as the centromedian nucleus and the pedunculopontine complex have been thought to be regulators of the basal ganglia.

Near the beginning of the 20th century, the basal ganglia system was first associated with motor functions, as lesions of these areas would often result in disordered movement in humans (chorea, athetosis, Parkinson's disease).

Terminology

The nomenclature of the basal ganglia system and its components has always been problematic. Early anatomists, seeing the macroscopic anatomical structure but knowing nothing of the cellular architecture or neurochemistry, grouped together components that are now believed to have distinct functions (such as the internal and external segments of the globus pallidus), and gave distinct names to components that are now thought to be functionally parts of a single structure (such as the caudate nucleus and putamen).

The term "basal" comes from the fact that most of its elements are located in the basal part of the forebrain. The term ganglia is a misnomer: In modern usage, neural clusters are called "ganglia" only in the peripheral nervous system; in the central nervous system they are called "nuclei". For this reason, the basal ganglia are also occasionally known as the "basal nuclei".[40] Terminologia anatomica (1998), the international authority for anatomical naming, retained "nuclei basales", but this is not commonly used.

The International Basal Ganglia Society (IBAGS)[41] informally considers the basal ganglia to be made up of the striatum, the pallidum (with two nuclei), the substantia nigra (with its two distinct parts), and the subthalamic nucleus, whereas Terminologia anatomica excludes the last two. Some neurologists have included the centromedian nucleus of the thalamus as part of the basal ganglia,[42][43] and some have also included the pedunculopontine nucleus.[44]

Other animals

The basal ganglia form one of the basic components of the forebrain, and can be recognized in all species of vertebrates.[45] Even in the lamprey (generally considered one of the most primitive of vertebrates), striatal, pallidal, and nigral elements can be identified on the basis of anatomy and histochemistry.[46]

The names given to the various nuclei of the basal ganglia are different in different species. In cats and rodents the internal globus pallidus is known as the entopeduncular nucleus.[47] In birds the striatum is called the paleostriatum augmentatum and the external globus pallidus is called the paleostriatum primitivum.

A clear emergent issue in comparative anatomy of the basal ganglia is the development of this system through phylogeny as a convergent cortically re-entrant loop in conjunction with the development and expansion of the cortical mantle. There is controversy, however, regarding the extent to which convergent selective processing occurs versus segregated parallel processing within re-entrant closed loops of the basal ganglia. Regardless, the transformation of the basal ganglia into a cortically re-entrant system in mammalian evolution occurs through a re-direction of pallidal (or "paleostriatum primitivum") output from midbrain targets such as the superior colliculus, as occurs in sauropsid brain, to specific regions of the ventral thalamus and from there back to specified regions of the cerebral cortex that form a subset of those cortical regions projecting into the striatum. The abrupt rostral re-direction of the pathway from the internal segment of the globus pallidus into the ventral thalamus—via the path of the ansa lenticularis—could be viewed as a footprint of this evolutionary transformation of basal ganglia outflow and targeted influence.

See also

References

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External links

Ansa lenticularis

The ansa lenticularis (ansa lentiformis in older texts) is a part of the brain, making up the superior layer of the substantia innominata. Its fibers, derived from the medullary lamina of the lentiform nucleus, pass medially to end in the thalamus and subthalamic region, while others are said to end in the tegmentum and red nucleus.

It is classified by NeuroNames as part of the subthalamus.

Basal ganglia disease

Basal ganglia disease is a group of physical problems that occur when the group of nuclei in the brain known as the basal ganglia fail to properly suppress unwanted movements or to properly prime upper motor neuron circuits to initiate motor function. Research indicates that increased output of the basal ganglia inhibits thalamocortical projection neurons. Proper activation or deactivation of these neurons is an integral component for proper movement. If something causes too much basal ganglia output, then the Ventral Anterior (VA) and Ventral Lateral (VL) thalamocortical projection neurons become too inhibited and one cannot initiate voluntary movement. These disorders are known as hypokinetic disorders. However, a disorder leading to abnormally low output of the basal ganglia leads to relatively no inhibition, and thus excitation, of the thalamocortical projection neurons (VA and VL) which synapse onto the cortex. This situation leads to an inability to suppress unwanted movements. These disorders are known as hyperkinetic disorders. Currently, reasons for abnormal increases or decreases of basal ganglia output are poorly understood. One possible factor could be the natural accumulation of iron in the basal ganglia, causing neurodegeneration due to its involvement in toxic, free-radical reactions. Though motor disorders are the most common associated with the basal ganglia, recent research shows that basal ganglia disorders can lead to other dysfunctions such as obsessive compulsive disorder (OCD) and Tourette syndrome.

Cerebrum

The cerebrum is a large part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In the human brain, the cerebrum is the uppermost region of the central nervous system. The prosencephalon is the embryonic structure from which the cerebrum develops prenatally. In mammals, the dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into approximately symmetric left and right cerebral hemispheres.

With the assistance of the cerebellum, the cerebrum controls all voluntary actions in the body.

Direct pathway

The direct pathway, sometimes known as the direct pathway of movement, is a neural pathway within the central nervous system (CNS) through the basal ganglia which facilitates the initiation and execution of voluntary movement. It works in conjunction with the indirect pathway. Both of these pathways are part of the cortico-basal ganglia-thalamo-cortical loop.

External globus pallidus

The external globus pallidus (GPe) (or lateral globus pallidus) combines with the internal globus pallidus (GPi) to form the globus pallidus, an anatomical subset of the basal ganglia. Globus pallidus means "pale globe" in Latin, indicating its appearance. The external globus pallidus is the segment of the globus pallidus that is relatively further (lateral) from the midline of the brain.

The GPe GABAergic neurons, allow for its inhibitory function and projects axons to the subthalamic nucleus (in the diencephalon), the striatum, internal globus pallidus (GPi) and substantia nigra pars reticulata.The GPe is particular in comparison to the other elements of the set by the fact that it does not work as an output base of the basal ganglia (not sending axons to the thalamus) but as the main regulator of the basal ganglia system. It is sometimes used as a target for deep brain stimulation as a treatment for Parkinson's disease.

Fahr's syndrome

Idiopathic basal ganglia calcification, also known as Fahr disease, is a rare, genetically dominant, inherited neurological disorder characterized by abnormal deposits of calcium in areas of the brain that control movement. Through the use of CT scans, calcifications are seen primarily in the basal ganglia and in other areas such as the cerebral cortex.

Ganglion

A ganglion is a nerve cell cluster or a group of nerve cell bodies located in the autonomic nervous system and sensory system. Ganglia house the cell bodies of afferent nerves (input nerve fibers) and efferent nerves (output/motor nerve fibers), or axons.

A pseudoganglion looks like a ganglion, but only has nerve fibers and has no nerve cell bodies.

Globus pallidus

The globus pallidus (Latin for "pale globe") also known as paleostriatum or dorsal pallidum, is a subcortical structure of the brain. It consists of two adjacent segments, one external, known in rodents simply as the globus pallidus, and one internal, known in rodents as the entopeduncular nucleus. It is part of the telencephalon, but retains close functional ties with the subthalamus in the diencephalon – both of which are part of the extrapyramidal motor system. The globus pallidus is a major component of the basal ganglia, with principal inputs from the striatum, and principal direct outputs to the thalamus and the substantia nigra. The latter is made up of similar neuronal elements, has similar afferents from the striatum, similar projections to the thalamus, and has a similar synaptology. Neither receives direct cortical afferents, and both receive substantial additional inputs from the intralaminar thalamus.

Hemiballismus

Ballismus or ballism (called hemiballismus or hemiballism in its unilateral form) is a very rare movement disorder. It is a type of chorea caused in most cases by a decrease in activity of the subthalamic nucleus of the basal ganglia, resulting in the appearance of flailing, ballistic, undesired movements of the limbs. It can also appear rarely due to certain metabolic abnormalities. It is a rare movement disorder, being 500 times rarer than Parkinson's disease. Hemiballismus can cause significant disability. Symptoms can decrease during sleep.

Indirect pathway

The indirect pathway sometimes known as the indirect pathway of movement is a neuronal circuit through the basal ganglia and several associated nuclei within the central nervous system (CNS) which helps to prevent unwanted muscle contractions from competing with voluntary movements. It operates in conjunction with the direct pathway.

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 globus pallidus. The internal capsule contains both ascending and descending axons, going to and coming from the cerebral cortex.

The corticospinal tract constitutes a large part of the internal capsule, carrying motor information from the primary motor cortex to the lower motor neurons in the spinal cord.

Above the basal ganglia the corticospinal tract is a part of the corona radiata, below the basal ganglia the tract is called cerebral crus (a part of the cerebral peduncle) and below the pons it is referred to as the corticospinal tract.

Internal globus pallidus

The internal globus pallidus (GPi) (or medial globus pallidus) and the external globus pallidus (GPe) make up the globus pallidus. The GPi is one of the output nuclei of the basal ganglia (the other being the substantia nigra pars reticulata). The GABAergic neurons send their axons to the ventral anterior nucleus (VA) and the ventral lateral nucleus (VL) in the dorsal thalamus, to the centromedian complex, and to the pedunculopontine complex.The efferent bundle is constituted first of the ansa and lenticular fasciculus, then crosses the internal capsule as the Edinger's comb system then arrives at the laterosuperior corner of the subthalamic nucleus and constitutes the field H2 of Forel, then H, and suddenly changes its direction to form field H1 that goes to the inferior part of the thalamus. The distribution of axonal islands is widespread in the lateral region of the thalamus. The innervation of the central region is done by collaterals.The internal globus pallidus contains GABAergic neurons, which allow for its inhibitory function. As the GPi, along with the substantia nigra pars reticulata, forms the output of the basal ganglia, these neurons extend to the thalamus, the centromedian complex and the pedunculopontine complex.

Nigrostriatal pathway

The nigrostriatal pathway or the nigrostriatal bundle (NSB), is a dopaminergic pathway that connects the substantia nigra pars compacta (SNc) with the dorsal striatum (i.e., the caudate nucleus and putamen). It is one of the four major dopamine pathways in the brain, and is particularly involved in the production of movement, as part of a system called the basal ganglia motor loop. Dopaminergic neurons of this pathway synapse onto GABAergic neurons.Loss of dopamine neurons in the SNc is one of the main pathological features of Parkinson's disease, leading to a marked reduction in dopamine function in this pathway. Depletion of neurons in this pathway lead to the symptomatic motor deficits of Parkinson's disease including tremors, rigidity, and postural imbalance.

Pallidothalamic tracts

The pallidothalamic tracts (or pallidothalamic connections) are a part of the basal ganglia. They provide connectivity between the internal globus pallidus (GPi) and the thalamus, primarily the ventral anterior nucleus and the ventral lateral nucleus.

Putamen

The putamen ( meaning: nutshell) is a round structure located at the base of the forebrain (telencephalon). The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that compose the basal ganglia. Through various pathways, the putamen is connected to the substantia nigra, the globus pallidus, the claustrum, and the thalamus, in addition to many regions of the cerebral cortex. A primary function of the putamen is to regulate movements at various stages (e.g. preparation and execution) and influence various types of learning. It employs GABA, acetylcholine, and enkephalin to perform its functions. The putamen also plays a role in degenerative neurological disorders, such as Parkinson's disease.

Striatum

The striatum, or corpus striatum (also called the neostriatum and the striate nucleus) is a nucleus (a cluster of neurons) in the subcortical basal ganglia of the forebrain. The striatum is a critical component of the motor and reward systems; receives glutamatergic and dopaminergic inputs from different sources; and serves as the primary input to the rest of the basal ganglia.

Functionally, the striatum coordinates multiple aspects of cognition, including both motor and action planning, decision-making, motivation, reinforcement, and reward perception. The striatum is made up of the caudate nucleus and the lentiform nucleus. The lentiform nucleus is made up of the larger putamen, and the smaller globus pallidus.In primates, the striatum is divided into a ventral striatum, and a dorsal striatum, subdivisions that are based upon function and connections. The ventral striatum consists of the nucleus accumbens and the olfactory tubercle. The dorsal striatum consists of the caudate nucleus and the putamen. A white matter, nerve tract (the internal capsule) in the dorsal striatum separates the caudate nucleus and the putamen. Anatomically, the term striatum describes its striped (striated) appearance of grey-and-white matter.

Substantia nigra

The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. Substantia nigra is Latin for "black substance", reflecting the fact that parts of the substantia nigra appear darker than neighboring areas due to high levels of neuromelanin in dopaminergic neurons. It was discovered in 1784 by Félix Vicq-d'Azyr, and Samuel Thomas von Sömmerring alluded to this structure in 1791. Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta.Although the substantia nigra appears as a continuous band in brain sections, anatomical studies have found that it actually consists of two parts with very different connections and functions: the pars compacta (SNpc) and the pars reticulata (SNpr). This classification was first proposed by Sano in 1910. The pars compacta serves mainly as an output to the basal ganglia circuit, supplying the striatum with dopamine. The pars reticulata, though, serves mainly as an input, conveying signals from the basal ganglia to numerous other brain structures.

Subthalamic fasciculus

The subthalamic fasciculus is a tract which connects the subthalamic nucleus (STN) and the globus pallidus (GP).

It is a bidirectional connection between the GP and the subthalamic nucleus, including (1) fibers running from the GPe to the subthalamic nucleus, and (2) fibers running from the STN back to the GP.

Subthalamic nucleus

The subthalamic nucleus is a small lens-shaped nucleus in the brain where it is, from a functional point of view, part of the basal ganglia system. In terms of anatomy, it is the major part of the subthalamus. As suggested by its name, the subthalamic nucleus is located ventral to the thalamus. It is also dorsal to the substantia nigra and medial to the internal capsule. It was first described by Jules Bernard Luys in 1865, and the term corpus Luysi or Luys' body is still sometimes used.

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