Basal ganglia
Basal ganglia | |
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Details | |
Part of | Cerebrum |
Identifiers | |
Latin | nuclei basales |
Acronym(s) | BG |
MeSH | D001479 |
NeuroNames | 224, 2677 |
NeuroLex ID | birnlex_826 |
TA98 | A14.1.09.501 |
TA2 | 5559 |
FMA | 84013 |
Anatomical terms of neuroanatomy |
The basal ganglia (BG) or basal nuclei are a group of
The main functional components of the basal ganglia include the
The basal ganglia are thought to play a key role in action selection, aiding in the choice of behaviors to execute. More specifically, they regulate motor and premotor cortical areas, facilitating smooth voluntary movements. [2][5] 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.[3][6] It has also been hypothesized that the basal ganglia are not only responsible for motor action selection, but also for the selection of more cognitive actions.[7][8][9] Computational models of action selection in the basal ganglia incorporate this.[10]
The basal ganglia are of major importance for normal brain function and behaviour. Their dysfunction results in a wide range of
Structure
In terms of development, the human
Primary division of the neural tube | Secondary subdivision | Final segments in a human adult |
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Prosencephalon
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Mesencephalon
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Rhombencephalon
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The basal ganglia form a fundamental component of the
In terms of anatomy, the basal ganglia are divided into four distinct structures, depending on how
Striatum
The striatum is a subcortical structure generally divided into the
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 (i.e. constantly releasing neurotransmitter unless inhibited) cholinergic interneurons, parvalbumin-expressing neurons and calretinin-expressing neurons.[18] 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.[19] 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.[20][21] 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.[22] 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.[2] 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
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
Circuit connections
Multiple models of basal ganglia circuits and function have been proposed, however there have been questions raised about the strict divisions of the
- 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:[24]
- 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 oralis.
- 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 pallidum and rostrodorsal SNr, followed by a loop back into the cortex through the posteromedial part of the medial dorsal nucleus.[25] However, more subdivisions of loops have been proposed, up to 20,000.[26]
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 consists of
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
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.[28][29] 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.[26]
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.[26]
Neurotransmitters
The basal ganglia contains many afferent
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.[31]
Function
Eye movements
One intensively studied function of the basal ganglia is its role in controlling
The SC receives a strong inhibitory projection from the basal ganglia, originating in the substantia nigra pars reticulata (SNr).[32] 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 state, medium levels with seeking, and low with aversion. The limbic basal ganglia circuits are influenced heavily by extracellular
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.[35] The CBGTC loop is also involved in reward discounting, with firing increasing with an unexpected or greater than expected reward.[36] 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.[37]
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.[26]
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.[38]
The following is a list of disorders, conditions, and symptoms that have been linked to the basal ganglia: [citation needed]
- Addiction[39][40]
- Athetosis[41]
- PAP syndrome)
- Attention-deficit hyperactivity disorder(ADHD)
- Blepharospasm
- Bruxism
- Cerebral palsy: basal ganglia damage during second and third trimester of pregnancy
- Chorea[41]
- Dystonia[41][42]
- Epilepsy[43]
- Fahr's disease[39]
- Foreign accent syndrome (FAS)
- Huntington's disease[39][42]
- Kernicterus
- Lesch–Nyhan syndrome
- Major depressive disorder[39][44]
- Other anxiety disorders[40][46]
- PANDAS (disorder)
- Parkinson's disease[39][42]
- Restless legs syndrome
- Schizophrenia[39][40]
- Spasmodic dysphonia
- Stuttering[47]
- Sydenham's chorea
- Tardive dyskinesia, caused by chronic antipsychotic treatment
- Tourette's syndrome[39]
- Wilson's disease[39][42]
History
The acceptance that the basal ganglia system constitutes one major cerebral system took time to arise. The first
A thorough reconsideration by
The anatomical link of the striatum with its primary targets, the
Additional structures that later became associated with the basal ganglia are the "body of Luys" (1865) (nucleus of Luys on the figure) or
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
The International Basal Ganglia Society (IBAGS)[52] 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,[53][54] and some have also included the pedunculopontine nucleus.[55]
Other animals
The basal ganglia form one of the basic components of the
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.[58] 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
See also
Additional images
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Basal ganglia highlighted in green on coronal T1 MRI images
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Basal ganglia highlighted in green on sagittal T1 MRI images
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Basal ganglia highlighted in green on transversal T1 MRI images
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External links
- Imaging of Basal Ganglia at USUHS
- Houk Jim (2007). "Models of Basal ganglia". .
- The International Basal Ganglia Society
- Basal ganglia – Official journal of LIMPE (Lega Italiana per la Lotta Contro la Malattia di Parkinson, le Sindromi Extrapiramidali e le Demenze, Italy), the German Parkinson Society (DPG, Deutsche Parkinson Gesellschaft), and the Japanese Basal Ganglia Society (JBAGS Japan Basal Ganglia Society)