Basal ganglia

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Brain: Basal ganglia
Basal ganglia labeled at top right.
Connectivity Diagram showing glutamatergic pathways as red, dopaminergic as magenta and GABA pathways as blue.
Latin	nuclei basales
Gray's	subject #
Part of
Components
Artery
Vein
BrainInfo/UW	hier-206
MeSH	A08.186.211.730.885.105

The basal ganglia are a group of nuclei in the brain interconnected with the cerebral cortex, thalamus and brainstem. Mammalian basal ganglia are associated with a variety of functions: motor control, cognition, emotions and learning.

History

The acceptance that the basal ganglia system constitutes one major cerebral system has been slow to appear. The first anatomical identification of distinct subcortical structures was published by Thomas Willis in 1664. For many years, the term corpus striatum was used to describe a large group of subcortical elements, some of which were later discovered to be functionally unrelated. Additionally, the putamen and the caudate nucleus were not linked together. The putamen was thought to be associated to the pallidum in what used to be called the "nucleus lenticularis" (see lentiform nucleus on the fig.). Pioneering work by Cécile and Oskar Vogt (1941) greatly 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 fundus. The striatum gets its name from the striated appearance created by radiating dense bundles of striato-pallido-nigral axons, described by anatomist Kinnear Wilson as "pencil-like". The anatomical link of the striatum with its primary targets, the pallidum and the substantia nigra was later discovered. Together, these structures constitute, the striato-pallido-nigral bundle, which is the core of the basal ganglia. This nerve bundle forms the so-called "comb bundle of Edinger" when it crosses the internal capsule. 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 central complex (centre médian-parafascicular) and the pedunculopontine complex have been thought to be regulators of the basal ganglia. At the beginning of the 20th century, the basal ganglia system was associated with motor functions, as lesions of these areas would often result in disordered movement in humans (chorea, athetosis, Parkinson's disease).

Anatomical subdivisions

Coronal slices of human brain showing the basal ganglia.
ROSTRAL: striatum, globus pallidus (GPe and GPi)
CAUDAL: subthalamic nucleus (STN), substantia nigra (SN)

Main article: Anatomical subdivisions and connections of the basal ganglia

The five individual nuclei that make up the primate basal ganglia, along with their major subdivisions, are:

• rostral
  • the striatum
    • putamen
    • caudate nucleus
    • nucleus accumbens
  • external segment of the globus pallidus (GPe)
  • internal segment of the globus pallidus (GPi)
• caudal
  • subthalamic nucleus (STN)
  • substantia nigra (SN)
    • pars compacta (SNc)
    • pars reticulata (SNr)
    • pars lateralis (SNl)

There are 2 sets of basal ganglia in the mammalian brain, mirrored in the left and right hemispheres. Two coronal sections are used to show the basal ganglia; the STN and substantia nigra lie deeper back in the brain (more caudal). Images show two schematic coronal cross-sections of the human brain with nuclei of the basal ganglia labeled on the right side.

As it refers to a group of nuclei, the term "basal ganglia" is plural (the singular of ganglia is ganglion). However this is a misnomer, as "ganglion" refers to a somatic cluster within the peripheral nervous system, whereas the basal ganglia are within the central nervous system (CNS). A somatic cluster within the CNS is referred to as a nucleus, so some neuroanatomists refer to the basal ganglia as the "basal nuclei".

Comparative anatomy and naming

"Basal ganglia"-like areas are found in the central nervous systems of many species. The striatal and pallidal components can be clearly identified in all amniotes (mammals, birds, and reptiles) and amphibians. The anatomical connections of these nuclei and their pharmacology also appear relatively conserved. Non-tetrapod vertebrates such as fish also display basal ganglia-like structures, although the data are less clear in this case.

The names given to the various nuclei of the basal ganglia are different in different species. For example, the "internal segment of the globus pallidus" in primates is called the "entopenduncular nucleus" in rodents. The "striatum" and "external segment of the globus pallidus" in primates are called the "paleostriatum augmentatum" and "paleostriatum primitivum" respectively in birds. 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. The evolutionary emergence of cortical re-entrant systems in the brain has been postulated by Gerald Edelman as a critical basis for the emergence of primary consciousness in the theory of Neural Darwinism.

Neuronal Classes in the Striatum

The striatum is a large nucleus located deep in the telencephalon in proximity to the lateral brain ventricles. In primates, it is composed of 4 neuronal classes: spiny (96%), leptodendritic (2%), spidery (1%) neurons and microneurons(1%) (Yelnik et al. 1991). In spiny neurons dendritic arborisations are spherical. Most of the dendritic spines in this cell type receive synaptic input from cortical afferents. Their axons have abundant collateral projections participating in local circuitry. The neurons are GABAergic. The leptodendritic neurons (or Deiter's neurons) have all the morphological properties of the pallidal neurons. The spidery neurons are specific to primates. They have a big soma and short dendritic and axonal branches. They are the cholinergic interneurons and are characterized by the fact that they are tonically active. The microneurons are local circuit neurons similar to those found in the thalamus and are GABAergic. Some may be also be dopaminergic (Rochette et al.).

Neurotransmitters

The different types of neuron of the basal ganglia biosynthesize different neurotransmitters.

Neostriatum

Medium neurons, the principal cells of the neostriatum, are inhibitory; they produce transmitter GABA (connections using GABA are shown in blue in the diagram below).

Substantia nigra

Dopamine is biosynthesized in the dopaminergic neurons, primarily in the substantia nigra pars compacta. Disruption in the biosynthesis or transmission of dopamine can lead to serious motor and cognitive deficits, such as occurs in Parkinson's disease. The substantia nigra pars compacta (SNc) primarily targets the striatum with this neurotransmitter (shown as the magenta connection in the classic connectivity diagram below).

Connections

Basal ganglia connectivity is illustrated in the figure.

The striatum is the main (but not the only) input zone for other brain areas to connect to the basal ganglia. Via the striatum, the basal ganglia receives input from the cortex, mainly from the motor and prefrontal cortices.

The circuitry of the basal ganglia is often divided into two major pathways, the direct pathway and the indirect pathway:

• Direct pathway: striatum ^-→ GPi/SNr ^-→ thalamus ⁺→ cortex
• Indirect pathway: striatum ^-→ GPe ^-→ STN ⁺→ GPi/SNr ^-→ thalamus ⁺→ cortex

Cortical activity that excites cells in the striatum that participate in the direct pathway leads to inhibition of areas of the GPi and SNr, which in turn removes their tonic inhibition from the thalamus. This removal of inhibition by inhibition is called disinhibition. In contrast, cortical activity that excites the striatal cells in the indirect pathway is thought to inhibit the thalamus. This is due to an odd number of the pathways in the indirect pathway being inhibition pathways (blue arrows), as opposed to an even number of inhibition pathways in the direct pathway.

Dopamine from the substantia nigra pars compacta stimulates all of the dopamine receptors : D1, D2, D3, D4, and D5. However, via the G proteins (Gs, Gi, Go) the dopamine receptors D2, D3 and D4 are inhibitory, and the dopamine receptors D1 and D5 are stimulatory. Striatal cells of the direct pathway have D1 receptors, while those of the indirect pathway have D2 receptors. Through this mechanism, the presence of dopamine selects for the direct pathway. And the absence of dopamine selects for the indirect pathway.

Disorders linked with the basal ganglia

• Attention-deficit hyperactivity disorder (ADHD)
• Athymhormic syndrome (PAP syndrome)
• Basal ganglia disease
• Cerebral palsy: basal ganglia damage during second and third trimester of pregnancy
• Fahr's disease
• Foreign accent syndrome (FAS)
• Huntington's disease
• Obsessive-compulsive disorder
• Parkinson's disease
• Progressive supranuclear palsy
• Tourette's disorder
• Tardive dyskinesia, caused by chronic antipsychotic treatment
• Stuttering
• Spasmodic Dysphonia
• Wilson's disease

See also

• Amygdaloid body
• Caudate nucleus
• Extrapyramidal symptoms
• Globus palladius
• Nucleus basalis magnocellularis
• Primate basal ganglia system

References

• Nolte, John, The Human Brain: An Introduction to its Functional Anatomy (Fifth Edition). (St. Louis: Mosby, Inc., 2002), 464-484. ISBN 0-323-01320-1
• Parent, André, Comparative Neurobiology of the Basal Ganglia (Wiley, New York, 1986), ISBN 0-471-80348-0
• Reynolds, J. et, al., "A cellular mechanism of reward-related learning", Nature (2001) volume 413, pages 67-70.
• Andrew Gilies, A brief history of the basal ganglia, retrieved on 27 June 2005
• Alm, Per A., Stuttering and the basal ganglia circuits, Journal of Communication Disorders (2004) volume 37, pages 325-369.

Additional images

Schematic representation of the chief ganglionic categories (I to V).

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External links

• Basal ganglia disease, adult-onset - Geneva Foundation for Medical Education and Research
• Dictionary at eMedicine Basal+ganglia
• Dictionary at eMedicine basal+nuclei