Basal ganglia

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Template:Infobox Brain The basal ganglia are a group of nuclei in the brain associated with motor and learning functions. However, no single function can be definitively assigned to the mammalian basal ganglia.

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History

The first anatomical identification of distinct subcortical structures was published by the English anatomist Thomas Willis in 1664. At that time, it was referred to as the corpus striatum (comprising only the globus pallidus segments and striatum). At the beginning of the 20th century, it was associated with motor functions, as lesions of these areas would often result in disordered movement in humans. In 1925, Kinnear Wilson described them as the "dark basement of the brain."

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Anatomical subdivisions

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

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”.

There are two 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).

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Evolution 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.

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Neurotransmitters

Neurons of the various basal ganglia nuclei use a variety of neurotransmitters. The most widely used is the inhibitory transmitter GABA (connections using GABA are shown in blue in the diagram below). Of particular interest is the neurotransmitter of the pigmented substantia nigra pars compacta neurons, called dopamine. Disruption in the production or transmission of dopamine can lead to serious motor and cognitive deficits (for example, see 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), and it is thought to play an important role in learning (see LTP/LTD).

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Connections

Image:Basal-ganglia-classic.png These nuclei are thought to be connected as shown (left). 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.

In primates, there is evidence that cells in the striatum that participate in the different pathways also differ in the type of dopamine receptor they express. Striatal cells in the direct pathway (i.e., that have axons terminating in the GPi and/or SNr) express the D1 class of dopamine receptor, and cells in the indirect pathway express the D2 class. It is generally thought that the nigral (substantia nigra) dopamine acts on these different receptor types in different ways. Dopamine may inhibit striatal cells that have D2 receptors and excite striatal cells that have D1 recepors, although there is little physiological evidence for these effects.

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Function

The exact function of the basal ganglia is unknown. Current theories suggest that they are involved in mediating between rival perceptions and/or rival motor actions.

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Disorders linked with the basal ganglia

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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 0471803480
  • 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
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External links

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