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Basal gangliaThe basal ganglia are a group of nucleiin the brainassociated with motor and learningfunctions. However, there is no single definitive function that can be assigned to the mammalianbasal ganglia.
HistoryThe first anatomicalidentification of distinct subcortical structures was published by the Englishanatomist Thomas Willisin 1664. At that time, it was referred to as the corpus striatum (comprising only the globus pallidussegments and striatum). At the beginning of the 20th century, it was associated with movement functions, as lesions of these areas would often result in disorders of motor function in humans. In 1925, Kinnear Wilsondescribed them as the "dark basement of the brain." Anatomical subdivisionsImage:Basal-ganglia-coronal-sections-large.png The five individual nuclei that make up the primatebasal ganglia are:
Some of these nuclei may be further subdivided: for example, the striatum is separated into the putamen, caudate nucleus, and nucleus accumbens; the substantia nigra is generally divided into the pars compacta (SNc), pars reticulata (SNr), and pars lateralis (SNl). These images show two schematic coronalcross-sections of the humanbrain 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 misnomeras ?ganglion? refers to a somaticcluster within the peripheral nervous systemwhereas the basal ganglia are located within the central nervous system(CNS). A somatic cluster within the CNS is referred to as a nucleus; it is for this reason some neuroanatomistsrefer to the basal ganglia as the ?basal nuclei?. There are two complete 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). Evolution and naming"Basal ganglia"-like areas are observed 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 pharmacologyalso appear relatively conserved. Non-tetrapodvertebrates such as fish also display basal ganglia-like structures, although the data is less clear in this case. The names given to the various nuclei comprising the basal ganglia can vary greatly depending on 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 named the paleostriatum augmentatum and paleostriatum primitivum respectively in birds. NeurotransmittersNeurons 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 connectivity 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 this transmitter 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). ConnectionsImage:Basal-ganglia-classic.pngThese nuclei have been considered to be connected as shown (left). The striatum is the primary (but not exclusive) input zone for other brain areas to connect to the basal ganglia. Via the striatum the basal ganglia receives input from the cortex, with a majority of projections from the motorand prefrontalcortices. The circuitryof the basal ganglia is often divided into two major pathways, the direct pathway and the indirect pathway:
Cortical activity that excites those 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 via inhibition is called disinhibition. In contrast, cortical activity that excites the striatal cells in the indirect pathway is thought to have an inhibitory effect on the thalamus. There is evidence in primates that cells in the striatum that participate in the different pathways also differ in the type of dopamine receptorthey express. Striatal cells that are involved in the direct pathway (i.e., that have axonsterminating in the GPi and/or SNr) express the D1 class of dopamine receptor, and cells involved 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 activity in striatal cells expressing D2 receptors and excite striatal cells expressing D1, although there is very limited physiologicalevidence for these effects. Disorders linked with the basal ganglia
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de:Basalganglien es:Ganglio basal he:?????? ???? nl:Basale ganglia no:Basalgangliene pl:J?dra podstawne Categories: Central nervous system| Cerebrum| Anatomy| Neuroscience
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