Basal ganglia

The basal ganglia are a group of nuclei in the brain associated with motor and learning functions. However, there is no single definitive function that can be assigned to the mammalian basal ganglia.

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 movement functions, as lesions of these areas would often result in disorders of motor function in humans. In 1925, Kinnear Wilson described them as the "dark basement of the brain."

Anatomical subdivisions
The five individual nuclei that make up the primate basal ganglia are the striatum, external segment of the globus pallidus (GPe), internal segment of the globus pallidus (GPi), subthalamic nucleus (STN), and substantia nigra. 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 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 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 neuroanatomists refer 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 pharmacology also appear relatively conserved. Non-tetrapod vertebrates 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.

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 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).

Connections
Classically, these nuclei were 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 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

The direct pathway is thought to disinhibit the thalamus - cortical activity exciting areas of the striatum 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. The indirect pathway, in contrast, is thought to have an inhibitory effect on the thalamus.

There is evidence in primate that cells in the striatum that participate in the different pathways also differ in the type of dopamine receptor they express. Striatal cells that are involved 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 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 physiological evidence for these effects.

Disorders linked with the basal ganglia

 * Huntington's disease
 * Parkinson's disease
 * Tourette's syndrome
 * Obsessive-compulsive disorder
 * Attention-deficit hyperactivity disorder (ADHD)
 * Athymhormic syndrome (PAP syndrome)
 * Cerebral palsy: basal ganglia damage during second and third trimester of pregnancy
 * Tardive dyskinesia, caused by chronic antipsychotic treatment