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Primate basal ganglia system
The basal ganglia form a major brain system in all species of vertebrates, but the basal ganglia of primates (including humans) have special features that justify a separate consideration. As in other vertebrates, the primate basal ganglia can be divided into striatal, pallidal, nigral, and subthalamic components. In primates, however, the two pallidal subdivisions are called the ''external'' and ''internal'' (or sometimes ''lateral'' and ''medial'') segments of the globus pallidus, whereas in other species they are called the ''globus pallidus'' and ''entopeduncular nucleus''. Also in primates, the striatum is divided by a large tract of white matter called the internal capsule into two masses of gray matter that early anatomists named the caudate nucleus and putamen—in most other species no such division exists, and only the striatum as a whole is recognized. Beyond this, the complex topography of connections between the striatum and cortex means that functions are segregated within the primate striatum in ways that do not apply to other species.
A separate consideration of the primate basal ganglia is also warranted by the fact that different types of information are available than for other species. Large areas of the primate brain are devoted to vision; consequently the role of the basal ganglia in controlling eye movements has been studied almost exclusively in primates. Functional imaging studies have been performed mainly using human subjects. Also, several major degenerative diseases of the basal ganglia, including Parkinson's disease and Huntington's disease, are specific to humans, although "models" of them have been proposed for other species.
==Corticostriatal connection==
The whole system starts as a major output of the cerebral cortex, about the same size as the corticopontine system opening the cerebellar system. The corticostriatal connection represents a significant portion of the whole cortical output. Almost every part of the cortex, except for the primary olfactory, visual and auditory cortices, sends axons to the striatum. The origin of the connection is in the pyramidal neurons of layer V of the cortex. Corticostriate contributors, of the motor cortex at least, may be collaterals of axons descending lower in the nervous system. However, in primates, the majority of corticostriate axons are monotarget, purely cortico-striate, thin and unbranched until they arrive in the striatum.〔Parent and Parent (2006)〕 The corticostriatal connection is glutamatergic and excitatory. This connection is not topologically as simple as was initially described by Kemp and Powell (1970), where the frontal lobe projected anteriorly and the occipitotemporal lobes posteriorly. Part of this distribution grossly remains, but the distribution is much more complex. One small cortical site can send terminal arborisations to several and distal striatal places.〔Goldman-Rakic and Nauta (1977)〕〔Selemon and Goldman-Rakic (1985)〕 The cortico-striatal connection is the substrate of cortical information separation and recombination: axons from distinct cortical areas can systematically end together or separately. There is also a spatial reorganisation, a "remapping".〔Flaherty and Graybiel, 1991〕
The corticostriate connection is the first in a chain of strong reduction in numbers between emitter and receiver neurons, i.e. a numerical convergence.〔Percheron ''et al.'' (1987)〕 The effect of this is that if each striatocortical neuron has its own message, this will be mixed or compressed, leading to lesser definition of the input map.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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