Primate basal ganglia

The basal ganglia form a major brain system in all species of vertebrates, but in primates (including humans) there are 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. Also in primates, the dorsal striatum is divided by a large tract called the internal capsule into two masses named the caudate nucleus and the putamen—in most other species no such division exists, and only the striatum as a whole is recognized. Beyond this, there is a complex circuitry of connections between the striatum and cortex that is specific to primates. This complexity reflects the difference in functioning of different cortical areas in the primate brain.

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.

A major output from the cortex, with axons from most of the cortical regions connecting to the striatum, is called the corticostriatal connection. In the primate most of these axons are thin and unbranched. The striatum does not receive axons from the primary olfactory, visual or auditory cortices. The corticostriatal connection is an excitatory glutamatergic pathway. One small cortical site can project many axon branches to several parts of the striatum.

The striatum is the largest structure of the basal ganglia.

Medium spiny neurons (MSN)s, account for up to 95 per cent of the striatal neurons. There are two populations of these projection neurons, MSN1 and MSN2, both of which are inhibitory GABAergic. There are also various groups of GABAergic interneurons and a single group of cholinergic interneurons. These few types are responsible for the reception, processing, and relaying of all the cortical input.

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