Globus pallidus

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Coronal slices of human brain showing the basal ganglia, globus pallidus: external segment (GPe), subthalamic nucleus (STN), globus pallidus: internal segment (GPi), and substantia nigra (SN).

The globus pallidus (Latin for "pale body") or pallidum (Terminologia anatomica since it is not even globoid) is a sub-cortical structure in the brain. It is a major element of the basal ganglia system. It is a major element of the basal ganglia core made of the striatum and its direct targets: pallidum and nigra. In primates, the globus pallidus is divided into two parts by the medial medullary lamina. These are termed the internal and external segments of the globus pallidus (medial and lateral pallidum), and they are two of the major nuclei of the basal ganglia. Both pallidal nuclei are made up of the same neuronal components. Almost all neurons in primates are very large, parvalbumin positive, with very large dendritic arborizations. These have the particularity of having the three dimensional shape of flat discs, parallel to one other, parallel to the border of the pallidum and perpendicular to the afferent striatopallidal axons (Percheron et al. 1984).There are only a few small local circuitry neurons. The globus pallidus is traversed by numerous myelinated nerve fibres that give it the pale appearance for which it is named. The two pallidal nuclei and the two nigral (lateralis and reticulata) constitute a high frequency pacemaker (Surmeier et al. 2005, see primate basal ganglia system)

External segment

The external segment of the globus pallidus, or lateral pallidum, consists mostly of neurons that send axons to the subthalamic nucleus, substantia nigra pars reticulata, and the internal segment of the globus pallidus, where they release the neurotransmitter GABA. It receives GABAergic inputs from the striatum and also a reciprocal glutamatergic projection from the subthalamic nucleus. It is sometimes used as a target for deep brain stimulation as a treatment for Parkinson's disease.

Internal segment

The internal segment of the globus pallidum, or medial pallidum, is one of the output nuclei of the basal ganglia (the other being the substantia nigra pars reticulata). The GABA-containing neurons send their axons to a specific thalamic nucleus, to the centremedian complex and to the pedunculopontine complex.

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