Globus pallidus

The globus pallidus (Latin for "pale globe") or pallidum (pallid), is a sub-cortical structure of the brain. It is a major element of the basal ganglia system. In this system, it is a major element of the basal ganglia core; made up of the striatum and its direct targets: pallidum and nigra. The last two are made up of the same neuronal elements, have a similar main afferent, the striatum, a similar synaptology and do not receive cortical afferents.

History
The origin of the name is not established. It was known by Dejerine (1906) but not by Ramon y Cajal (1909-1911). As the elements in no way have the shape of a globe, Foix and Nicolesco (1925), the Vogts (1941), Crosby et all.(1962) followed by the Terminologia anatomica proposed the simpler term (neutral adjective) of pallidum (pale). During a long period the pallidum was unduly linked to the putamen in the nucleus lenticularis or lentiformis. This was a heterogeneous anatomical entity that should no longer be considered a part of the pallidum. The link with the substantia reticulata was stressed very early on due to the similarities in dendritic arborisation, but in spite of solid arguments this link is still not widely accepted. The two however constitute a particular set of the basal ganglia system (the pallidonigral set).

Parts
In primates, the pallidum is divided into two parts (and not segments, which are geometrically different things) by the medial medullary lamina.A frequent nomenclature use the adjective internus and externus, which in many languages may be interpreted as meaning inside and outside. The anatomically usual medialis and lateralis are preferable. The medial pallidum(GPi) and lateral pallidum (GPe) are thus the two parts of the pallidum that are two closed nuclei surrounded everywhere by myelinic walls. In primates an accessory lamina in the medial pallidum separates the last into a lateral (PaML) and a medial (PaMM) subparts, which do not correspond to known functional subdivisions.

The later isolation of a ventral subdivision by Heimer (1978) has not the same strength and value, with no clear border. Its isolation is not accepted by all authors.

Structure
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 peculiarity of having the three dimensional shape of flat discs, parallel one to the other, parallel to the border of the pallidum and perpendicular to the afferent striatopallidal axons. There are only a few small local circuitry neurons.

The globus pallidus is traversed by the numerous myelinated axons of the striato-pallidonigral bundle that give it the pale appearance from which it is named.

The ultrastructure is very peculiar as the long dendrites are everywhere without discontinuity, covered by synapses.

Pallidonigral pacemaker
The two pallidal nuclei and the two nigral (lateralis and reticulata) parts constitute a high frequency autonomous pacemaker (see primate basal ganglia system)

Common afferences
The two parts receive successively a large quantity of GABAergic axonal terminal arborisations from the striatum through the dense striato-pallidonigral bundle. The synaptology is very peculiar (see primate basal ganglia system). The striatal afference contribute for more than 90% of synapses. The two pallidal nuclei receives dopaminergic axons from the pars compacta of the substantia nigra.

Lateral pallidum (GPe)
The lateral pallidum receives a strong glutamatergic projection from the subthalamic nucleus. The two form a particular system: a coupled pacemaker.

The axons of the lateral pallidum go essentially to the subthalamic nucleus. They go also to other elements of the basal ganglia system, the striatum, the substantia nigra pars reticulata and the medial pallidum, where they release the neurotransmitter GABA. The lateral pallidum is particular in comparison to the other elements of the set by the fact that it does not work as an output base of the basal ganglia (not sending axons to the thalamus) but as the main regulator of the basal ganglia system. It is sometimes used as a target for deep brain stimulation as a treatment for Parkinson's disease.

Medial pallidum (GPi)
The medial pallidum (internal segment of the globus pallidus; GPi) 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 (VO), to the centremedian complex and to the pedunculopontine complex.

The efferent bundle is constituted first of the ansa and fasciculus lenticularis, then crosses the internal capsule as the Edinger's comb system then arrives at the laterosuperior corner of the subthalamic nucleus and constitutes the Forel's field H2, then H and suddendly changes its direction to form H1 that goes to the inferior part of the thalamus. The distribution of axonal islands is widespread in the lateral region of the thalamus. The innervation of the central region is done by collaterals.