Pallidum

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Brain: Pallidum

Globus pallidus labeled at bottom right.
NeuroNames	hier-213
MeSH	A08.186.211.730.885.105.487.397
Dorlands/Elsevier	g_07/12394364

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 afference, the striatum, a similar synaptology and do not receive cortical afferent.

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

1 History
2 Parts
3 Structure
4 Pallidonigral pacemaker
5 Common afferences
6 Other connections and subsystems
- 6.1 Lateral pallidum (GPe)
- 6.2 Medial pallidum (GPi)
7 References
8 External links

[edit] 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 unduely linked to the putamen in the nucleus lenticularis or lentiformis. This was a heterogeneous anatomical entity that should no more be considered. The link with the substantia reticulata was stressed very early on the similar aspect of dendritic arborisations but in spite of solid arguments, this link is still difficily accepted. The two however constitute a particular set of the basal ganglia system (the pallidonigral set).

[edit] 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.

[edit] 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 particularity of having the three dimensional shape of flat discs, parallel one to the other, parallel to the border of the pallidum (Yelnik et al. 1984) 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 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 sysmapses (Fox et al.1974, di Figlia et al. 1982).

[edit] Pallidonigral pacemaker

The two pallidal nuclei and the two nigral (lateralis and reticulata) parts constitute a high frequency autonomous pacemaker (Surmeier et al. 2005, see primate basal ganglia system)

[edit] 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, Fox et al.1974 and di Figlia et al.1982). 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.

[edit] Other connections and subsystems

[edit] 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.

[edit] 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 (Nauta and Mehler (1966) Percheron et al. (1996).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 (Arrechi-Bouchhiouia et al. 1997)

[edit] References

Arrechi-Bouchhioua, P., Yelnik, K., François, C..,Percheron. G., Tandé, D. (1997) Three-dimensional morphology and distribution of pallidal axons projecting to both the lateral region of the thalamus and the central complex in primates. Brain Res. 754: 311-314
di Figlia, M., Pasik, P., Pasik, T. (1982) A Golgi and ultrastructural study of the monkey globus pallidus. J. Comp. Neurol. 212: 53-75
Fox, C.A., Andrade, A.N. Du Qui, I.J., Rafols, J.A. (1974) The primate globus pallidus. A Golgi and electron microscopic study. J. Hirnforsch. 15: 75-93
François, C., Percheron, G., Yelnik, J. and Heyner, S. (1984) A Golgi analysis of the primate globus pallidus. I- Inconstant processes of large neurons, other neuronal types and afferent axons. J. Comp. Neurol. 227:182-199
Nauta, W.J.H. and Mehler, W.R. (1966) Projections of the lentiform nucleus in the monkey. Brain Res. 1: 3-42
Percheron, G.,Yelnik, J. and François. C. (1984) A Golgi analysis of the primate globus pallidus. III-Spatial organization of the striato-pallidal complex. J. Comp. Neurol. 227: 214-227
Percheron, G., François, C., Talbi, B., Yelnik, J., Fenelon, G. (1996) The primate motor thalamus. Brain Res. Rev. 22: 93-181
Yelnik, J., Percheron, G., and François, C. (1984) A Golgi analysis of the primate globus pallidus. II- Quantitative morphology and spatial orientation of dendritic arborisations. J. Comp. Neurol. 227:200-213