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2.3 The Subcortical Structures Extrapyramidal System (anatomy, functioning and disorders)
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2.3 The Subcortical Structures Extrapyramidal System (anatomy, functioning and disorders)

 

 

Figure 2.4. Basal ganglia of the hemispheres (J.D. Fix, 1995)

The extrapyramidal system consists of the following gray structures: caudate nucleus, putamen, pallidum, subthalamic nucleus, substantia nigra and red nucleus.

The basal ganglia of the hemispheres

Except of the grey cortex on the surface of the hemisphere, masses of grey matter are present in the depth of its tissue. These are called basal, central, or subcortical nuclei. They include three conglomerates of subcortical nuclei: corpus striatum, claustrum, and the amygdaloid nucleus (Fig. 2.4).

1. Corpus striatum consists of two parts, the caudate and lentiform nuclei, which are incompletely separated one from the other. The caudate nucleus (nucleus caudatus) is located above and medial to the lentiform nucleus and is separated from it by a layer of white matter called the internal capsule (capsula interna). The thickened anterior part of the nucleus, its head (caput nuclei caudati) forms the lateral wall of the anterior horn of the lateral ventricle, the body forms the inferior wall of the central part of the lateral ventricle; the cauda forms the superior wall of the inferior horn. The caudate nucleus is separated from thalamus by the stria semicircularis (stria terminalis). Anteriad and deeper head of the nucleus approaches the anterior perforated substance where it is united with the lentiform nucleus (with the part called the putamen). The lentiform nucleus (nucleus lentiformis) is located laterally to the caudate nucleus and the thalamus and is separated from them by the internal capsule. On a frontal section the lentiform nucleus is wedge-shaped, the apex of the wedge is directed medially, and the base laterally. Two parallel white layers called the medullary laminae (laminae medullares) divide the lentiform nucleus into three segments, one lateral grey segment called the putamen and two medial lighter coloured segments united under the term globus pallidus.

The globus pallidus has a distinctive macroscopic appearance and also differs from the other parts of the corpus striatum histologically. It is phylogenetically older (palaeostriatum) than the putamen of caudate nucleus (neostriatum).

2. The claustrum is a thin sheet of grey matter in the region of the insula between it and the putamen. It is separated from the putamen by a thin layer of white matter, the external capsule (capsula externa) and from the cortex of the insula by a similar layer called the capsula extrema.

3. The amygdaloid nucleus (corpus amygdaloideum), or the epistriatum is located under the putamen in the anterior end of the temporal lobe. It does not reach the temporal pole, but lies in front of the apex of the inferior horn of the lateral ventricle. Morphologically, the amygdaloid nucleus is a posteroventral continuation of the claustrum.

As the formation of the cerebral cortex expands, the phylogenetically older motor centers (paleostriatum and neostriatum) become increasingly controlled by the new motor system, the system of the pyramidal tracts.

Major afferent and efferent connections of the striatal system. The striatum receives major input from three sources: the thalamus, the neocortex, and the substantia nigra. The striatum projects to the globus pallidus and the substantia nigra. The globus pallidus is the effector nucleus of the striatal system; it projects to the thalamus and to the subthalamic nucleus. The substantia nigra also projects to the thalamus. The striatal motor system expresses itself via the corticobulbar and corticospinal tracts. Major neurotransmitters of the striatal motor system. Within the striatum, globus pallidus, and pars reticularis of the substantia nigra, ?-aminobutyric acid (GABA) is the predominant neurotransmitter. GABA may coexist in the same neuron with enkephalin or substance P. Dopamine-containing neurons are found in the pars compacta of the substantia nigra. Acetylcholine is found in local circuit neurons of the striatum. The subthalamic nucleus projects excitatory glutaminergic fibers to the globus pallidus.



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