The basal ganglia is traditionally composed by three groups of brain structures called the "striatum," "pallidum," and "substantia nigra," and an additional structure termed the "subthalamic nucleus" (STN). The striatum can be separated into two general components, the dorsal striatum, which consists of the caudate and putamen, and the ventral striatum, which consists of the nucleus accumbens, septum, and olfactory tubercle. In a similar manner, the pallidum can also be divided into multiple divisions, including a lateral or external segment of the globus pallidus (GPe), a medial or internal segment of the globus pallidus (GPi), and a portion that lies ventral and anterior to the anterior commissure, designated the ventral pallidum (VP). Finally, the substantia nigra is also composed of more than one component, a cell group rich in neuromelanin called the pars compacta (SNpc), which is responsible for the black appearance of the nucleus in gross specimens, and an unpigmented celI group known as the pars reticulata (SNpr). According to the simplest classification scheme, the nuclei within the basal ganglia can be subdivided into sets of "input," "output," and"intermediate" structures. Input structures include the caudate, putamen, and nucleus accumbens. Collectively, these input structures receive direct projections from nearly the entire cerebral cortex. These input structures then project to intermediate structures as well as output structures. The three principal output structures of the basal ganglia include the GPi, SNpr, and VP. For the most part, these output structures send their efferent projections to different subdivisions of the ventroanteriorventrolateral (VAIVL), mediodorsal (M,D), and intralaminar (IL) groups ofthalamic nuclei (particularly the centrum medianum and parafasicularis intralaminar nuclei (CM/PF). The VA/VL and MD nuclei of the thalamus, in turn, project largely back upon the cerebral cortex. Thus, one of the major features of basal ganglia anatomy is their participation in what has become known as cortical- basal gangliathalamo-cortical circuits (hereafter referred to as simply cortical-basal ganglia circuits). Many of these same cortical areas also contain cells that project directly to the STN and, thus, constitute a subthalamic path. We present the results of different studies on the topic, and discuss the effect in PD. © 2013 Nova Science Publishers, Inc. All rights reserved.
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