The basal ganglia, or basal nuclei, are SUBCORTICAL grey masses lateral to the THALAMUS in the basal parts of CEREBRAL HEMISPHERES. In the strict sense, they include the CORPUS STRIATUM, the CLAUSTRUM and the amygdaloid nuclear complex (see AMYGDALA). However, common usage tends not to group the amygdala with the basal ganglia and instead includes the diencephalic SUBTHALAMIC NUCLEUS and mesencephalic SUBSTANTIA NIGRA within a functional and connectional, rather than a topographical, definition. The corpus striatum is divided traditionally into the CAUDATE NUCLEUS and LENTIFORM NUCLEUS, the latter itself being divided into the medial GLOBUS PALLIDUS (or pallidum/dorsal pallidum) and a more laterally placed PUTAMEN. This topographical delineation is also misleading. The caudate nucleus and putamen are in fact the same structure, being only separated in primate species by the large size of the INTERNAL CAPSULE, which artificially separates them into two separately named ‘nuclei’.
The caudate and putamen are often grouped together as the dorsal STRIATUM, to distinguish them from the ventral striatum which is largely comprised of the NUCLEUS ACCUMBENS and underlying OLFACTORY TUBERCLE.
The basal ganglia are interpolated within what is generally referred to as cortico-striatopallido-thalamo-cortical loop circuitry (see CORTICOSTRIATAL LOOPS). The major AFFERENTS to the basal ganglia are from widespread areas of the CEREBRAL CORTEX and they arrive in the striatum in a topographically ordered pattern. The striatum receives two other major sources of afferents, namely a DOPAMINERGIC input from the midbrain (substantia nigra pars compacta which innervates the dorsal striatum; more medial dopaminergic neurons of the ventral tegmental area which innervate the ventral striatum) and the midline and intralaminar thalamic nuclei (so-called non-specific thalamic nuclei; see THALAMUS). EFFERENTS from the striatum reach various parts of the globus pallidus, which in turn projects to the thalamus (ventral anterior and medial dorsal nuclei) and then back to the FRONTAL and PREFRONTAL CORTEX, hence partially closing a cortico-cortical loop. In fact, discrete areas of the cortex have quite specific connections with sub-regions of the striatum, pallidum, thalamus and frontal lobes and this has been interpreted to indicate the existence of several anatomically segregated cortico-striato-pallido-thalamo-cortical loops with different motor, cognitive and affective functions. Diseases of the basal ganglia are associated with motor disorders and disturbances in cognitive processes. For example PARKINSON’S DISEASE is characterized by degeneration of the midbrain DOPAMINE neurons and hence dopamine depletion of the striatum, giving rise to difficulties in initiating movement (BRADYKINESIA or AKINESIA). Huntington’s disease is an inherited disorder associated with degeneration of striatal neurons and is associated with CHOREA (involuntary movements, especially of the limbs) and also with a progressive DEMENTIA.
BARRY J.EVERITT
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