Basal Ganglia, also known as basal nuclei, are a group of subcortical nuclei in the brains of vertebrates, involved in various processes like cognitive, motor, and mnemonic functions (J. P. Bolam, 1999). It has been shown that basal ganglia contribute to habit and stimulus-response learning, which, in the case of humans, occur without conscious awareness. The memory and learning functions of basal ganglia are core features of the same in influencing motor as well as cognitive pattern generators (Graybiel, 1198). Basal ganglia likewise play a key role in many neurological conditions, which include movement disorders.
Most notable of these are Parkinson's disease and Huntington's disease. The movement disorders that are linked to basal ganglia dysfunction involve a gamut of abnormalities ranging from hypokinetic disorders (like Parkinson's disease) to hyperkinetic disorders (like Huntington's disease). Hypokinetic disorders have the characteristics of significant impairments in the movement initiation and the reduction in the velocity and amplitude of voluntary movements.
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They are often accompanied by muscular rigidity as well as tremor at rest. Hyperkinetic disorders, on the other hand, are characterized by "excessive motor activity in the form of involuntary movements," and degrees of hypotonia. These extremes in the movement disorder spectrum are attributable to specific disturbances in the basal ganglia-thalamocortical motor circuit (DeLong, 1990). Roger Albin, et al. (1989) postulated that hypokinetic disorders result from a complex series of changes in the workings of striatal projection neuron subpopulations that result in the increase in basal ganglia output. Hyperkinetic disorders are, on the other hand, postulated to have resulted from selective impairment of striatal neurons that project to lateral globus pallidus. This model associating basal ganglia disorders with changes in function of striatal projection neurons is based on "synthesis of experimental animal and post-mortem human anatomic and neurochemical data."