The basal ganglia (or basal nuclei) are a group of nuclei of varied origin (mostly telencephalic embryonal origin, with some diencephalic and mesencephalic elements) in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and strongly connected with the cerebral cortex, thalamus and other brain areas. The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or "habits," eye movements, and cognitive, emotional functions. Currently popular theories implicate the basal ganglia primarily in action selection, that is, the decision of which of several possible behaviors to execute at a given time. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which is widely believed to play a key role in executive functions.
The main components of the basal ganglia are the striatum (also called neostriatum) composed of caudate and putamen, globus pallidus or pallidum composed of globus pallidus externa (GPe) or globus pallidus interna (GPi), substantia nigra composed of both substantia nigra pars compacta (SNc) & substantial nigra pars reticulata (SNr), and the subthalamic nucleus (STN). The largest component, the striatum, receives input from many brain areas but sends output only to other components of the basal ganglia. The pallidum receives its most important input from the striatum (either directly or indirectly), and sends inhibitory output to a number of motor-related areas, including the part of the thalamus that projects to the motor-related areas of the cortex. One part of substantia nigra, the reticulata (SNr), functions similarly to the pallidum, and another part (compacta or SNc) provides the source of the neurotransmitter dopamine's input to the striatum. The subthalamic nucleus (STN) receives input mainly from the striatum and cortex, and projects to the a portion of the pallidum (interna portion or GPi). Each of these areas has a complex internal anatomical and neurochemical organization.
The basal ganglia play a central role in a number of neurological conditions, including several movement disorders. The most notable are, Parkinson's disease, which involves degeneration of the melanin-pigmented dopamine-producing cells in the substantia nigra par compacta (SNc), and Huntington's disease, which primarily involves damage to the striatum. Basal ganglia dysfunction is also implicated in some other disorders of behavior control such as the Tourette's syndrome, ballismus (particularly hemibalismus), obsessive–compulsive disorder (OCD), and Wilson's disease (Hepatolenticular degeneration); except for Wilson's disease and hemiballismus, the neuropathological mechanisms underlying diseases of ganglia such as Parkinsons' and Huntington's are not very well understood or are at best still developing theories.
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