From the initial descriptions of its major constituents the basal ganglia emerged in the history of neurology and psychiatry as a term to describe all nuclei located in the base of the brain. Later, the clinical and pathologic findings in patients with Parkinson's, Huntington's, Athetosis, and Wilson's disease were instrumental in establishing the concept of an extrapyramidal system that is centered in the basal ganglia. From this original view, the basal ganglia evolved, and now are related not only to motor execution but also to more complex aspects of behavior: motor planning and sequencing, motor learning, and cognitive and motivational drives. This list of functions continues to grow as recent research in patients with Parkinson's disease has unveiled that the striatum is also important for acquiring nonmotor dispositions and tendencies that depend on new associations (nommotor habit learning.)<sup>57 Knowlton B.J., Mangels J.A., Squire L.R. A neostriatal habit learning system in humans Science 1996 ;  273 : 1399-1402

Cliquez ici pour aller à la section Références</sup> Anatomically, the basal ganglia is a set of intimately connected structures composed by the caudate and the putamen (striatum), the globus pallidus, the substantia nigra, and the subthalamic nucleus. These structures are linked to one another by multiple intrinsic loops, most of which are reciprocal, and to other brain structures (cortex and thalamus) by a parallel circuit.

To understand how a dysfunctional given nucleus or circuit in the basal ganglia might originate the psychiatric and neurologic symptoms present in the above-mentioned diseases, we need to address its basic anatomic features. Topographically organized input from the neocortex project to the medium spiny neurons of the striatum, which project to the globus pallidus and substantia nigra, and these give rise to projections to thalamic nuclei (ventroanterior, ventrolateral, and centromedian), which project back to the neocortex closing the circuit.