The basal ganglia in extrapyramidal dysfunction.

The rapid advances in knowledge of basal ganglia circuitry and function in recent years have allowed the construction of a functional scheme to explain many facets of known pathologic states. The dichotomy of Parkinson's disease; akinesia with increased tone, and the mirror effects in Huntington's disease; hemiballismus and tardive dyskinesia, hyperkinesia with decreased tone are explained as due to two outputs of the system with an intervening inhibitory neuron which reverses the sign. The two outputs control different motor functions; pallidothalamic involved primarily with movements and nigrobrainstem involved primarily with muscle tone.