Anatomy and Functional Models of the Basal Ganglia

It has long been observed that diseases of the basal ganglia result in disorders of movement, both hypo- and hyperkinetic. An understanding of the anatomy, physiology, and neurochemistry of the component structures of the basal ganglia has greatly facilitated our understanding of control of movement in both health and disease. The direct-indirect pathway model developed in the late 1980s described two parallel channels of neuronal information converging to influence the output nuclei of the basal ganglia. According to this model, alterations in levels of excitation or inhibition produce changes in the absolute level of neuronal activity in the target structures and can be used to explain a number of observations from both laboratory and clinical settings. This model greatly aided our understanding of basal ganglia function and has contributed to the development of new therapies for movement disorders. However, there are a number of findings which are not satisfactorily explained, and a higher level of complexity of basal ganglia function is clearly implicated. Additional anatomically defined projections may serve as internal autoregulatory loops connecting external pathways. These are taken into account in a recently developed model and may help elucidate some of the contradictory evidence from both laboratory experiments and clinical results of neurosurgical interventions. KEYWORD Basal ganglia - models - movement disorders