Synaptic organisation of the basal ganglia

The basal ganglia are a group of subcortical nuclei involved in a variety o f processes including motor, cognitive and mnemonic functions. One of their major roles is to integrate sensorimotor, associative and limbic informati on in the production of context-dependent behaviours. These roles are exemp lified by the clinical manifestations of neurological disorders of the basa l ganglia. Recent advances in many fields, including pharmacology, anatomy, physiology and pathophysiology have provided converging data that have led to unifying hypotheses concerning the functional organisation of the basal ganglia in health and disease. The major input to the basal ganglia is der ived from the cerebral cortex. Virtually the whole of the cortical mantle p rojects in a topographic manner onto the striatum, this cortical informatio n is 'processed' within the striatum and passed via the so-called direct an d indirect pathways to the output nuclei of the basal ganglia, the internal segment of the globus pallidus and the substantia nigra pars reticulata. T he basal ganglia influence behaviour by the projections of these output nuc lei to the thalamus and thence back to the cortex, or to subcortical 'premo tor' regions. Recent studies have demonstrated that the organisation of the se pathways is more complex than previously suggested. Thus the cortical in put to the basal ganglia, in addition to innervating the spiny projection n eurons, also innervates GABA interneurons, which in turn provide a feed-for ward inhibition of the spiny output neurons. Individual neurons of the glob us pallidus innervate basal ganglia output nuclei as well as the subthalami c nucleus and substantia nigra pars compacta. About one quarter of them als o innervate the striatum and are in a position to control the output of the striatum powerfully as they preferentially contact GABA interneurons. Neur ons of the pallidal complex also provide an anatomical substrate, within th e basal ganglia, for the synaptic integration of functionally diverse infor mation derived from the cortex. It is concluded that the essential concept of the direct and indirect pathways of information flow through the basal g anglia remains intact but that the role of the indirect pathway is more com plex than previously suggested and that neurons of the globus pallidus are in a position to control the activity of virtually the whole of the basal g anglia.