Abstract
The basal ganglia are a richly interconnected set of subcortical nuclei that provide a feedback loop to wide areas of the cortex as well as descending influences to brainstem motor regions. The input nucleus of the basal ganglia is the striatum (caudate, putamen, and nucleus accumbens), which receives dense innervation from the cortex and subcortical structures. The primary output nuclei of the basal ganglia in primates are the internal globus pallidus (GPi) and the related substantia nigra pars reticulata (SNr). In non primates, the GPi is referred to as the entopeduncular nucleus (EPN). Recent models of flow of cortical information through the basal ganglia suggest that striatal neurons project to GPi and SNr through two major routes, a direct pathway and an indirect pathway (Albin et al., 1989; Bergman et al., 1990; Delong, 1990; Ciliax et al., 1997). Striatal neurons giving rise to both of these pathways provide inhibitory GABAergic projections. The direct pathway originates from a subpopulation of GABAergic neurons that project directly to GPi/SNr and thereby inhibit activity in these output nuclei. The indirect pathway originates from a different population of GABAergic striatal neurons that project to the external globus pallidus (GPe) in primates. In turn, GPe (referred to as globus pallidus or GP in non primates) sends a GABAergic projection to the subthalamic nucleus (STN), which provides an important excitatory glutamatergic drive to GPi/SNr. Thus, the output nuclei, GPi/SNr, receive a balance of opposing inhibitory and excitatory signals from the direct and indirect pathways respectively.
Published Version
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