Functional Architecture of the Rodent Substantia Nigra Pars Reticulata: Evidence for Segregated Channels

Abstract

While it is widely accepted that basal ganglia are involved in motor processes, the precise role of this system in motor functions is still unclear. Since the basal ganglia receive afferents from all the major functional sectors of the cortical mantle, it has been suggested that they constitute a suitable system to integrate motivation, cognition and perception for the elaboration of movement. A pertinent question however is to know exactly how the various cortical products are processed by the basal ganglia. Until recent past years, current views on the functional organization of basal ganglia were strongly influenced by their apparent funnel-like structure. Due to the reduction in nuclear volume along the cortico-striato-nigral and striato-pallidal circuits, the basal ganglia were regarded as an integrative device where the multiple cortical processings are compiled for the elaboration of a motor act. However, cumulative evidences in primate for a parallel arrangement of the transbasal ganglia pathways has led to the idea that basal ganglia maintain rather than mingle the functional heterogeneity of the cortical mantle (Alexander et al., 1986; Alexander and Crutcher, 1990). It is in fact well documented, in a variety of mammalian species, that the cortical functional mosaic is orderly mapped onto the striatal nuclear complex. While the sensorimotor cortices innervate predominantly the putamen (Kunzle, 1977; Flaherty and Graybiel, 1991), the associative frontal, parietal and temporal areas project mostly to the caudate (Goldman and Nauta, 1977; Yeterian and Pandya, 1991) and the allocortex to the accumbens (McGeorge and Faull, 1989; Berendse et al., 1992).