Neural circuits and functional organization of the striatum

The basal ganglia and motor thalamic nuclei are functionally and anatomical ly divided into the sensorimotor, supplementary motor, premotor, associativ e and limbic territories. There exist both primary segregated basal ganglia -thalamocortical loops and convergence of functionally related information from different cortical areas onto these cortical basal ganglia-thalamocort ical loops. The basal ganglia-thalamocortical loop arising from the sensori motor area, supplementary motor area (SMA), premotor area and cingulate mot or area provides distinct segregated subloops through the functionally dist inct striatal, pallidal and thalamic regions with partial overlap. The subt halamic nucleus (STN) is also topographically organized. The ventrolateral part of the caudal 2/3 levels of the medial pallidal segment (GPi) projects to the primary motor area via the oral part of the ventral lateral thalami c nucleus (VLo) (Voa, Vop by Hassler's nomenclature). The thalamic relay nu clei of the GPi projection to SMA are identified in the transitional zone o f the VApc (parvicellular part of the anterior ventral nucleus)-VLo and in the rostromedial part of the VLo. The thalamic nuclei relaying the cingulat e subloop are not yet clearly defined. The supplementary motor subloop appe ars to be divided into the pre-SMA and SMA proper subloops. The premotor ar ea is also divided into the dorsal premotor area subloop and the ventral pr emotor area subloop. It is suggested that the limbic loop consists of a num ber of subloops in the monkey as indicated by Haber et al. [67] and in rats [64]. We review here the microcircuitry of the striatum, as well as the co nvergence and integration between the functionally segregated loops. Finall y, we discuss the functional implications of striatal connections.