Origin of thalamic inputs to the primary, premotor, and supplementary motor cortical areas and to area 46 in macaque monkeys: a multiple retrograde tracing study.

The origin of thalamic inputs to distinct motor cortical areas was established in five monkeys to determine whether the motor areas receive inputs from a common thalamic nucleus and the extent to which the territories of origin overlap. To not rely on the rough definition of cytoarchitectonic boundaries in the thalamus, monkeys were subjected to multiple injections of tracers (four to seven) in the primary (M1), premotor (PM), and supplementary (SMA) motor cortical areas and in area 46. The cortical areas were distributed into five groups, each receiving inputs from a specific set of thalamic nuclei: 1) M1; 2) SMA-proper and the caudal part of the dorsal PM (PMdc); 3) the rostral and caudal parts of the ventral PM (PMvr and PMvc); 4) the rostral part of the dorsal PM (PMdr); and 5) the superior and inferior parts of area 46 (area 46sup and area 46inf). A major degree of overlap was obtained for the origins of the thalamocortical projections directed to areas 46inf and 46sup and for those terminating in SMA-proper and PMdc. PMvc and PMvr received inputs from adjacent and/or common thalamic regions. In contrast, the degree of overlap between M1 and SMA was smaller. The projection to M1 shared relatively limited zones of origin with the projections directed to PM. Thalamic inputs to the motor cortical areas (M1, SMA, PMd, and PMv), in general, were segregated from those directed to area 46, except in the mediodorsal nucleus, in which there was clear overlap of the territories sending projections to area 46, SMA-proper, and PMdc.