Thalamo-cortical connections of areas 3a and M1 in marmoset monkeys

The present investigation is part of a broader effort to examine cortical a reas that contribute to manual dexterity, reaching, and grasping. In this s tudy we examine the thalamic connections of electrophysiologically de fined regions in area 3a and architectonically defined primary motor cortex (M1) . Our studies demonstrate that area 3a receives input from nuclei associate d with the somatosensory system: the superior, inferior, and lateral divisi ons of the ventral posterior complex (VPs, VPi, and VPl, respectively). Sur prisingly, area 3a receives the majority of its input from thalamic nuclei associated with the motor system, posterior division of the ventral lateral nucleus of the thalamus (VL), the mediodorsal nucleus (MD), and intralamin ar nuclei including the central lateral nucleus (CL) and the centre median nucleus (CM). In addition, sparse but consistent projections to area 3a are from the anterior pulvinar (Pla). Projections from the thalamus to the cor tex immediately rostral to area 3a, in the architectonically defined M1, ar e predominantly from VL, VA, CL, and MD. There is a conspicuous absence of inputs from the nuclei associated with processing somatic inputs (VP comple x). Our results indicate that area 3a is much like a motor area, in part be cause of its substantial connections with motor nuclei of the thalamus and motor areas of the neocortex (Huffman et al. [2000] Sec. Neurosci. Abstr. 2 5:1116). The indirect input from the cerebellum and basal ganglia via the v entral lateral nucleus of the thalamus supports its role in proprioception. Furthermore, the presence of input from somatosensory thalamic nuclei sugg ests that it plays an important role in somatosensory and motor integration . (C) 2001 Wiley-Liss, Inc.