Beyond Barrels: Diverse Thalamocortical Projection Motifs in the Mouse Ventral Posterior Complex.

Thalamocortical pathways fromthe rodent ventral posterior (VP) thalamic complex to the somatosensory cerebral cortex areas are a key model in modern neuroscience. However, beyond the intensively studied projection from medial VP (VPM) to the primary somatosensory area (S1), the wiring of these pathways remains poorly characterized. We combined micropopulation tract-tracing and single-cell transfection experiments to map the pathways arising from different portions of the VP complex inmalemice. We found that pathways originating fromdifferent VP regions show differences in area/lamina arborization pattern and axonal varicosity size. Neurons fromthe rostral VPM subnucleus innervate trigeminal S1 in point-to-point fashion. In contrast, a caudal VPMsubnucleus innervates heavily and topographically second somatosensory area (S2), but not S1. Neurons in a third, intermediate VPM subnucleus innervate through branched axons both S1 and S2, with markedly different laminar patterns in each area. A small anterodorsal subnucleus selectively innervates dysgranular S1. The parvicellularVPMsubnucleus selectively targets the insular cortex and adjacent portions of S1 and S2. Neurons in the rostral part of the lateral VP nucleus (VPL) innervate spinal S1, while caudal VPL neurons simultaneously target S1 and S2. Rostral and caudal VP nuclei show complementary patterns of calcium-binding protein expression. In addition to the cortex, neurons in caudal VP subnuclei target the sensorimotor striatum. Our finding of a massive projection fromVP to S2 separate fromthe VP projections to S1 adds critical anatomical evidence to the notion that different somatosensory submodalities are processed in parallel in S1 and S2. [ABSTRACT FROM AUTHOR]