Whole-brain input architecture of primary and secondary somatosensory cortices in mice.

The primary and secondary somatosensory cortices (S1 and S2) play crucial roles in processing sensory inputs from various body regions, encompassing tactile, pressure, thermal, and nociceptive stimuli. These cortices are anatomically distinct, with S1 primarily involved in mechanical and cold stimulus discrimination and S2 in the interpretation of mechanical and thermal inputs, particularly in pain perception. However, the upstream innervation patterns of the somatosensory system remain less explored. In this study, we employed a modified rabies virus (RV)-mediated transsynaptic retrograde tracing system to map and compare the whole-brain input patterns of S1 and S2 in mice. Our results revealed that both S1 and S2 receive inputs from diverse brain regions, including the cortical plate, thalamus, cortical subplate, striatum, and pallidum. Notably, the cortical plate emerged as the primary source of input neurons for both S1 and S2, while the thalamus demonstrated preferential projections to S1. Through quantitative analysis, we identified distinct input distribution patterns across 64 brain subregions, revealing that S1 and S2 exhibit complex internal circuitry, including abundant local projections. Furthermore, we observed notable variations in the proportional contributions of inputs from diverse subregions to S1 and S2. This comprehensive anatomical framework provides new insights into the neural circuits underlying somatosensory perception and modulation, with potential implications for the development of targeted therapeutic strategies for pain and other somatosensory disorders.