(A) Top: Flat-mount section through L4 of the cortex of a Scnn1a-Cre;Ai14 mouse, showing the L4 labeling pattern across S1 cortex. This image is of the right side of the brain, as are all dorsal-view images in the figures unless indicated otherwise. A, anterior; L, lateral. Bottom: Enlarged view of the hand region. Septa (arrows) separate the hand region from the neighboring face and hindlimb regions. Labeling of S1 somatotopic subfields is based on prior studies in mice and rats (Waters et al., 1995; Sigl-Glöckner et al., 2019) and standard atlases (Dong, 2008). (B) Left: Same, additionally showing corticospinal neurons (green; their dendrites within the section mainly through L4), labeled by cervical injection of AAVretro-GFP. Dashed rectangle: region of interest used to quantify fluorescence profile. Arrow: region of overlap. Right: Coronal section (different animal), showing laminar labeling patterns. This image is of the right side of the brain (midline is to the left), as are all coronal images in the figures unless indicated otherwise. (C) Fluorescence intensity profiles across the anteromedial edge of the S1 area (marked by dashed rectangle in image in panel B), for individual animals (lighter traces) and group average (darker, n = 5 animals), showing hand area (gray) bordered by septa (red arrows), with region of corticospinal labeling (green arrow) located medially, in the putative hypothenar/ulnar subregion. Intensity profiles were aligned to the hand-hindlimb septum (x = 0). (D) Somatosensory responses mapped by transcranial GCaMP6s imaging in Scnn1a-Cre;Ai96 mice, showing the average responses to stimulation of the fifth (D5) and second (D2) digits, with the centroids of the responses marked (red ‘+'), which are also shown superimposed on the average transcranial image of corticospinal labeling, from a subset of the same mice that were injected with AAVretro-GFP in the spinal cord. Maps are aligned to bregma (white ‘+').