Fig. 3. Functional subdivisions of PNs.

a Schematic depiction of the selected DRG (highlighted in red) to study the spatial distribution of PNs along the rostral-caudal axis of the trunk. b Spatial distribution of the three major types of PNs in representative DRG (see A) (n = 3 animals). Data are presented as mean ± SEM; dots represent values from individual animals. See “Methods” for details. c Comparison of cross-nucleus soma sizes of Ia-PNs subtypes depicted in box and whisker plot. Lower and upper hinges: first and third quartiles; the horizontal line: median; the whiskers extend to the value no further than 1.5 * IQR from the hinge; large dots: outliers. Each small dot represents a cell: Ia₁ (n = 342 cells), Ia₂ and Ia₃ together (n = 148 cells). Two-tailed t-test, ****p < 0.0001. d Spatial distribution of Ia-PNs subtypes in representative DRG (see A). Data are presented as mean ± SEM (n = 3 animals); dots represent values from individual animals. See Methods for details. e Ventral spinal cord section of P54 Calb1^dgCre;Ai14 mice labeled for CHAT (motor neurons) and VGLUT1 (sensory terminals), showing RFP⁺ sensory boutons onto motor neurons in LMC but not MMC. Scale bar: 100 µm. Scale bars of the micrographs: 20 µm. f Schematic depiction of the over-representation of Ia₃-PNs innervation in the distal-dorsal region of the mouse forelimb. g Proportion of MSs innervated by RFP⁺ afferents in P30 Calb2^Cre;Ai14 mice (n = 3 animals). P: proximal; DD: distal-dorsal; DV: distal-ventral; ECR: extensor carpi radialis; Tri: triceps. Data are presented as mean ± SEM; dots represent values from individual animals. Two-tailed t-test, *p < 0.05. h MS of P30 Calb2^Cre;Ai14 mice stained for RFP, VGLUT1 and DAPI, showing that RFP⁺ afferents innervate only the nuclear bag but not chain fibers. Bag and chain fibers are distinguishable by their sizes and organization of nuclei: chain fibers have nuclei aligned in a chain, while bag fibers have many nuclei stacked in bags. Right panel: schematic representing the finding. Scale bar: 20 µm. i Quantification of cross-nucleus soma sizes of II-PNs subtypes depicted in box and whisker plot. Lower and upper hinges: first and third quartiles; the horizontal line: median; the whiskers extend to the value no further than 1.5 * IQR from the hinge; large dots: outliers. Each small dot represents a cell: II₁ (n = 25 cells), II₂ (n = 43 cells), II₃ (n = 37 cells), II₄ (n = 65 cells). Two-tailed t-test, ****p < 0.0001. j Spatial distribution of II-PNs subtypes in representative DRG (see a). Data are presented as mean ± SEM (n = 3 animals); dots represent values from individual animals. See “Methods” for details. k Dot plot summarizing the spatial distribution of all PN subtypes. The size of the circle reflects how a subtype is distributed along the rostro-caudal axis where the presence of the subtype in each DRG is proportional to the highest represented DRG. The color intensity reflects the percentage of different subtypes among all PNs in the same DRG. l Table summarizing main characteristics of PN subtypes. The illustrated soma sizes of the subtypes are proportional to the average soma sizes of the respective PN subtypes observed in situ. Source data are provided as a Source Data file.