Fig. 1. Drawings depicting the CST and anatomical structures relevant to this study. A, Sagittal view of an early postnatal rat brain illustrating the pathway of corticospinal axons, which originate from cortical layer 5 (L5) neurons, and the location of collateral branches related to some of their major subcortical targets. The locations and planes of section of subsequent figures are indicated. The shaded boxessurrounding the basilar pons (BP) and the pyramidal decussation (pd) indicate the location of the sagittal sections shown in Figure 2A–D andE, F, respectively. The arrow markedT approximates the location of the transverse sections shown in Figures 3 and 6. The arrow markedH approximates the location of the horizontal sections shown in Figure 7. B, Schematic of a transverse section through the spinal cord (marked by the arrow T inA). The lines extending from thetwo black dots depict branches (lines) extending laterally into the spinal gray (SG) from parent corticospinal axons (black dots) located in the corticospinal tract (cst), which is in the ventral part of the dorsal funiculus (df). The dashed box and the shaded box indicate the approximate locations of the panels illustrated in Figures 3 and 6, respectively.C, Schematic of a horizontal section through the spinal cord (marked by the arrow H in A) as in Figure 7. Depicted are four branches extending laterally into the spinal gray from two parent corticospinal axons coursing rostrocaudally through the left CST. Abbreviations: BP, basilar pons;cp, cerebral peduncle; cst, corticospinal tract; df, dorsal funiculus; ic, internal capsule; lat, lateral; L5, layer 5;mid, midline; pd, pyramidal decussation;pt, pyramidal tract; SG, spinal gray;SpCd, spinal cord.