Corticospinal fiber orientation shifts from longitudinal alignment to isotropic organization along the dorsoventral axis in the spinal gray matter

Corticospinal (CS) neurons form a major descending pathway linking the cerebral cortex to spinal circuits and contribute to the regulation of voluntary movement and somatosensory processing. However, the spatial organization of CS fiber arborization within the spinal gray matter remains poorly understood. Here, we investigated the local orientation of CS fibers in the horizontal plane of the cervical spinal cord via population-level labeling combined with quantitative orientation analysis. We compared CS fibers originating from the primary somatosensory cortex (S1) and primary motor cortex (M1) to determine whether local fiber orientation is associated primarily with cortical origin or the dorsoventral position within the spinal gray matter. S1-derived CS fibers exhibited strong longitudinal (rostrocaudal) alignment in dorsal regions, whereas M1-derived CS fibers distributed more ventrally showed largely isotropic organization. At intermediate dorsoventral positions where the S1 and M1 projections overlapped, the degree of longitudinal alignment was comparable between the two origins. Furthermore, spinal dendrites exhibited a similar dorsoventral organization, and their orientation indices were positively correlated with those of CS fibers. These findings demonstrate that local CS fiber orientation is strongly associated with the dorsoventral position within the spinal gray matter and further suggest that presynaptic and postsynaptic geometries are closely coordinated within spinal circuits.