Functional Gradients of the Spinal Cord and Corticospinal System

Diffusion map embedding has been successfully applied to restingstate fMRI to reveal spatial gradients that reflect functional organization. However, this approach has not yet been applied to investigate spinal cord organization, despite its central role in sensorimotor control. Prior resting-state spinal fMRI studies have described intrinsic spinal connectivity, but gradient-style organization and interactions with the brain have not been characterized. Here, we applied the gradient approach to resting-state corticospinal fMRI to derive functional gradients linking the somatosensory cortex and cervical spinal cord. Corticospinal gradients projected to the cortex revealed a lateralized axis distinct from the canonical somatotopic gradients, suggesting the known lateralization effect of cortical-spinal coupling. Furthermore, intrinsic spinal gradients uncovered a rostrocaudal axis across the cord and segment-level gradients reflecting left-right and ascending-descending circuitry consistent with spinal neuroanatomy. In contrast, corticospinal gradients, projected to the spine showed a distinct organization, likely shaped by heterogeneous cortical-spinal coupling. These findings provide the first evidence of macroscale functional gradients in the human spinal cord and introduce gradient mapping as a powerful framework to study corticospinal organization.