Quantitative comparison of uncrossed corticospinal tracts arising from different cortical areas in humans.

The uncrossed corticospinal tract (CST) has garnered interest as a potential compensatory neural pathway for recovering motor function after stroke-induced damage to the crossed CST. However, the area of origin of the uncrossed CST in humans remain unclear. This study aimed to identify the area of origin of the uncrossed CST in healthy adults via fibre tractography and diffusion-weighted magnetic resonance imaging. We identified uncrossed corticospinal (CS) streamlines arising from seven cortical areas: the primary motor cortex (M1: mean ± SE = 71.55 ± 2.19 %), primary somatosensory cortex (13.77 ± 2.18 %), supplementary motor area (SMA: 8.02 ± 1.49 %), dorsal (2.51 ± 0.52 %), and ventral (1.39 ± 0.31 %) aspects of the premotor cortex, preSMA (1.50 ± 0.42), and Brodmann area 5 (1.26 ± 0.21 %). Even though there was a fair amount of individual variation (min - max: 17.98-99.73 %), on average, 74.04 % of the CS streamlines passed through the dorsolateral funiculus of the spinal cord. Our findings enhance the anatomical understanding of the uncrossed CST and may inform neurorehabilitation strategies aimed at optimizing motor recovery following neurological injury.