Somatotopic homunculus profiles of motor cortex and corticospinal tract in aging

Motor performance declines with aging, yet how somatotopic subregions of the primary motor cortex (M1) and corticospinal tract (CST) show region-specific structural and functional changes during aging remains unclear. We analyzed T1- and T2-weighted MRI, multi-delay arterial spin labeling, and multi-shell diffusion data from 339 right-handed adults in the Human Connectome Project-Aging database. Measures included M1 cerebral blood flow (CBF), arterial transit time (ATT), intracortical myelin (T1/T2 ratio), and CST integrity. M1 was divided into face, upper-limb, and lower-limb subregions using mrGrad, and corresponding CST subdivisions were reconstructed. Across age groups, CBF was highest in upper-limb M1 and lowest in lower-limb M1, while myelin content was highest in lower-limb regions of both M1 and CST. Aging was associated with prolonged ATT, reduced CBF, and altered T1/T2 ratio across all subregions, whereas the upper-limb CST showed relatively preserved microstructure. These imaging features of M1 and CST were linked to somatic motor performance. M1 and CST exhibit distinct somatotopic gradients in structure and function. Aging affects motor homunculus subregions and their pathways in region-specific ways. Multiparametric MRI has potential to characterize early motor decline and provides insights into neural mechanisms underlying somatotopic motor functions in older adults.