Topographical differences in the developmental profile of central motor conduction time

Objectives: To study the topographical difference in the developmental profile of the central motor conduction time (CMCT) in upper extremity muscles, electromyographic (EMG) responses to transcranical magnetic stimulation (TMS) were examined in the first dorsal interosseous, extensor carpi radialis (ECR), biceps (BCP), and deltoid (DT) muscles of 25 neurologically normal subjects aged from 2 to 26 years. Methods: The motor cortex and cervical spinal roots were magnetically stimulated, and CMCT was measured as the onset latency difference between these EMG responses. Results: CMCT in children was shorter in the more proximal muscle of each adjacent muscle pair, despite the tendency of a higher threshold intensity for TMS of the more proximal muscle. This topographical difference tended to be more distinct in younger children, whereas CMCT in adults did not show such a topographical difference. Consequently, the linear decrease in CMCT during maturation was less pronounced in the proximal muscles. Conclusions: We speculate that direct activation of corticospinal neurons to the more proximal muscles was preferentially produced by TMS in younger children, depending on the relationship between the spatial direction of axons, head circumference, and stimulating coil diameter.