• axonal excitability;
  • cortical excitability;
  • hypothenar muscles;
  • thenar muscles;
  • TMS


Introduction: Differences in cortical and axonal excitability may underlie preferential atrophy of abductor pollicis brevis (APB) and first dorsal interosseous (FDI) in amyotrophic lateral sclerosis, termed the split-hand. Consequently, this study aimed to determine whether differences in excitability follow a split-hand pattern across the intrinsic hand muscles. Methods: Excitability studies were undertaken using threshold tracking techniques in 26 healthy controls with responses recorded over APB, FDI, and abductor digiti minimi. Results: Short interval intracortical inhibition was significantly greater from the APB and FDI. In addition, motor evoked potential amplitude was greater, while cortical silent period was longer from APB and FDI. At a peripheral level, the strength-duration time constant was greater when recorded over APB. Conclusions: This study establishes that differences in cortical excitability follow the split-hand pattern in healthy controls, a finding potentially explained by evolution of specialized activity of APB/FDI in complex hand tasks. Muscle Nerve 49: 836–844, 2014