Cortical excitability differences between flexor pollicis longus and APB

Although abductor pollicis brevis (APB) and flexor pollicis longus (FPL) share a common peripheral nerve supply, these muscles subserve different functions and may be differently affected in neurodegenerative disease such as amyotrophic lateral sclerosis (ALS). As a consequence, differences in cortical excitability may potentially develop in relation to these functional differences. Cortical excitability was assessed using the threshold tracking transcranial magnetic stimulation (TMS) technique in 15 healthy controls with motor responses recorded over the APB and FPL using surface electrode recordings. Short-interval intracortical inhibition (SICI) was significantly reduced from the FPL compared to APB (SICIFPL 6.9 ± 1.8%; SICIAPB 10.7 ± 1.4%, P < 0.01). In addition, the FPL motor evoked potential amplitude (MEPFPL 14.7 ± 2.3%; MEPAPB 21.7 ± 3.9%; P < 0.01) and cortical silent period duration (CSPFPL 174.7 ± 6.7 ms; CSPAPB 205.4 ± 3.9 ms, P < 0.01) were significantly smaller. The findings in the present study indicate that cortical inhibition and corticomotoneuronal output is reduced when recording over the FPL. The differences in cortical excitability may develop as a consequence of varied function and could potentially explain the dissociated muscle atrophy evident in ALS.