Short-latency afferent inhibition on cortical motor representation in healthy humans.

The motor system continuously receives sensory inputs and uses this information to perform purposeful movements in a process known as sensorimotor integration. As a biomarker of sensorimotor integration efficacy, short-latency afferent inhibition (SAI), the phenomenon whereby afferent sensory inputs inhibit cortical motor outputs in a given muscle, has been widely studied in humans. However, it remains unclear how the (sensory) nerve-muscle relationship, that is, anatomical proximity and homotopy (nerve supply to muscles), affects SAI magnitude. To address this question, we assessed SAI magnitudes in cortical motor excitability by examining the size of the motor representations of two intrinsic hand muscles when afferent inputs were provided to the nerves either innervating or noninnervating the muscles. In 16 healthy adults, we measured the effect of conditioning electrical stimuli to the median nerve (MN) or ulnar nerve (UN) at the wrist on motor evoked potentials induced by transcranial magnetic stimulation in the first dorsal interosseous (innervated by UN) and abductor pollicis brevis (innervated by MN) muscles, both of which are anatomically located closer to MN than to UN. Conditioning MN stimulation resulted in a significant SAI in both muscles, with no significant difference in SAI between the muscles. No clear SAI was found in either muscle with the UN stimulation. These results suggest that SAI magnitude may depend on anatomical proximity rather than on homotopy. Given the inhibition of the motor representation size of both muscles, the specific nature of such SAI may contribute to the synergistic coordination between muscles. We found the significant SAI in both cortical broad (motor map) and local (hotspot) areas of the FDI and APB hand muscles only when a conditioning stimulus was delivered to the MN not to the UN. These results suggest that the SAI magnitude may depend on anatomical proximity rather than homotopic interactions between the nerve-muscle relationship, which may contribute to the synergistic coordination of muscles in response to afferent sensory inputs.