Changes in muscle responses to stimulation of the motor cortex induced by peripheral nerve stimulation in human subjects

The aim of this study was to determine whether prolonged, repetitive mixed nerve stimulation (duty cycle 1 s, 500 ms on-500 ms off, 10 Hz) of the ulna r nerve leads to a change in excitability of primary motor cortex in normal human subjects. Motor-evoked potentials (MEPs) generated in three intrinsi c hand muscles [abductor digiti minimi (ADM), first dorsal interosseous (FD I) and abductor pollicis brevis (APB)] by focal transcranial magnetic stimu lation were recorded during complete relaxation before and after a period o f prolonged repetitive ulnar nerve stimulation at the wrist. Transcranial m agnetic stimuli were applied at seven scalp sites separated by I cm: the op timal scalp site for eliciting MEPs in the target muscle (FDI), three sites medial to the optimal site and three sites lateral to the optimal stimulat ion site, The area of the MEPs evoked in the ulnar- (FDI, ADM) but not the median-innervated (APB) muscles was increased after prolonged ulnar nerve s timulation. Centre of gravity measures demonstrated that there was no signi ficant difference in the distribution of cortical excitability after the pe ripheral stimulation. F-wave responses in the intrinsic hand muscles were n ot altered after prolonged ulnar nel ve stimulation, suggesting that the ch anges in MEP areas were not the result of stimulus-induced increases in the excitability of spinal motoneurones. Control experiments employing transcr anial electric stimulation provided no evidence for a spinal origin for the excitability changes. These results demonstrate that in normal human subje cts the excitability of the cortical projection to hand muscles can be alte red in a manner determined by the peripheral stimulus applied.