Mc. Ridding et al., Changes in muscle responses to stimulation of the motor cortex induced by peripheral nerve stimulation in human subjects, EXP BRAIN R, 131(1), 2000, pp. 135-143
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.