Abstract
The purpose of this study was to study neurophysiological mechanisms underlying the facilitatory effect of voluntary contraction upon the motor evoked potential (MEP) by transcranial electrical stimulation of human motor cortex. The MEPs were recorded from the lower limb muscles while applying percutaneous electrical stimulation to the cervical spinal cord. The spinal cord stimulation was expected to elicit identical amount of synchronous descending input to motoneuron pool. Spinal cord stimulation reduced the threshold intensity to evoke MEP of the tibialis anterior muscle and enhanced the amplitude of MEP during voluntary contraction than when relaxed. However, voluntary contraction did not change the MEP latency. Thus, voluntary contraction coupled with spinal cord stimulation increased the excitability of spinal motoneuron but did not facilitate effect on the latency of the MEP. The author postulates that the amplitude enhancement of MEP to cortical stimulation during a voluntary contraction occurs at least in part at the spinal level, but the latency decrement of MEP could not be explicable by the neural event solely at the spinal cord level.
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