Spinal motor neuron excitability during the silent period after cortical stimulation

During tonic voluntary muscle contraction, a period of electromyographic silence follows the motor evoked potential produced by transcranial stimulation of the contralateral motor cortex. We studied the silent period in the wrist flexors of 3 normal volunteers and a deafferented patient during 20% of maximal contraction. To test the excitability of the spinal motor neuron pool during the period of silence, the H-reflex was evoked in the normal subjects at different intervals after cortical stimulation. The amplitude of the H-reflex in the silent period was expressed as a percentage of the amplitude during complete muscle relaxation. The H-reflex was profoundly depressed at the beginning of the silent period (13.5-27% of the control measurement), but showed a clear tendency to recover toward the end of the silent period despite continued absence of muscle activation (71-84% of the control). Moreover, the silent period in the deafferented patient was of longer duration than can be accounted for by segmental mechanisms. These findings imply, at least in the late part of the silent period, that a reduction in the excitability of the spinal motor neuron pool plays only a minor role in determining the phenomenon and that it is probably caused by lack of cortical drive.