The excitability of cortical inhibitory interneurones responsible for silent period after transcranial brain stimulation

Transcranial brain stimulation produces an interruption of the voluntary EMG activity for somel 00-200 ms by activating inhibitory interneurones located in the cerebral cortex. We studied the excitability of these interneurones by the technique of paired magnetic stimulation of the motor cortex in 8 normal subjects. Two magnetic shocks were delivered over the motor cortex with a round coil connected to a Bistim module and two Magstim stimulators. MEPs were recorded in the first interosseous muscle during voluntary contraction. We have seen that at interstimulus interval beween 200 and 500 ms the cortical silent period following the second stimulus is prolonged by about 20% of control silent period duration. The SP duration was unaffected at interstimulus interval of 750 ms. MEP size was unchanged at all stimulus intervals. We conclude that the excitability of inhibitory cortical interneurones responsible for the silent period is enhanced by a preceding cortical stimulus. A syncronization of elements responsible for cortical inhibition could possibly explain this phenomenon.