On the origin of the postexcitatory inhibition seen after transcranial magnetic brain stimulation in awake human subjects

Non-invasive transcranial magnetic stimulation (TMS) of motor cortex induces motor evoked potentials in contralateral muscles which are thought to be conducted by the corticospinal tract. Furthermore, inhibitory actions can be elicited by TMS which appear directly after the motor evoked potential (postexcitatory inhibition, PI) and can be visualized by blockade of tonic voluntary EMG activity. It was the aim of the present study to answer the questions of whether this inhibitory action is mainly of cortical or of spinal origin, which brain area generates this inhibition, and whether the duration of PI differs between proximal and distal muscles. Experiments were performed on a total of 34 healthy volunteers. Brain stimuli were delivered with a Novametrix Magstim 200HP with a maximum output of 2.0 T, and stimulation was performed during tonic voluntary activation of the muscle under study. Stimulation strength was 1.5 times threshold level. Duration of PI was defined as the time from the onset of the motor evoked potential to the reoccurrence of the EMG background activity. PI was found more pronounced in distal hand muscles than in proximal arm and leg muscles. The largest PI values were observed when the primary motor cortex was stimulated. To test the excitability of the spinal motoneurones during PI, cortical double stimulation at various intervals was performed and the soleus H-reflex was evoked at different intervals after cortical stimulation. Neither test revealed a decrease in the excitability of the spinal motoneurones during PI. These findings imply that spinal segmental inhibitory action cannot account for PI and that, most probably, inhibitory actions within the motor cortex play a major role in the genesis of PI.