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 i nduces motor evoked potentials in contralateral muscles which are thou ght to be conducted by the corticospinal tract. Furthermore, inhibitor y actions can be elicited by TMS which appear directly after the motor evoked potential (postexcitatory inhibition, PI) and can be visualize d by blockade of tonic voluntary EMG activity. It was the aim of the p resent 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 proxi mal and distal muscles. Experiments were performed on a total of 34 he althy volunteers. Brain stimuli were delivered with a Novametrix Magst im 200HP with a maximum output of 2.0 T, and stimulation was performed during tonic voluntary activation of the muscle under study. Stimulat ion 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 reocc urrence of the EMG background activity. PI was found more pronounced i n distal hand muscles than in proximal arm and leg muscles. The larges t PI values were observed when the primary motor cortex was stimulated . To test the excitability of the spinal motoneurones during PI, corti cal double stimulation at various intervals was performed and the sole us H-reflex was evoked at different intervals after cortical stimulati on. Neither test revealed a decrease in the excitability of the spinal motoneurones during Pl. These findings imply that spinal segmental in hibitory action cannot account for PI and that, most probably, inhibit ory actions within the motor cortex play a major role in the genesis o f PI.