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

Abstract

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.

Effects of muscle action type on corticospinal excitability and triceps surae muscle-tendon mechanics

“…Furthermore, Roick et al (1993) also demonstrated that SP duration was not modulated by any changes of finger or ankle joint position. It was identical irrespective of whether muscle actions were isometric or dynamic, with or without joint fixation.…”

“…SP duration progressively increases with increasing TMS intensity. Roick et al (1993) reported that for first dorsal interosseous muscle, SP increased concomitantly with stimulation intensity from 30 to 100% of maximum stimulator output, unlike the MEPs which plateaued at 60-70%. Interestingly, muscle activation had a negligible role on the duration of SP; Säisänen et al (2008) reported that varying from 20 to 80% of MVC for the thenar musculature had no effect on SP.…”

“…In both measures, additional problem exists in defining the reappearance of the EMG background especially during low muscle forces. Considerable interindividual variation in SP has been reported for first dorsal interosseous and tibialis anterior muscles, but no differences in SP between homonymous muscles of both sides (Roick et al 1993). …”

Inhibition of the cortex using transcranial magnetic stimulation in psychiatric populations: current and future directions

“…[15][16][17][18] For instance, administration of tiagabine, a GABA reuptake inhibitor, leads to an increased concentration of GABA in the synaptic cleft and predominantly activates GABA B receptors, 19 which has been reported to result in a dose-dependent prolongation of the cortical silent period. 16 Furthermore, baclofen, a potentiator of GABA B receptor-mediated inhibitory neurotransmission, has also been found to lengthen the cortical silent period.…”

A new concept in the electrophysiological evaluation of syringomyelia

“…10,38 Evidence has been given for the existence of supraspinal inhibition due to intracortical inhibitory interneurons because MEPs and CoSPs are variable in different cortical and subcortical lesions. 2,4,34 The CoSPs can be used for prediction of recovery in deficient motor systems poststroke and for judgment of callosal function. 18,43 The present study demonstrates that CoSPs can correspond well to motor deficits in syringomyelia and are superior in specificity than MEPs (86% vs 72%, respectively), although this is statistically not significant.…”