Suppression of voluntary motor activity revealed using transcranial magnetic stimulation of the motor cortex in man.

Abstract

1. Suppression of voluntary muscle activity of hand and arm muscles in response to transcranial magnetic stimulation (TMS) of the motor cortex has been investigated in man. 2. Suppression could be elicited by low levels of TMS without any prior excitatory response.The latency of the suppression was 3-8 ms longer than the excitation observed at a higher stimulus intensity. The duration of the suppression ranged from 8 to 26 ms. 3. A circular stimulating coil was used to determine threshold intensity for excitation and suppression of contraction of thenar muscles in response to TMS at different locations over the motor cortex. The locations for lowest threshold excitation coincided with those for lowest threshold suppression. Suppression was elicited at a lower threshold than excitation at all locations. 4. A figure-of-eight stimulating coil was positioned over the left motor cortex at the lowest threshold point for excitation of the right thenar muscles. The orientation for the lowest threshold excitatory and inhibitory responses was the same for all subjects. That orientation induced a stimulating current travelling in an antero-medial direction. Suppression was invariably elicited at lower thresholds than excitation. 5. When antagonistic muscles (second and third dorsal interosseus) were co-contracted, TMS evoked coincident suppression of voluntary EMG in the two muscles without prior excitation of either muscle. This suggests that the suppression is not mediated via corticospinal activation of spinal interneurones. 6. Test responses to electrical stimulation of the cervical spinal cord were evoked in both relaxed and activated thenar muscles. In the relaxed muscle, prior TMS at an intensity that would suppress voluntary activity failed to influence the test responses, suggesting absence of inhibition at a spinal level. However, in the activated muscle, prior TMS could reduce the test response. This may be explained by disfacilitation of motoneurones due to inhibition of corticospinal output. 7. We propose that suppression of voluntary muscle activity by TMS is due in large part to activation of a mechanism within the motor cortex that reduces the corticospinal output to the muscle. It is concluded that TMS evokes excitation and inhibition via neuronal structures lying close to one another and having similar orientations.

Specific interpretation of augmented feedback changes motor performance and cortical processing

“…To avoid fatigue, a break of 3 min was given between the trials. To analyze whether TMS caused a facilitation or suppression in the FDI EMG, the rectified and then averaged 40 sweeps without stimulation (control EMG) were subtracted from 40 sweeps with stimulation (Davey et al 1994;Petersen et al 2001;Zuur et al 2010;Lauber et al 2012). The intensity of the TMS during the first trial was chosen to be high enough to evoke MEPs in the FDI (0.9 times motor threshold at rest).…”

Magnetic brain stimulation can improve clinical outcome in incomplete spinal cord injured patients

“…1,2 This change occurs within a month of injury 3 and has been identified in patients with iSCI in the cervical spinal cord by recording electromyographic (EMG) responses in hand muscles to single-pulse transcranial magnetic stimulation (TMS) of the motor cortex. Such downregulation of cortical inhibition might not only be associated with natural recovery following iSCI; it may also be a useful mechanism to promote active recovery of motor function.…”

Postural challenge affects motor cortical activity in young and old adults

“…Rest periods of 30 s were given between blocks and rest periods of 5 min between conditions. As in previous studies using subTMS (Davey et al, 1994;Petersen et al, 2001;Zuur et al, 2010), two young and five old adults were excluded because they did not exhibit suppression without a preceding facilitation in all conditions.…”

“…The subTMS method was first described by Davey et al (1994), and subsequent experiments using a variety of techniques supported the idea that the TMS-induced EMG suppression originates from activation of intracortical inhibitory circuits, reducing motor cortical output (Davey et al, 1994;Petersen et al, 2001;Di Lazzaro et al, 1998;Classen and Benecke, 1995). For example, transcranial electrical stimulation, thought to activate the axons of the corticospinal neurons directly, does not suppress ongoing EMG (Petersen et al, 2001).…”

“…To examine motor cortical activity, we applied transcranial magnetic stimulation pulses at subthreshold intensities (subTMS). Such pulses suppress ongoing electromyographic (EMG) activity through the activation of inhibitory intracortical circuits (Davey et al, 1994).…”