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.

In summary, this study has revealed that corticospinal excitability, driving ES muscles close to the site of pain, is lowered in patients with chronic low back pain.

Study design: Preliminary longitudinal clinical trial.Objectives: To test the efficacy of repetitive transcranial magnetic stimulation (rTMS) in modulating corticospinal inhibition and improving recovery in stable incomplete spinal cord injury (iSCI). Setting: National Spinal Injuries Centre, Stoke Mandeville Hospital, Bucks, UK and Division of Neuroscience, Imperial College Faculty of Medicine, Charing Cross Hospital, London, UK. Methods: Four stable iSCI patients were treated with rTMS over the occipital cortex (sham treatment) and then over the motor cortex (real treatment). Patients were assessed using electrophysiological, clinical and functional measures before treatment, during sham treatment, during the therapeutic treatment and during a 3-week follow-up period. Results: Cortical inhibition was reduced during the treatment week. Perceptual threshold to electrical stimulation of the skin, ASIA clinical measures of motor and sensory function and time to complete a peg-board improved and remained improved into the follow-up period. Conclusion: In this preliminary trial, rTMS has been shown to alter cortical inhibition in iSCI and improve the clinical and functional outcome.

The size of compound motor evoked potentials (cMEPs) to transcranial magnetic stimulation of the motor cortex was measured in the relaxed first dorsal interosseous muscle of the nondominant hand (ndFDI) during different levels of voluntary contraction in the homonymous muscle of the dominant hand (dFDI). cMEP responses in the ndFDI became larger when the dFDI was contracted to forces ranging 10–70% of maximum voluntary contraction. Variability in the amplitude of the cMEP responses in ndFDI decreased when dFDI was contracted. Comparison with cMEPs to spinal cord stimulation suggested a large component of the facilitation was occurring at a cortical level. The amplitude of cMEP responses in ndFDI also increased when the tibialis anterior muscle of the leg on the contralateral side was contracted. The observed facilitation of motoneurons during contraction of contralateral muscles might involve a transcallosal pathway modulating the excitability of one cortex when the other is activated. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:1033–1039, 1998

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