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Are the after-effects of low-frequency rTMS on motor cortex excitability due to changes in the efficacy of cortical synapses?
Clin Neurophysiol. 2001 Nov; 112(11):2138-45.CN

Abstract

OBJECTIVES

To investigate the mechanisms responsible for suppressing the amplitude of electromyogram (EMG) responses to a standard transcranial magnetic stimulus (TMS) after prior conditioning of the motor cortex with repetitive subthreshold TMS (rTMS) at a frequency of 1 Hz.

METHODS

EMG responses from the first dorsal interosseous, abductor pollicis brevis and flexor carpi radialis (FCR) muscles were recorded after suprathreshold TMS of the motor cortex. In some experiments, H-reflexes were also obtained in the FCR. The amplitude of these responses was compared before and after applying from 150 to 1500 rTMS pulses to motor cortex at an intensity of 95% resting motor threshold through the same figure-of-8 coil.

RESULTS

When tested with subjects relaxed, rTMS conditioning reduced the amplitude of motor evoked potentials (MEPs) to approximately 60% of pre-conditioning values for 2-10 min after the end of the conditioning train, depending on the number of pulses in the train. There was more suppression with 1500 rTMS pulses than with 150 pulses. There was no effect on H-reflexes. There was no effect on MEPs if the test stimuli were given during active contraction of the target muscle.

CONCLUSIONS

The findings confirm previous observations that low-frequency, low-intensity rTMS to motor cortex can produce transient depression of MEP excitability. Since there was no effect on spinal H-reflexes, this is consistent with the idea that some of the suppression occurs because of an effect on the motor cortex itself. The lack of any conditioning effect on MEPs evoked in actively contracting muscle is not readily consistent with the idea that rTMS depresses transmission in synaptic connections to pyramidal cells activated by the test TMS pulse. An alternative explanation is that rTMS reduces the excitability of cortical neurones in relaxed subjects, so that responses to a given input are smaller than before conditioning. Voluntary contraction normalises excitability levels so that the effect is no longer seen.

Authors+Show Affiliations

Third Department of Internal Medicine, Kagawa Medical University, Kagawa, Japan.No affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Comparative Study
Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

11682353

Citation

Touge, T, et al. "Are the After-effects of Low-frequency rTMS On Motor Cortex Excitability Due to Changes in the Efficacy of Cortical Synapses?" Clinical Neurophysiology : Official Journal of the International Federation of Clinical Neurophysiology, vol. 112, no. 11, 2001, pp. 2138-45.
Touge T, Gerschlager W, Brown P, et al. Are the after-effects of low-frequency rTMS on motor cortex excitability due to changes in the efficacy of cortical synapses? Clin Neurophysiol. 2001;112(11):2138-45.
Touge, T., Gerschlager, W., Brown, P., & Rothwell, J. C. (2001). Are the after-effects of low-frequency rTMS on motor cortex excitability due to changes in the efficacy of cortical synapses? Clinical Neurophysiology : Official Journal of the International Federation of Clinical Neurophysiology, 112(11), 2138-45.
Touge T, et al. Are the After-effects of Low-frequency rTMS On Motor Cortex Excitability Due to Changes in the Efficacy of Cortical Synapses. Clin Neurophysiol. 2001;112(11):2138-45. PubMed PMID: 11682353.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Are the after-effects of low-frequency rTMS on motor cortex excitability due to changes in the efficacy of cortical synapses? AU - Touge,T, AU - Gerschlager,W, AU - Brown,P, AU - Rothwell,J C, PY - 2001/10/30/pubmed PY - 2002/1/5/medline PY - 2001/10/30/entrez SP - 2138 EP - 45 JF - Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology JO - Clin Neurophysiol VL - 112 IS - 11 N2 - OBJECTIVES: To investigate the mechanisms responsible for suppressing the amplitude of electromyogram (EMG) responses to a standard transcranial magnetic stimulus (TMS) after prior conditioning of the motor cortex with repetitive subthreshold TMS (rTMS) at a frequency of 1 Hz. METHODS: EMG responses from the first dorsal interosseous, abductor pollicis brevis and flexor carpi radialis (FCR) muscles were recorded after suprathreshold TMS of the motor cortex. In some experiments, H-reflexes were also obtained in the FCR. The amplitude of these responses was compared before and after applying from 150 to 1500 rTMS pulses to motor cortex at an intensity of 95% resting motor threshold through the same figure-of-8 coil. RESULTS: When tested with subjects relaxed, rTMS conditioning reduced the amplitude of motor evoked potentials (MEPs) to approximately 60% of pre-conditioning values for 2-10 min after the end of the conditioning train, depending on the number of pulses in the train. There was more suppression with 1500 rTMS pulses than with 150 pulses. There was no effect on H-reflexes. There was no effect on MEPs if the test stimuli were given during active contraction of the target muscle. CONCLUSIONS: The findings confirm previous observations that low-frequency, low-intensity rTMS to motor cortex can produce transient depression of MEP excitability. Since there was no effect on spinal H-reflexes, this is consistent with the idea that some of the suppression occurs because of an effect on the motor cortex itself. The lack of any conditioning effect on MEPs evoked in actively contracting muscle is not readily consistent with the idea that rTMS depresses transmission in synaptic connections to pyramidal cells activated by the test TMS pulse. An alternative explanation is that rTMS reduces the excitability of cortical neurones in relaxed subjects, so that responses to a given input are smaller than before conditioning. Voluntary contraction normalises excitability levels so that the effect is no longer seen. SN - 1388-2457 UR - https://www.unboundmedicine.com/medline/citation/11682353/Are_the_after_effects_of_low_frequency_rTMS_on_motor_cortex_excitability_due_to_changes_in_the_efficacy_of_cortical_synapses L2 - https://linkinghub.elsevier.com/retrieve/pii/S1388-2457(01)00651-4 DB - PRIME DP - Unbound Medicine ER -