title: Repetitive activation of the corticospinal pathway by means of rTMS may reduce the efficiency of corticomotoneuronal synapses.
abstract: Low-frequency rTMS applied to the primary motor cortex (M1) may produce depression of motor-evoked potentials (MEPs). This depression is commonly assumed to reflect changes in cortical circuits. However, little is known about rTMS-induced effects on subcortical circuits. Therefore, the present study aimed to clarify whether rTMS influences corticospinal transmission by altering the efficiency of corticomotoneuronal (CM) synapses. The corticospinal transmission to soleus α-motoneurons was evaluated through conditioning of the soleus H-reflex by magnetic stimulation of either M1 (M1-conditioning) or the cervicomedullary junction (CMS-conditioning). The first facilitation of the H-reflex (early facilitation) was determined after M1- and CMS-conditioning. Comparison of the early facilitation before and after 20-min low-frequency (1 Hz) rTMS revealed suppression with M1- (-17 ± 4%; P = 0.001) and CMS-conditioning (-6 ± 2%; P = 0.04). The same rTMS protocol caused a significant depression of compound MEPs, whereas amplitudes of H-reflex and M-wave remained unaffected, indicating a steady level of motoneuronal excitability. Thus, the effects of rTMS are likely to occur at a premotoneuronal site-either at M1 and/or the CM synapse. As the early facilitation reflects activation of direct CM projections, the most likely site of action is the synapse of the CM neurons onto spinal motoneurons.
authors.affiliation: Department of Medicine, Movement and Sport Science, University of Fribourg, 1700 Fribourg, Switzerland.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Taube
authors.affiliation: Department of Medicine, Movement and Sport Science, University of Fribourg, 1700 Fribourg, Switzerland Department of Sport Science, University of Freiburg, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Leukel
authors.affiliation: Department of Nutrition, Exercise and Sports, Department of Neuroscience and Pharmacology, University of Copenhagen, DK-1165 Copenhagen, Denmark.
authors.forename: Jens Bo
authors.initials: JB
authors.lastname: Nielsen
authors.affiliation: Department of Nutrition, Exercise and Sports, Department of Neuroscience and Pharmacology, University of Copenhagen, DK-1165 Copenhagen, Denmark.
authors.forename: Jesper
authors.initials: J
authors.lastname: Lundbye-Jensen
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001703:Biophysics; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D010525:Peripheral Nerves / Q000502:physiology; D011712:Pyramidal Tracts / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology; D013569:Synapses / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
keywords: H-reflex conditioning; corticospinal tract; synaptic plasticity