Repetitive activation of the corticospinal pathway by means of rTMS may reduce the efficiency of corticomotoneuronal synapses

Cereb Cortex. 2015 Jun;25(6):1629-37. doi: 10.1093/cercor/bht359. Epub 2014 Jan 9.


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.

Keywords: H-reflex conditioning; corticospinal tract; synaptic plasticity.

MeSH terms

  • Adult
  • Analysis of Variance
  • Biophysics
  • Electric Stimulation
  • Electromyography
  • Evoked Potentials, Motor / physiology*
  • Female
  • H-Reflex / physiology
  • Humans
  • Male
  • Motor Neurons / physiology*
  • Peripheral Nerves / physiology
  • Pyramidal Tracts / physiology*
  • Recruitment, Neurophysiological / physiology
  • Synapses / physiology*
  • Transcranial Magnetic Stimulation*
  • Young Adult