LONG-LASTING DEPRESSION OF SOLEUS MOTONEURONS EXCITABILITY FOLLOWING REPETITIVE MAGNETIC STIMULI OF THE SPINAL-CORD IN MULTIPLE-SCLEROSIS PATIENTS

The effect of repetitive magnetic stimulation at the spinel level on t he soleus H-reflex amplitude was evaluated in I I MS patients with low er limb spasticity and in nine healthy subjects. In MS patients stimul ation with a train of 16 stimuli at 25 Hz induced a decrease in amplit ude to 61.2 +/- 25.7% of the unconditioned H-reflex amplitude at inter stimulus interval (ISI) of 10-1000 ms (P<0.01). The amount of decrease in H-reflex amplitude was highly dependent on the stimulation intensi ty and the placement of the coil, and to a lesser extent influenced by the stimulation frequency No decrease in motor evoked potentials (MEP s) evoked by transcranial magnetic stimulation was seen following trai ns of 16 stimuli at mid-thoracic in contrast to the post-stimulation d epression in H-reflex amplitude which could imply that mechanisms acti ng at presynaptic level were involved. In response to repetitive magne tic stimuli for 5 min, a long-lasting decrease in H-reflex amplitude t o a level of about 70% of the pre-stimulation H-reflex amplitude occur red in MS patients (P<0.01). A similar although not significant decrea se was observed in healthy subjects. We Propose that long-lasting depr ession of the soleus H-reflex amplitude after repetitive magnetic stim uli is due to long-term depression of the synoptic transmission.