Modulation of the soleus H reflex by electrical subcortical stimuli in humans

Over the past two decades, the H reflex has been used as a neural tool to assess the effect on the motoneuronal pool of conditioning volleys in supraspinal descending tracts elicited by transcranial magnetic stimulation (TMS) or auditory stimuli. However, mechanisms mediating such modulation are unclear. These hypothesized neural pathways are likely to be affected by single electrical stimulus applied through the electrodes implanted in the subthalamic nucleus for deep brain stimulation (sSTNDBS). To improve our knowledge on such mechanisms, we examined in 11 Parkinson's disease patients the effects of conditioning sSTNDBS applied contralateral and ipsilateral to the H reflex recording on the amplitude of the soleus H reflex, at interstimulus intervals (ISIs) between 0 and 110 ms. There was a significant main effect of the ISI (P<0.001) and of the sSTNDBS stimulation side (P=0.019) on the percentage change in the soleus H-reflex amplitude. Contralateral sSTNDBS modulation of the soleus H reflex resembles that of TMS in healthy subjects with two facilitation phases (at 5-20 ms and at 60 ms), while after ipsilateral sSTNDBS, there is only a single facilitation phase peaking up at 5 ms later than the first facilitation period observed with contralateral stimulation. These findings contribute to the discussion of the mechanisms underlying the excitability of the spinal alpha motoneuron pool and the modulation of the H reflex by supraspinal stimuli.