Use of transcutaneous spinal cord stimulation to explore inhibitory and facilitatory circuits in muscles of the human lower limb.

The aim of this study was to explore the primary afferent depolarization mechanism, to determine whether the soleus transspinal evoked potential (TEP), elicited through transcutaneous spinal cord stimulation over the L1-L2 level, is modulated by presynaptic inhibition and heteronymous facilitation, similar to the Hoffmann (H) reflex, elicited by posterior tibial nerve stimulation. Twenty subjects participated in two experiments. Experiment 1 assessed D1 and D2 inhibition by conditioning the H reflex and TEP with peroneal nerve stimulation at different interstimulus intervals (ISIs; ranging from 1 to 200 ms). Experiment 2 examined heteronymous facilitation of responses using femoral nerve conditioning stimulation (ISIs ranging from -1 to -10 ms). Conditioned responses (HPSI or TEPPSI and HFAC or TEPFAC) were compared with unconditioned ones (HTEST or TEPTEST). Concerning D1 and D2 inhibition, results did not reveal any significant difference between the two responses (p = 0.89 and p = 0.51 for D1 and D2, respectively). Inhibition was observed at all ISIs for D1 and at the 100 and 200 ms ISIs for D2. Facilitation patterns were also comparable between the two responses. Moreover, a negative correlation was observed between the modulation of soleus TEP and tibialis anterior TEP (conditioning muscle during inhibition), whereas a positive correlation was obtained between soleus TEP and quadriceps TEP (conditioning muscle during facilitation). The similar modulations between the two responses suggest that TEP can be an alternative to the H reflex for studying spinal circuits, with the advantage of offering insight into the activity of multiple lower-limb muscles.