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

• Published in: Experimental physiology
• Main Authors: Sordet, Julia, Papaiordanidou, Maria, Amiez, Nicolas, Amiridis, Ioannis, Quenot, Jean‐Pierre, Martin, Alain
• Format: Journal Article
• Language: English
• Published: England Wiley-Blackwell 21.07.2025
• Subjects: Human health and pathology; Life Sciences; Tissues and Organs; posterior root muscle reflex; presynaptic inhibition; primary afferent depolarization; heteronymous facilitation; H reflex
• ISSN: 0958-0670; 1469-445X; 1469-445X
• DOI: 10.1113/EP093023

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 D 1 and D 2 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 (H PSI or TEP PSI and H FAC or TEP FAC ) were compared with unconditioned ones (H TEST or TEP TEST ). Concerning D 1 and D 2 inhibition, results did not reveal any significant difference between the two responses ( p  = 0.89 and p  = 0.51 for D 1 and D 2 , respectively). Inhibition was observed at all ISIs for D 1 and at the 100 and 200 ms ISIs for D 2 . 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. What is the central question of this study? Is the transspinal evoked potential, elicited by transcutaneous spinal cord stimulation, sensitive to inhibitory and facilitatory circuits, similar to the H reflex evoked by peripheral nerve stimulation? What is the main finding and its importance? Both soleus H reflex and transspinal evoked potential are sensitive to D 1 and D 2 inhibition, mediated by activation of the antagonist tibialis anterior afferents, and to heteronymous facilitation, induced by projections of vastus lateralis afferents. These findings suggest that transcutaneous spinal cord stimulation is a valuable tool for investigating spinal circuits, offering the advantage of assessing multiple lower‐limb muscles simultaneously.