Evaluation of H-reflex recruitment curve after application of TENS on the desensitised skin of vertebral column

Electrical stimulation of neuromuscular system has been used in a variety of research and therapeutic applications. Although tri-polar transcutaneous electrical stimulation (TENS) is commonly used to change motoneuron excitabi1ity, but the effect of TENS on synaptic activities through dorsal column stimulation or cutaneous pathways is unknown. So, the aim of this research study was to determine the role of cutaneous receptors in conductance of TENS. For this purpose, 10 healthy non-athlete volunteers were tested in three separated sessions, i.e. control 1 (placebo spray and silent TENS), control 2 {lidocaine spray and silent TENS), and experimental (lidocaine spray and TENS) groups. Tri-polar TENS was used on vertebral column (cathode on Tll, one anode 3cm above and another 6cm below the cathode). For evaluation of motoneuron activity, soleus H-reflex and Mh wave recruitment curve were evaluated. We sprayed water (Placebo) and 10% lidocaine on the vertebral column skin under the electrodes for 20 sec. In the experimental session, tripolar TENS on desensitized skin of the vertebral column was applied. TENS was applied for 15 minutes with a frequency of 100 Hz and a pulse width of 300 µs. The results showed that Hmax evocation intensity decreases after application of placebo and lidocaine spray (p<0.05). The positive slope of H-reflex recruitment curve also increased after application of placebo and lidocaine spray (p<0.05). Based on the above results, we suggest a theory to explain the change of synaptic activities of spinal cord as foIlows: Low-threshold cutaneous receptors diminish the pre-synaptic inhibition of la afferent fibers and application of TENS on the desensitized skin increases fit L5 slope and decreases fit L3 slope of H-reflex recruitment curve. In this study, since only low-threshold cutaneous receptors were inhibited, therefore, the role of cutaneous receptors in conductance of TENS could not be ignored.