Effect Of Transcutaneous Electrical Spinal Cord Stimulation On The Functional Activity Of Reciprocal And Presynaptic Inhibition In Healthy Subjects

The objective of our research was to examine an effect of 20-minute electrical stimulation session of the spinal cord (tESCS) on the manifestation of reciprocal and presynaptic inhibition in the system of shin antagonist muscles in healthy individuals in a state of a relative muscle rest and weak muscle effort, along with possible physiological mechanisms of these manifestations. Material and Methods — The study involved 10 healthy men 27 to 35 years of age. Reciprocal and presynaptic inhibition was evaluated by suppressing the amplitude of testing H-reflex of m. soleus under conditions of conditioning stimulation of n. peroneus profundus and testing stimulation of n. tibialis with interstimulus intervals of 3 ms and 100 ms, respectively. Reciprocal inhibition and presynaptic inhibition were recorded during a 20-minute tESCS in the area of T11-T12 thoracic vertebrae at rest, in combination with voluntary muscular effort (5% of the maximum voluntary contraction), and after the stimulation. Results — During 20-minute electrical stimulation of the spinal cord at rest, the severity of reciprocal inhibition decreased, inverting to reciprocal facilitation, while presynaptic inhibition weakened only at the twentieth minute of stimulation; and in the postactivation period, the activity of spinal inhibition processes in the antagonist muscle system corresponded to background values. The most pronounced effect of prolonged tESCS was observed when performing a weak isometric contraction, which was accompanied by an increase in reciprocal inhibition activity during 10 minutes of exposure and 10 minutes after the end of stimulation. The severity of presynaptic inhibition during and after the spinal cord stimulation remained unchanged and complied with background values. The presynaptic inhibition activity was expressed to a greater extent during prolonged stimulation of the spinal cord at rest, and when performing a weak voluntary effort, as well as during the postactivation period. Conclusion — Changes in the spinal inhibitory interactions in the system of antagonist muscles on the basis of tESCS effects during arbitrary muscle tension could probably be explained by the fact that in this case, the inhibitory interneuron circuits of spinal cord were exposed to a wider range supraspinally descending and ascending peripheral effects, compared with spinal cord stimulation in a state of a relative muscle rest.