Effects of transcutaneous spinal direct current stimulation on presynaptic inhibition during control tasks of the ankle joint

Abstract

Transcutaneous spinal direct current stimulation (tsDCS) has been shown to modulate spinal excitability even though the underlying neural mechanisms are not known. To increase the knowledge about the plasticity of spinal cord circuitry by tsDCS, we will investigate the effects of stimulating an important neural circuit responsible for regulating the presynaptic inhibition (PSI) of soleus Ia afferents. Although there have been recent studies assessing how tsDCS influences the H-reflex (a specific spinal reflex) amplitude, none has assessed the effects of this technique over both PSI and different sensorimotor tasks. Because PSI influences these tasks, the potential clinical significance of this study lies in the tsDCS-induced plastic changes that we hope to find both in the spinal circuitry that generates PSI and in the functional aspects of sensorimotor tasks. Thus, in the experimental design, subjects (in a seated position) will control: 1) the force level exerted by the foot against a rigid pedal (force task, FT); or 2) the angular position of the ankle when sustaining inertial loads (position task, PT) that requires the same level of muscle activation exerted in FT. Subjects will be instructed to maintain their force/position at target levels set at ~10% of maximum isometric voluntary contraction for FT and 90° for PT, while unconditioned H-reflexes (i.e. control reflexes) and H-reflexes conditioned by electrical pulses applied to the common peroneal nerve with conditioning-to-test intervals of 21 ms and 100 ms (corresponding to D1 and D2 inhibitions, respectively, both associated with the PSI) will be evoked. To analyze tsDCS-induced plastic changes in spinal excitability, the stimulus-response curves of the soleus H-reflex will be determined before and after anodal, cathodal, and sham tsDCS. Also, the influences of the tsDCS on torque/position variability and muscles behavior during the TF and TP will be investigated. It is expected that the results may provide a better understanding of the specific neurophysiological mechanisms and functional aspects of the tasks affected by tsDCS. It is also expected that the new knowledge may result in the development of intervention strategies for rehabilitation and may also enhance the improvements obtained from conventional therapeutic interventions. (AU)