Repeated transspinal stimulation decreases soleus H-reflex excitability and restores spinal inhibition in human spinal cord injury

Abstract

Transcutaneous spinal cord or transspinal stimulation over the thoracolumbar enlargement, the spinal location of motoneurons innervating leg muscles, modulates neural circuits engaged in the control of movement. The extent to which daily sessions (e.g. repeated) of transspinal stimulation affects soleus H-reflex excitability in individuals with chronic spinal cord injury (SCI) remains largely unknown. In this study, we established the effects of repeated cathodal transspinal stimulation on soleus H-reflex excitability and spinal inhibition in individuals with and without chronic SCI. Ten SCI and 10 healthy control subjects received monophasic transspinal stimuli of 1-ms duration at 0.2 Hz at subthreshold and suprathreshold intensities of the right soleus transspinal evoked potential (TEP). SCI subjects received an average of 16 stimulation sessions, while healthy control subjects received an average of 10 stimulation sessions. Before and one or two days post intervention, we used the soleus H reflex to assess changes in motoneuron recruitment, homosynaptic depression following single tibial nerve stimuli delivered at 0.1, 0.125, 0.2, 0.33 and 1.0 Hz, and postactivation depression following paired tibial nerve stimuli at the interstimulus intervals of 60, 100, 300, and 500 ms. Soleus H-reflex excitability was decreased in both legs in motor incomplete and complete SCI but not in healthy control subjects. Soleus H-reflex homosynaptic and postactivation depression was present in motor incomplete and complete SCI but was of lesser strength to that observed in healthy control subjects. Repeated transspinal stimulation increased homosynaptic depression in all SCI subjects and remained unaltered in healthy controls. Postactivation depression remained unaltered in all subject groups. Lastly, transspinal stimulation decreased the severity of spasms and ankle clonus. The results indicate decreased reflex hyperexcitability and recovery of spinal inhibitory control in the injured human spinal cord with repeated transspinal stimulation. Transspinal stimulation is a noninvasive neuromodulation method for restoring spinally-mediated afferent reflex actions after SCI in humans.

Combined Supra- and Sub-Lesional Epidural Electrical Stimulation for Restoration of the Motor Functions after Spinal Cord Injury in Mini Pigs

“…Increase the threshold and decrease the amplitude of H-response in treated animals may also indicate on decrease in the excitability of corresponding motoneurons. Previous electrophysiological studies demonstrated that spinal cord stimulation may alter the properties of motoneurons, reducing their excitability [ 80 , 81 ]. In addition, the stimulation of the sciatic nerve may activate most of the afferents, including skin afferents and afferents of antagonist muscles [ 82 ], which may have inhibitory influence on recorded activity, and activation of intraspinal inhibitory mechanisms following SCI contributes to the restoration of locomotor activity [ 83 ].…”

Interfacing With Alpha Motor Neurons in Spinal Cord Injury Patients Receiving Trans-spinal Electrical Stimulation

“…In a rehabilitation scenario, knowing these relationships will enable building direct associations between electrical stimulation patterns and the resulting modulation in an individual patient's spinal cord excitability. This will permit using non-invasive electrical stimulation to restore physiological spinal excitability for treating a variety of related motor disorders associated with hyperexcitability of spinal neuronal structures, such as spasticity (37). Moreover, the ability to modulate spinal cord excitability may help understand how to best induce activity-dependent neuro-plasticity, thereby increasing the efficacy of rehabilitation programs (38).…”

A Comparison of FES and SCS for Neuroplastic Recovery After SCI: Historical Perspectives and Future Directions

“…Activation of supraspinal circuitry is also thought to contribute substantially to the attenuation of neural hyperexcitability via descending inhibitory tracts [for review see ( 72 )]. SCS has also been reported to attenuate neural hyperexcitability and recover spinal inhibitory control in people with SCI ( 115 – 117 ), and has been successfully used in the treatment of spasticity ( 118 , 119 ). The effects of tonic, subthreshold SCS on spinal hyperexcitability may be an important mechanism enabling voluntarily-driven movements in people with chronic SCI.…”