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.
Highlights
Electrical stimulation is commonly used to promote recovery of motor function in upper motoneuron lesions
The soleus H reflexes, combined from both legs, remained unaltered after transspinal stimulation (Fig 1F; F1,426 = 2.04, p = .153). These results suggest that repeated transspinal stimulation decreased soleus H-reflex excitability in both motor incomplete and complete spinal cord injury (SCI), but remained unaltered in healthy control subjects whom have physiological reflex excitability
The major findings of this study are that transspinal stimulation 1) reduced soleus H-reflex excitability in both motor incomplete and complete SCI but not in healthy control subjects, 2) restored the amplitude of monosynaptic motoneuron responses following repetitive excitation of muscle spindle group Ia afferents at low stimulation frequencies regardless the severity of SCI, 3) did not affect postactivation depression in any of the subject groups, and 4) decreased the severity and frequency of spasms and ankle clonus
Summary
Electrical stimulation is commonly used to promote recovery of motor function in upper motoneuron lesions. Stimulation of the primary motor cortex, spinal cord, peripheral nerve (s), and muscle(s) has been employed to improve sensorimotor function after spinal cord injury (SCI) [1,2,3,4,5,6]. The soleus TEP summates with the soleus H reflex when these two action potentials interact at the peripheral nerve axons [11], while at long stimulation delays the H reflex is depressed or completely abolished [7,11]. A single session of transspinal stimulation decreased ankle clonus and spasticity in a few cases of incomplete or complete SCI [12,13,14], while multiple sessions improved responsiveness of motoneurons spanning multiple spinal segments after SCI in humans [10]
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