Transcutaneous spinal cord stimulation of the cervical cord modulates lumbar networks

Abstract

It has been established that coordinated arm and leg (A&L) cycling facilitates corticospinal drive and modulation of cervico-lumbar connectivity and ultimately improves overground walking in people with incomplete spinal cord injury or stroke. This study examined the effect of noninvasive transcutaneous spinal cord stimulation (tSCS) on the modulation of cervico-lumbar connectivity. Thirteen neurologically intact adults participated in the study. The excitability of the Hoffmann (H) reflex elicited in the soleus muscle was examined under multiple conditions involving either the arms held in a static position or rhythmic arm cycling while tSCS was applied to either the cervical or lumbar cord. As expected, soleus H-reflex amplitude was significantly suppressed by 19.2% during arm cycling (without tSCS) relative to arms static (without tSCS). Interestingly, tSCS of the cervical cord with arms static significantly suppressed the soleus H-reflex (−22.9%), whereas tSCS over the lumbar cord did not suppress the soleus H-reflex (−3.8%). The combination of arm cycling with cervical or lumbar tSCS did not yield additional suppression of the soleus H-reflex beyond that obtained with arm cycling alone or cervical tSCS alone. The results demonstrate that activation of the cervical spinal cord through both rhythmic arm cycling and tonic tSCS significantly modulates the activity of lumbar networks. This highlights the potential for engaging cervical spinal cord networks through tSCS during rehabilitation interventions to enhance cervico-lumbar connectivity. This connectivity is influential in facilitating improvements in walking function after neurological impairment. NEW & NOTEWORTHY This is the first study to investigate the modulatory effects of transcutaneous spinal cord stimulation (tSCS) on cervico-lumbar connectivity. We report that both rhythmic activation of the cervical spinal cord through arm cycling and tonic activation of the cervical cord through tSCS significantly modulate the activity of lumbar networks. This suggests that engaging cervical spinal cord networks through tSCS during locomotor retraining interventions may not only enhance cervico-lumbar connectivity but also further improve walking capacity.

Simultaneous Cervical and Lumbar Spinal Cord Stimulation Induces Facilitation of Both Spinal and Corticospinal Circuitry in Humans

“…Enhancing cervico-lumbar connectivity by pairing A&L cycling with tSCS may further improve mobility outcomes (Pradarelli et al, 2020). Recently, we showed that cervical tSCS significantly suppresses the soleus H-reflex (the remote limb) (Barss et al, 2019), similarly to the effect of arm cycling on the soleus H-reflex. Benavides and colleagues showed that after 20 min of tSCS, the amplitude of subcortical MEP (i.e., cervicomedullary evoked potentials or CMEP) increased, but not the amplitude of MEPs (Benavides et al, 2020).…”

“…It may be possible that non-invasive spinal stimulation activates the spinal motor pools by increasing sensory inputs through Ia afferents (Sayenko et al, 2019). Our previous investigation determined that tonic activation of the cervical region through tSCS suppresses the amplitude of the soleus H-reflex (Barss et al, 2019) to a similar extent as that produced by rhythmic arm cycling (Zhou et al, 2018b), indicating that tSCS may also engage propriospinal interneuronal connections exerting effects on multiple segments of the spinal cord. Therefore, it was tempting to assume this suppressive effect would be bidirectional: tonic activation of the lumbar networks of the spinal cord by tSCS would reduce the amplitude of the H-reflex in FCR similarly to the suppressive effect rhythmic leg cycling has on the FCR H-reflex.…”

“…Building on our previous work (Barss et al, 2019), this project aimed to determine the effect of cervical, lumbar, or combined tSCS on spinal (H-reflex) and corticospinal (MEP) excitability during a static or cycling task. The neuromodulatory effects of tSCS were assessed in neurologically intact study participants who were seated in a recumbent leg cycling system.…”

“…M-wave in the arm or leg due to tSCS in the current work. This approach to identifying the maximal tolerable intensity was used to ensure relative similarity in stimulation intensity between individuals compared to the threshold intensity used in our previous investigation (Barss et al, 2019).…”

Neuromodulation in the treatment of symptoms of spinal cord injury

“…A recent study by Barss et al emphasize the significant impact of percutaneous stimulation on the improvement of cervical-lumbar connectivity of the spinal cord, which is one of the goals of rehabilitation procedures in patients with spinal cord injury. It is reported that both the rhythmic activation of the cervical spinal cord by cyclic arm movements, with the use of specially adapted ergometers, and the tonic activation of the cervical spinal cord by tSCS significantly modulate the activity of the lumbar neural networks [27]. This action may also lead to the improvement of locomotor functions, mainly walking [5,27].…”