Abstract
Coupling between cervical and lumbar spinal networks (cervico-lumbar coupling) is vital during human locomotion. Impaired cervico-lumbar coupling after neural injuries or diseases can be reengaged via simultaneous arm and leg cycling training. Sensorimotor circuitry including cervico-lumbar coupling may further be enhanced by non-invasive modulation of spinal circuity using transcutaneous spinal cord stimulation (tSCS). This project aimed to determine the effect of cervical, lumbar, or combined tSCS on spinal reflex (Hoffmann [H-]) and corticospinal (motor evoked potential [MEP]) excitability during a static or cycling cervico-lumbar coupling task. Fourteen neurologically intact study participants were seated in a recumbent leg cycling system. H-reflex and MEP amplitudes were assessed in the left flexor carpi radialis (FCR) muscle during two tasks (Static and Cycling) and four conditions: (1) No tSCS, (2) tSCS applied to the cervical enlargement (Cervical); (3) tSCS applied to the lumbar enlargement (Lumbar); (4) simultaneous cervical and lumbar tSCS (Combined). While cervical tSCS did not alter FCR H-reflex amplitude relative to No tSCS, lumbar tSCS significantly facilitated H-reflex amplitude by 11.1%, and combined cervical and lumbar tSCS significantly enhanced the facilitation to 19.6%. Neither cervical nor lumbar tSCS altered MEP amplitude alone (+4.9 and 1.8% relative to legs static, No tSCS); however, combined tSCS significantly increased MEP amplitude by 19.7% compared to No tSCS. Leg cycling alone significantly suppressed the FCR H-reflex relative to static, No tSCS by 13.6%, while facilitating MEP amplitude by 18.6%. When combined with leg cycling, tSCS was unable to alter excitability for any condition. This indicates that in neurologically intact individuals where interlimb coordination and corticospinal tract are intact, the effect of leg cycling on cervico-lumbar coupling and corticospinal drive was not impacted significantly with the tSCS intensity used. This study demonstrates, for the first time, that tonic activation of spinal cord networks through multiple sites of tSCS provides a facilitation of both spinal reflex and corticospinal pathways. It remains vital to determine if combined tSCS can influence interlimb coupling after neural injury or disease when cervico-lumbar connectivity is impaired.
Highlights
The recent surge of investigations in modulating the circuitry of the spinal cord by means of non-invasive transcutaneous spinal cord stimulation suggests that this approach has the potential to facilitate improved sensorimotor rehabilitation (Balykin et al, 2017; Inanici et al, 2018)
H-reflexes and motor evoked potential (MEP) were assessed in the flexor carpi radialis (FCR) muscle of the left arm during two tasks and four conditions: (1) No transcutaneous spinal cord stimulation (tSCS), (2) tSCS applied to the cervical enlargement (Cervical); (3) tSCS applied to the lumbar enlargement (Lumbar); and (4) simultaneous cervical and lumbar tSCS (Combined) (Figure 1)
TSCS provides functional improvements in the upper and lower limbs in people with a spinal cord injury (SCI), there is a continued lack of knowledge regarding the neuromodulation in sensorimotor circuitry that occurs with its use
Summary
The recent surge of investigations in modulating the circuitry of the spinal cord by means of non-invasive transcutaneous spinal cord stimulation (tSCS) suggests that this approach has the potential to facilitate improved sensorimotor rehabilitation (Balykin et al, 2017; Inanici et al, 2018). ESCS applied to the lumbar spinal cord, in conjunction with intensive locomotor training, allowed persons with clinically complete SCI to walk over ground for short distances (Angeli et al, 2018; Gill et al, 2018). This demonstrated that dormant neurons that survived the injury may be reengaged with spinal neuromodulation and produce stepping-like movements (Courtine et al, 2009; Angeli et al, 2014). To quadrupedal mammals, a bidirectional linkage between the cervical and lumbar segments of the spinal cord during rhythmic movements is present in humans (Dietz, 2002; Zehr et al, 2009), facilitated primarily by propriospinal connections (Frigon et al, 2004; Ferris et al, 2006)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
