When Spinal Neuromodulation Meets Sensorimotor Rehabilitation: Lessons Learned From Animal Models to Regain Manual Dexterity After a Spinal Cord Injury

The effect of transcutaneous spinal cord stimulation on the balance and neurophysiological characteristics of young healthy adults

Transcutaneous spinal cord stimulation (TSCS) is gaining popularity as a noninvasive alternative to epidural stimulation. However, there is still much to learn about its effects and utility in assisting recovery of motor control. In this study, we applied TSCS to healthy subjects concurrently performing a functional training task to study its effects during a training intervention. We first carried out neurophysiological tests to characterize the H-reflex, H-reflex recovery, and posterior root muscle reflex thresholds, and then conducted balance tests, first without TSCS and then with TSCS. Balance tests included trunk perturbations in forward, backward, left, and right directions, and subjects' balance was characterized by their response to force perturbations. A balance training task involved the subjects playing a catch-and-throw game in virtual reality (VR) while receiving trunk perturbations and TSCS. Balance tests with and without TSCS were conducted after the VR training to measure subjects' post-training balance characteristics and then neurophysiological tests were carried out again. Statistical comparisons using t-tests between the balance and neurophysiological data collected before and after the VR training intervention found that the immediate effect of TSCS was to increase muscle activity during forward perturbations and to reduce balance performance in that direction. Muscle activity decreased after training and even more once TSCS was turned off. We thus observed an interaction of effects where TSCS increased muscle activity while the physical training decreased it.

Challenges in Translating Regenerative Therapies for Spinal Cord Injury

Regenerating the injured spinal cord is a substantial challenge with many obstacles that need to be overcome to achieve robust functional benefits. This abundance of hurdles can partly explain the limited success when applying regenerative intervention treatments in animal models and/or people. In this article, we elaborate on a few of these obstacles, starting with the applicability of animal models and how they compare to the clinical setting. We then discuss the requirement for combinatorial interventions and the associated problems in experimental design, including the addition of rehabilitative training. The article expands on differences in lesion sizes and locations between humans and common animal models, and how this difference can determine the success or failure of an intervention. An additional and frequently overlooked problem in the translation of interventions that applies beyond the field of neuroregeneration is the reporting bias and the lack of transparency in reporting findings. New data mandates are tackling this problem and will eventually result in a more balanced view of the field. Finally, we will discuss strategies to negotiate the challenging course of successful translation to facilitate successful translation of regeneration promoting interventions.

Cervical transcutaneous spinal stimulation for spinal motor mapping

Transcutaneous spinal stimulation (TSS) is a promising approach to restore upper-limb (UL) functions after spinal cord injury (SCI) in humans. We sought to demonstrate the selectivity of recruitment of individual UL motor pools during cervical TSS using different electrode placements. We demonstrated that TSS delivered over the rostrocaudal and mediolateral axes of the cervical spine resulted in a preferential activation of proximal, distal, and ipsilateral UL muscles. This was revealed by changes in motor threshold intensity, maximum amplitude, and the amount of post-activation depression of the evoked responses. We propose that an arrangement of electrodes targeting specific UL motor pools may result in superior efficacy, restoring more diverse motor activities after neurological injuries and disorders, including severe SCI.

•

Cervical spinal motor maps are created by transcutaneous spinal stimulation

•

Upper limb muscle-specific stimulation coincides with segmental motor pool locations

•

Lateral stimulation can be advantageous depending on rostral or caudal sites

•

Site-specific stimulation can help determine therapeutic outcomes after SCI