Neural Substrates of Transcutaneous Spinal Cord Stimulation: Neuromodulation across Multiple Segments of the Spinal Cord

Abstract

Transcutaneous spinal cord stimulation (tSCS) has the potential to promote improved sensorimotor rehabilitation by modulating the circuitry of the spinal cord non-invasively. Little is currently known about how cervical or lumbar tSCS influences the excitability of spinal and corticospinal networks, or whether the synergistic effects of multi-segmental tSCS occur between remote segments of the spinal cord. The aim of this review is to describe the emergence and development of tSCS as a novel method to modulate the spinal cord, while highlighting the effectiveness of tSCS in improving sensorimotor recovery after spinal cord injury. This review underscores the ability of single-site tSCS to alter excitability across multiple segments of the spinal cord, while multiple sites of tSCS converge to facilitate spinal reflex and corticospinal networks. Finally, the potential and current limitations for engaging cervical and lumbar spinal cord networks through tSCS to enhance the effectiveness of rehabilitation interventions are discussed. Further mechanistic work is needed in order to optimize targeted rehabilitation strategies and improve clinical outcomes.

Preferential activation of proprioceptive and cutaneous sensory fibers compared to motor fibers during cervical transcutaneous spinal cord stimulation: a computational study

“…These implications should be carefully considered when other electrode configurations (e.g. the anode electrode is placed on the iliac crests [1][2][3][4]) are used while continuous stimulation is delivered, since the excitability of the sensory and motor fibers could be different [16,64]. For instance, de Freitas et al [16] suggested that nerve fiber activation thresholds are higher when the anode is placed at the iliac crests compared to the anterior neck anode placement.…”

“…Cervical transcutaneous spinal cord stimulation (tSCS) has recently emerged as a non-invasive rehabilitation technology for recovery of upper-limb motor function after spinal cord injury [1][2][3][4]. Neuromodulation of cervical spinal neural circuitries may occur when cervical tSCS is combined with supraspinal descending drive, promoting neuroplasticity across the damaged spinal regions [1,[5][6][7].…”

Modulations in neural pathways excitability post transcutaneous spinal cord stimulation among individuals with spinal cord injury: a systematic review

“…In recent times, transcutaneous spinal cord stimulation (TSCS), a non-invasive method of spinal stimulation, has emerged as a successful neuromodulation technique for enhancing motor function post-SCI ( Megía García et al, 2020 ; Taylor et al, 2021 ; Rehman et al, 2023 ). TSCS is generally divided into two different categories: direct current (DC-TSCS) and alternating current (AC-TSCS) ( Taylor et al, 2021 ; Barss et al, 2022 ; Rahman et al, 2022 ). In DC-TSCS (also called transspinal direct current stimulation: tsDCS), a constant electrical current, usually in the range of 1–2.5 mA, flows in a constant direction and the polarity of stimulation is defined based on the polarity of the electrode placed on the spine (cathodal or anodal) ( Grecco et al, 2015 ; Rahman et al, 2022 ).…”

Transcutaneous spinal stimulation in people with and without spinal cord injury: Effect of electrode placement and trains of stimulation on threshold intensity

“…Transcutaneous spinal cord stimulation (TSS) has emerged as a complementary therapy to restore motor function in people with spinal cord injury (SCI; Megía‐García et al, 2020 ; Rahman et al, 2022 ; Shackleton et al, 2022 ). By stimulating afferent nerve fibers in the dorsal roots, TSS is thought to reflexively increase motoneurone excitability, which improves the ability of descending motor drive to recruit additional motoneurones below the level of the SCI (Barss et al, 2022 ; Mayr et al, 2016 ; Taccola et al, 2018 ). Despite its growing popularity, some fundamental aspects of TSS methodology have yet to be explored systematically.…”