Abstract
Cervical transcutaneous spinal cord stimulation (tSCS) has been utilized in applications for improving upper-limb sensory and motor function in patients with spinal cord injury. Although therapeutic effects of continuous cervical tSCS interventions have been reported, neurophysiological mechanisms remain largely unexplored. Specifically, it is not clear whether sub-threshold intensity and 10-min duration continuous cervical tSCS intervention can affect the central nervous system excitability. Therefore, the purpose of this study was to investigate effects of sub-motor-threshold 10-min continuous cervical tSCS applied at rest on the corticospinal and spinal reflex circuit in ten able-bodied individuals. Neurophysiological assessments were conducted to investigate (1) corticospinal excitability via transcranial magnetic stimulation applied on the primary motor cortex to evoke motor-evoked potentials (MEPs) and (2) spinal reflex excitability via single-pulse tSCS applied at the cervical level to evoke posterior root muscle (PRM) reflexes. Measurements were recorded from multiple upper-limb muscles before, during, and after the intervention. Our results showed that low-intensity and short-duration continuous cervical tSCS intervention applied at rest did not significantly affect corticospinal and spinal reflex excitability. The stimulation duration and/or intensity, as well as other stimulating parameters selection, may therefore be critical for inducing neuromodulatory effects during cervical tSCS.
Highlights
The Friedman test showed that motor-evoked potentials (MEPs) peak-to-peak amplitudes were not statistically significantly different in any of the recorded muscles (FCR: χ2 (2) = 1.38, p = 0.500; extensor carpi radialis (ECR): χ2 (2) = 4.20, p = 0.122; first dorsal interosseous (FDI): χ2 (2) = 5.60, p = 0.061; abductor pollicis brevis (APB): χ2 (2) = 0.60, p = 0.741)
We investigated the effects of relatively short-duration (10 min) and low-intensity continuous cervical transcutaneous spinal cord stimulation (tSCS) applied at rest on the upper-limb muscle MEPs elicited by transcranial magnetic stimulation (TMS) as well as posterior root muscle (PRM) reflexes elicited by singlepulse cervical tSCS
We investigated effects of continuous cervical tSCS on corticospinal and spinal reflex excitability
Summary
It has been demonstrated that lumbar epidural stimulation can contribute to restoration of voluntary motor control of lower-limb muscles, postural stability, and gait function in chronic SCI individuals [2,3,4]. Consistent with epidural SCS approaches, noninvasive transcutaneous spinal cord stimulation (tSCS) has been utilized successfully in applications for improving sensory and motor function during lower-limb voluntary movement [6] and walking using lumbar stimulation [7,8,9,10,11,12], trunk stability and standing with lower thoracic and lumbar stimulation [13,14], and gripping and upper-limb function with cervical stimulation [15,16,17,18,19]. Since the stimulated afferent fibers activated by tSCS have synaptic connections to the spinal interneurons and motoneurons, tSCS application can be utilized to modulate spinal motor excitability
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