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Transcutaneous spinal cord stimulation (TSCS) has demonstrated potential to beneficially modulate spinal cord motor and autonomic circuitry. We are interested in pairing cervical TSCS with other forms of nervous system stimulation to enhance synaptic plasticity in circuits serving hand function. We use a novel configuration for cervical TSCS in which the anode is placed anteriorly over ~C4–C5 and the cathode posteriorly over ~T2–T4. We measured the effects of single pulses of TSCS paired with single pulses of motor cortex or median nerve stimulation timed to arrive at the cervical spinal cord at varying intervals. In 13 participants with and 15 participants without chronic cervical spinal cord injury, we observed that subthreshold TSCS facilitates hand muscle responses to motor cortex stimulation, with a tendency toward greater facilitation when TSCS is timed to arrive at cervical synapses simultaneously or up to 10 milliseconds after cortical stimulus arrival. Single pulses of subthreshold TSCS had no effect on the amplitudes of median H-reflex responses or F-wave responses. These findings support a model in which TSCS paired with appropriately timed cortical stimulation has the potential to facilitate convergent transmission between descending motor circuits, segmental afferents, and spinal motor neurons serving the hand. Studies with larger numbers of participants and repetitively paired cortical and spinal stimulation are needed.

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ObjectiveTo increase our knowledge of corticospinal excitability changes in persons with spinal cord injury (SCI) by comparing the cortical silent periods (CSPs) of persons with chronic, incomplete cervical SCI to the CSPs of able‐bodied (AB) volunteers.MethodsThe CSP is a corticospinal inhibitory response (1). It appears as gap in electromyographic (EMG) activity of a voluntarily contracting muscle due to transcranial magnetic stimulation (TMS) delivered to the motor cortex (see Figure A). We measured TMS‐induced CSPs in 5 participants with chronic, incomplete cervical SCI and 5 AB volunteers as an exploratory outcome of a larger study assessing the effects of non‐invasive stimulation in persons with and without SCI. EMG data was collected from the abductor pollicis brevis muscle of the dominant hand in each subject. We recorded a total of 139 CSPs in the SCI participants and 176 in the AB participants. Further analysis of 4 more SCI subjects and 10 more AB subjects is ongoing.ResultsThe preliminary results suggest that the average CSP duration of the SCI group (89 ± 34 ms) was less than that of the AB group (116 ± 30 ms). While statistically insignificant, (p = 0.230; Figure B), our results corroborate previous findings of shortened CSPs in persons with SCI (2). Moreover, the proportion of CSPs with at least one “interruption,” visualized as a momentary spike in EMG activity lasting 5–10 ms (see Figure C), was higher in the SCI group (14.48% of CSPs) than in the AB group (2.56% of CSPs; ns; Figure D).ConclusionsOur preliminary findings, of shorter CSP duration and higher frequency of CSP interruptions in subjects with SCI, may imply that cortical inhibitory pathways are damaged or less excitable in persons with SCI. CSP parameters may be valuable outcome measures in the study of SCI.Support or Funding InformationNew York Spinal Cord Injury Research Board (DOH01‐CARTID‐2015‐00037)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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Objective: We aim to better understand the silent period (SP), an inhibitory counterpart to the well-known motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS), in individuals with spinal cord injury (SCI). Methods: Electromyographic responses were measured in the target abductor pollicis brevis at rest (TMS at 120% of resting motor threshold (RMT)) and during maximal effort (TMS at 110% of RMT). Participants with chronic cervical SCI (n=9) and able-bodied volunteers (n=12) underwent between 3-7 sessions of stimulation on separate days. The primary outcomes were the magnitude and reliability of SP duration, resting and active MEP amplitudes, and RMT. Results: SCI participants showed significantly increased RMT, decreased MEP amplitudes, and non-significantly longer SP duration compared to AB participants. In contrast to high inter-participant variability, SP duration demonstrated reduced intra-participant variability within and across sessions compared with resting and active MEP amplitudes. SCI participants also demonstrated a higher prevalence of SP "interruptions" compared to AB participants. Conclusions: SP reflects a balance between corticospinal excitatory and inhibitory processes. SP duration is more reliable within and across multiple sessions than MEP amplitude. Significance: The higher reliability of SP duration may make it a useful outcome measure for future trials of SCI interventions.