Modulation of Motor Evoked Potentials via the Cutaneous Silent Period within Proximal–Distal Muscles in the Upper-Limb

• Published in: Neuroscience Vol. 495; pp. 15 - 24
• Main Authors: Eckert, Nathanial R., Greenwell, Davin, Poston, Brach, Riley, Zachary A.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 15.07.2022
• Subjects: cutaneous silent period; motor evoked potentials; spinal reflex; CSP; AD; TRI; APB; spinal reflex; FCR; PD; ECR; cutaneous silent period; motor evoked potentials; TMS; BIC; EMG; MEPs; Cutaneous Silent Period; Motor Evoked Potentials; Spinal Reflex
• ISSN: 0306-4522; 1873-7544; 1873-7544
• DOI: 10.1016/j.neuroscience.2022.05.026

•The cutaneous silent period (CSP) is traditionally presented to occur within extensor muscles.•This study demonstrates CSP activation in both extensor and flexor muscles of the upper limb.•Results demonstrate a differential level of descending drive suppression via the CSP.•This differential level of suppression may serve to help coordinate grasp versus reach control. A single pulse of high intensity electrical current delivered to the digits of the hand during voluntary contractions produces a period of decreased electromyographic (EMG) activity, known as a cutaneous silent period (CSP) (Caccia and Violini, 1973; Inghilleri et al., 1997; Uncini et al., 1991). Pairing transcranial magnetic stimulation (TMS) with digit stimulation results in motor evoked potentials (MEPs) with reduced amplitudes in a thenar muscle (Kofler, 2008). It is not known if similar behavior can be observed in more proximal upper-limb muscles. The current study investigated the CSP on several muscles throughout the upper-limb. Fourteen subjects performed isometric contractions with the following muscles: abductor pollicis brevis (APB), flexor carpi radialis (FCR), extensor carpi radialis (ECR), biceps brachii (BIC), triceps brachii (TRI), anterior deltoid (AD), and posterior deltoid (PD). During the isometric contractions, subjects experienced three different stimulation conditions: electrical stimulation (10x perceptual threshold) of digit II only (CSP), transcranial magnetic stimulation only (TMS), and a pairing of digit II stimulation and TMS (TMS+). The TMS evoked MEP was significantly greater than the TMS+ MEP for APB (p < 0.001), FCR (p = 0.006), and BIC (p < 0.049) muscles. The opposite relationship was seen within the PD (p < 0.047) muscle. An ANOVA test of normalized MEP values (TMS+/TMS) showed significant differences in APB vs TRI (p = 0.004) and PD (p = 0.003), and in FCR vs TRI (p = 0.046) and PD (p = 0.037) muscles. The results suggest that the CSP modulates descending drive differentially across upper-limb muscles.