Motoneuron responsiveness to corticospinal tract stimulation during the silent period induced by transcranial magnetic stimulation

Abstract

Based on H-reflex data, spinal mechanisms are proposed to be responsible for the first 50-80 ms of the transcranial magnetic stimulation (TMS)-induced silent period. As several methodological issues can compromise H-reflex validity as a measure of motoneuron excitability, this study used transmastoid stimulation to elicit cervicomedullary motor evoked potentials (CMEPs) during the silent period. Eleven subjects made 1-3 visits which involved 32 or 44 brief (~3 s) isometric elbow flexor contractions at 25 % of maximal torque. During each contraction, transmastoid stimulation was delivered in isolation to elicit an unconditioned CMEP and at interstimulus intervals (ISIs) ranging from 50 to 150 ms after TMS to elicit a conditioned CMEP. Stimulus intensities for TMS and transmastoid stimulation were set to elicit a silent period of ~200 ms and an unconditioned CMEP of 15, 50, or 85 % of the maximal compound muscle action potential (M ), respectively. At all ISIs and intensities of transmastoid stimulation, the conditioned CMEP was significantly smaller than the unconditioned CMEP (p< 0.001). However, suppression of the conditioned CMEP was significantly less at 85 % compared to 15 or 50 % M (p = 0.001). Contrary to published H-reflex data, the conditioned CMEP did not recover within 50-80 ms, remaining significantly suppressed at the longest ISI tested (150 ms). These data suggest the spinal portion of the TMS-evoked silent period is considerably longer than reported previously. Transmastoid stimulation, unlike peripheral nerve stimulation, does not impact proprioceptive inflow to motoneurons. Hence, relative to the H-reflex, the CMEP will be subjected to greater afferent-mediated disfacilitation and inhibition due to the TMS-induced muscle twitch.

Corticospinal excitability during fatiguing whole body exercise

“…Specifically, the early portion of the silent period (~50ms) has traditionally been related to spinal mechanisms, while the latter portion (>50ms) is thought to result from inhibition within the cortex (Chen et al, 1999; Inghilleri et al, 1993). More recent studies have now suggested that the spinal portion of the silent period may account for a longer segment than originally believed, up to 150ms or even more (Yacyshyn et al, 2016). Finally, several paired pulse paradigms have been used to quantify intracortical excitatory or inhibitory processes.…”

Myths and Methodologies: How loud is the story told by the transcranial magnetic stimulation‐evoked silent period?

“…It should be noted, however, that Yacyshyn et al. () compared CMEPs elicited during a contraction with those elicited during the SP, thus not comparing CMEPs at the same level of neural drive. Although within‐condition results were not reported, it appears that CMEPs were reduced in size in the first ∼80–90 ms, after which they plateaued.…”

“…Notably, the behaviour of the H reflex, which has primarily been used to infer the spinal contribution to the SP, might be confounded by the marked influence of presynaptic inhibition (Zehr, 2002). Recent evidence obtained using direct subcortical activation of corticospinal axons, which is devoid of classical presynaptic influence (Nielsen & Petersen, 1994) and thus arguably a more robust technique to assess motoneuron excitability, suggested that the spinal motoneuronal component of the SP might be substantially greater than previously demonstrated (Yacyshyn et al, 2016). Specifically, when electrical stimulation of the cervicomedullary junction was delivered during a ∼200 ms SP, the index of motoneuron excitability (CMEP) was depressed throughout the 150 ms investigated (Yacyshyn et al, 2016).…”

“…This period of EMG silence is known as the silent period (SP), typically lasts ∼100–300 ms, and historically has been assumed to reflect intracortical inhibition (Säisänen et al., ). Despite the limitations of current methodological approaches, it has been suggested that spinal mechanisms might have a much more prolonged and influential contribution to the SP than previously thought (Yacyshyn, Woo, Price, & McNeil, ). Nevertheless, the relative contributions of supraspinal and spinal factors and the exact underlying mechanisms that modulate SP duration are still subject to considerable debate.…”

“…Despite the limitations of current methodological approaches, it has been suggested that spinal mechanisms might have a much more prolonged and influential c 2019 The Authors. Experimental Physiology c 2019 The Physiological Society contribution to the SP than previously thought (Yacyshyn, Woo, Price, & McNeil, 2016). Nevertheless, the relative contributions of supraspinal and spinal factors and the exact underlying mechanisms that modulate SP duration are still subject to considerable debate.…”