Author(s): GERBE, S., PAPAIORDANIDOU, M., MARTIN, A., Institution: UNIVERSITÉ BOURGOGNE EUROPE, Country: FRANCE, Abstract-ID: 747

INTRODUCTION:
It is well established that muscle strength training effectively increases strength in humans. However, the neural mechanisms underlying these strength gains remain unclear (Tallent et al., 2021). Evidence from studies on primates suggests that these adaptations would be predominantly mediated by the reticulospinal tract (RST) rather than the corticospinal tract (CST) (Glover & Baker, 2020). In humans, RST function can be assessed indirectly using a loud auditory stimulus (>110 dB), combined with magnetic or electrical stimulations at different levels of the nervous system (Furubayashi et al., 2000). Evoked responses conditioned by such stimuli are known to be modulated by the interstimulus interval (ISI), between the auditory and electrical of magnetic stimulation. The aim of this study is to identify the ISI that induces the greatest modulation of these evoked responses.
METHODS:
Fourteen healthy volunteers participated in three randomized sessions. During the first session, motor evoked potentials (MEPs), obtained by transcranial magnetic stimulation, were recorded under three conditioning conditions: a startle auditory stimulus of 115 dB + a red LED (SAS), a 70 dB auditory stimulus + a red LED (AS), and a no-sound condition, red LED only (V). During the second and third sessions, evoked responses by electrical stimulation at the thoracic (TMEPs) and peripheral levels (H-reflexes) were recorded under the same conditions. During the three sessions, five different ISIs were tested (10, 50, 75, 100 and 125 ms). All evoked responses were obtained during 10% of maximal voluntary contraction. The area of the evoked responses was normalized to the area of the muscle compound potential (Mmax).
RESULTS:
Statistical analysis revealed no significant interaction between conditioning and ISIs on the MEP/Mmax ratio (F (3.021, 33.234) = 0.557, p = 0.649, np2 = 0.048). In contrast, significant interactions were observed on the TMEP/Mmax ratio (F (2.982, 38.771) = 8.326, p < 0.001, np2 = 0.390) and the H/Mmax ratio (F (3 .146, 37.752) = 4.092, p = 0.012, np2 = 0.254), indicating that, at ISIs ranging from 75 to 125 ms, these ratios were significantly higher in SAS condition relative to AS and V.
CONCLUSION:
TMEPs and H-reflexes were facilitated at ISIs between 75 and 125 ms without any modulation of MEPs. These results indicate that a startle auditory stimulus increases the excitability of subcortical structures projecting onto alpha-motoneuron pool, particularly the reticulospinal tract. These findings provide a basis for future investigations into the respective contributions of corticospinal and reticulospinal tracts to motor activation.

References:
Furubayashi et al., (2000), Clin. Neurophysiol.
Glover & Baker (2020), J. Neurosci.
Tallent et al., (2021), EJAP.