Can local vibration alter the contribution of persistent inward currents to human motoneuron firing?. (22nd March 2023)

• Record Type: Journal Article
• Authors: Lapole, T.; Mesquita, R. N. O.; Baudry, S.; Souron, R.; Brownstein, C. G.; Rozand, V.
• Is Part Of: Journal of physiology. Volume 601:Number 8(2023)
• Page Start: 1467
• Page End: 1482
• Publication Date: 2023-03-22
• Subjects: high‐density electromyography -- Ia afferents -- motoneuron excitability -- muscle spindles -- neuromodulation; Physiology -- Periodicals; 612.005
• DOI: 10.1113/JP284210
• Languages: English
• ISSNs: 0022-3751

Abstract

Abstract : Abstract: The response of spinal motoneurons to synaptic input greatly depends on the activation of persistent inward currents (PICs), which in turn are enhanced by the neuromodulators serotonin and noradrenaline. Local vibration (LV) induces excitatory Ia input onto motoneurons and may alter neuromodulatory inputs. Therefore, we investigated whether LV influences the contribution of PICs to motoneuron firing. This was assessed in voluntary contractions with concurrent, ongoing LV, as well as after a bout of prolonged LV. High‐density surface electromyograms (HD‐EMG) of the tibialis anterior were recorded with a 64‐electrode matrix. Twenty males performed isometric, triangular, dorsiflexion contractions to 20% and 50% of maximal torque at baseline, during LV of the tibialis anterior muscle, and after 30‐min of LV. HD‐EMG signals were decomposed, and motor units tracked across time points to estimate PICs through a paired motor unit analysis, which quantifies motor unit recruitment–derecruitment hysteresis (Δ F ). During ongoing LV, Δ F was lower for both 20% and 50% ramps. Although significant changes in Δ F were not observed after prolonged LV, a differential effect across the motoneuron pool was observed. This study demonstrates that PICs can be non‐pharmacologically modulated by LV. Given that LV leads to reflexive motor unit activation, it is postulated that lower PIC contribution to motoneuron firing during ongoing LV results from decreased neuromodulatory inputs associated with lower descending corticospinal drive. A differential effect in motoneurons of different recruitment thresholds after prolonged LV is provocative, challenging the interpretation of previous observations and motivating future investigations. Key points: Neuromodulatory inputs from the brainstem influence motoneuron intrinsic excitability through activation of persistent inward currents (PICs). PICs make motoneurons more responsive to excitatory input. We demonstrate that vibration applied on the muscle modulates the contribution of PICs to motoneuron firing, as observed through analysis of the firing of single motor units. The effects of PICs on motoneuron firing were lower when vibration was concurrently applied during voluntary ramp contractions, likely due to lower levels of neuromodulation. Additionally, prolonged exposure to vibration led to differential effects of lower‐ vs . higher‐threshold motor units on PICs, with lower‐threshold motor units tending to present an increased and higher‐threshold motor units a decreased contribution of PICs to motoneuron firing. These results demonstrate that muscle vibration has the potential to influence the effects of neuromodulation on motoneuron firing. The potential of using vibration as a non‐pharmacological neuromodulatory intervention should be further investigated. Abstract : Abstract figure legend Neuromodulatory inputs from the brainstem influence motoneuron intrinsic excitability through activation of persistent inward currents (PICs), making motoneurons more responsive to synaptic input. The Δ F method was used to estimate the contribution of PICs to the prolongation of human motoneuron firing. In experiment 1, we observed a lower contribution of PICs to motoneuron firing when vibration was concurrently applied during voluntary ramp dorsiflexions (participants were asked to follow a force trace). An increased contribution of vibration‐induced reflex activation to force output (i.e. greater Ia afferent discharge) likely led to lower magnitudes of descending voluntary drive and neuromodulatory inputs. In experiment 2, prolonged exposure to vibration had different effects on the contribution of PICs to lower‐ vs . higher‐threshold motor units in subsequent ramp contractions. Our results suggest that prolonged vibration can increase contribution of PICs to motoneuron firing in lower‐threshold motor units, but decrease this contribution in higher‐threshold motor units.