Gomes Matheus M, Jenz Sophia T, Beauchamp James A, Negro Francesco, Heckman C J, Pearcey Gregory E P
Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, USA.
School of Physical Education and Sport of Ribeirão Preto, University of São Paulo, Ribeirão Preto, Brazil.
bioRxiv. 2024 Mar 3:2024.02.28.582534. doi: 10.1101/2024.02.28.582534.
Motoneuronal persistent inward currents (PICs) are both facilitated by neuromodulatory inputs and highly sensitive to local inhibitory circuits (e.g., Ia reciprocal inhibition). Methods aimed to increase group Ia reciprocal inhibition from the antagonistic muscle have been successful in decreasing PICs, and the diffuse actions of neuromodulators released during activation of remote muscles have increased PICs. However, it remains unknown how motoneurons function in the presence of simultaneous excitatory and inhibitory commands. To probe this topic, we investigated motor unit (MU) discharge patterns and estimated PICs during voluntary co-contraction of ankle muscles, which simultaneously demands the contraction of agonist-antagonist pairs. Twenty young adults randomly performed triangular ramps (10s up and down) of both co-contraction (simultaneous dorsiflexion and plantarflexion) and isometric dorsiflexion to a peak of 30% of their maximum muscle activity from a maximal voluntary contraction. Motor unit spike trains were decomposed from high-density surface electromyography recorded over the tibialis anterior (TA) using blind source separation algorithms. Voluntary co-contraction altered motor unit discharge rate characteristics, decreasing estimates of PICs by 20% (4.47 pulses per second (pps) vs 5.57 pps during isometric dorsiflexion). These findings suggest that, during voluntary co-contraction, the inhibitory input from the antagonist muscle overcomes the additional excitatory and neuromodulatory drive that may occur due to the co-contraction of the antagonist muscle, which constrains PIC behavior.
运动神经元持续性内向电流（PICs）既受神经调节输入的促进，又对局部抑制性回路（如Ia类交互抑制）高度敏感。旨在增强拮抗肌Ia类交互抑制的方法已成功降低了PICs，而在远隔肌肉激活过程中释放的神经调质的弥散作用则增加了PICs。然而，运动神经元在同时存在兴奋性和抑制性指令时如何发挥作用仍不清楚。为了探究这个问题，我们研究了在踝关节肌肉自主协同收缩期间的运动单位（MU）放电模式并估计了PICs，踝关节肌肉自主协同收缩同时需要主动肌-拮抗肌对的收缩。20名年轻成年人随机进行了协同收缩（同时背屈和跖屈）和等长背屈的三角斜坡运动（上下各10秒），收缩强度达到其最大自主收缩时最大肌肉活动的30%峰值。使用盲源分离算法从胫骨前肌（TA）表面记录的高密度肌电图中分解出运动单位放电序列。自主协同收缩改变了运动单位放电率特征，使PICs估计值降低了20%（等长背屈时为每秒5.57个脉冲（pps），协同收缩时为每秒4.47个脉冲）。这些发现表明，在自主协同收缩期间，拮抗肌的抑制性输入克服了由于拮抗肌协同收缩可能产生的额外兴奋性和神经调节驱动，从而限制了PIC行为。
