A geometric approach to quantifying the neuromodulatory effects of persistent inward currents on individual motor unit discharge patterns.
A geometric approach to quantifying the neuromodulatory effects of persistent inward currents on individual motor unit discharge patterns.
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DOI：
10.1088/1741-2552/acb1d7
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发表时间：
2023-01-30
影响因子：
4
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目标。所有的运动指令都流经运动神经元，运动神经元控制它们所支配的肌肉纤维，形成一个运动单位(MU)。由于MU内动作电位的高保真，它们的放电曲线详细说明了运动神经元的电离性兴奋/抑制以及代谢性神经调制命令的组织。神经调节性输入(如去甲肾上腺素、5-羟色胺)增强运动神经元的兴奋性并促进持续的内向电流(PIC)。PIC通过增加激活时的去极化电流(即PIC放大)，并在低于招募所需的兴奋性输入水平(即PIC延长)时促进放电，从而在MU放电曲线中引入可量化的特性。接近。在这里，我们介绍了一种新的几何方法，通过利用PIC放大在时变线性任务中引入的放电非线性来估计神经调节和抑制对MU放电的贡献。具体地说，我们量化了与线性流量的偏差(支撑高度)和流量的变化率(即加速斜率、衰减斜率、角度)。我们在一个具有已知兴奋性、抑制性和神经调节性输入的模拟运动神经元池上，以及在人类MU(MU数量：胫骨前肌：1448，腓肠肌内侧：2100，比目鱼肌：1062，第一背侧骨间肌：2296)上进一步描述了这些指标。主要结果。在模拟的运动池中，我们发现支架高度和衰减斜率分别一致地指示神经调节和抑制模式(兴奋-抑制耦合)的变化，而配对MU分析(ΔF)取决于神经调节和抑制模式。此外，我们在人类MU中提供了对这些指标的估计，并显示了在多个试验中匹配的MU的ΔF和支架高度测量的可比性。意义重大。跨越两个数据集，我们发现支架高度量化提供了一种直观的方法来实现对单个MU的神经调节和抑制驱动的分级估计。这是对常用技术的补充，并为神经调节水平和抑制性运动指令模式的变化提供了一条脱钩的途径。
Objective. All motor commands flow through motoneurons, which entrain control of their innervated muscle fibers, forming a motor unit (MU). Owing to the high fidelity of action potentials within MUs, their discharge profiles detail the organization of ionotropic excitatory/inhibitory as well as metabotropic neuromodulatory commands to motoneurons. Neuromodulatory inputs (e.g. norepinephrine, serotonin) enhance motoneuron excitability and facilitate persistent inward currents (PICs). PICs introduce quantifiable properties in MU discharge profiles by augmenting depolarizing currents upon activation (i.e. PIC amplification) and facilitating discharge at lower levels of excitatory input than required for recruitment (i.e. PIC prolongation). Approach. Here, we introduce a novel geometric approach to estimate neuromodulatory and inhibitory contributions to MU discharge by exploiting discharge non-linearities introduced by PIC amplification during time-varying linear tasks. In specific, we quantify the deviation from linear discharge (‘brace height’) and the rate of change in discharge (i.e. acceleration slope, attenuation slope, angle). We further characterize these metrics on a simulated motoneuron pool with known excitatory, inhibitory, and neuromodulatory inputs and on human MUs (number of MUs; Tibialis Anterior: 1448, Medial Gastrocnemius: 2100, Soleus: 1062, First Dorsal Interosseus: 2296). Main results. In the simulated motor pool, we found brace height and attenuation slope to consistently indicate changes in neuromodulation and the pattern of inhibition (excitation–inhibition coupling), respectively, whereas the paired MU analysis (ΔF) was dependent on both neuromodulation and inhibition pattern. Furthermore, we provide estimates of these metrics in human MUs and show comparable variability in ΔF and brace height measures for MUs matched across multiple trials. Significance. Spanning both datasets, we found brace height quantification to provide an intuitive method for achieving graded estimates of neuromodulatory and inhibitory drive to individual MUs. This complements common techniques and provides an avenue for decoupling changes in the level of neuromodulatory and pattern of inhibitory motor commands.
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