Coupling of neural oscillations is essential for the transmission of cortical motor commands to motoneuron pools through direct and indirect descending motor pathways. Most studies focus on iso-frequency coupling between brain and muscle activities, i.e., cortico-muscular coherence, which is thought to reflect motor command transmission in the mono-synaptic corticospinal pathway. Compared to this direct pathway, indirect corticobulbospinal motor pathways involve multiple intermediate synaptic connections via spinal interneurons. Neuronal processing of synaptic inputs can lead to modulation of inter-spike intervals which produces cross-frequency coupling. This theoretical study aims to evaluate the effect of the number of synaptic layers in descending pathways on the expression of cross-frequency coupling between supraspinal input and the cumulative output of the motoneuron pool using a computer simulation. We simulated descending pathways as various layers of interneurons with a terminal motoneuron pool using Hogdkin–Huxley styled neuron models. Both cross- and iso-frequency coupling between the supraspinal input and the motorneuron pool output were computed using a novel generalized coherence measure, i.e., n:m coherence. We found that the iso-frequency coupling is only dominant in the mono-synaptic corticospinal tract, while the cross-frequency coupling is dominant in multi-synaptic indirect motor pathways. Furthermore, simulations incorporating both mono-synaptic direct and multi-synaptic indirect descending pathways showed that increased reliance on a multi-synaptic indirect pathway over a mono-synaptic direct pathway enhances the dominance of cross-frequency coupling between the supraspinal input and the motorneuron pool output. These results provide the theoretical basis for future human subject study quantitatively assessing motor command transmission in indirect vs. direct pathways and its changes after neurological disorders such as unilateral brain injury.

中文翻译：

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下行运动通路中的交叉频率耦合：理论与仿真

神经振荡的耦合对于通过直接和间接下行运动通路将皮层运动命令传输到运动神经元池至关重要。大多数研究集中在大脑和肌肉活动之间的等频耦合上，即皮质肌肉一致性，这被认为反映了单突触皮质脊髓通路中的运动命令传递。与这种直接通路相比，间接皮质球脊髓运动通路涉及通过脊髓中间神经元的多个中间突触连接。突触输入的神经元处理可导致产生交叉频率耦合的尖峰间间隔的调制。本理论研究旨在使用计算机模拟评估下行通路中突触层的数量对脊髓上输入与运动神经元池累积输出之间的交叉频率耦合表达的影响。我们使用 Hogdkin-Huxley 风格的神经元模型将下行通路模拟为具有终端运动神经元池的各层中间神经元。脊髓上输入和运动神经元池输出之间的交叉和等频耦合都使用一种新的广义相干性测量来计算，即 n:m 相干性。我们发现等频耦合仅在单突触皮质脊髓束中占主导地位，而跨频耦合在多突触间接运动通路中占主导地位。此外，结合单突触直接和多突触间接下行通路的模拟表明，与单突触直接通路相比，对多突触间接通路的依赖增加增强了脊髓上输入和运动神经元池输出之间交叉频率耦合的优势。这些结果为未来人类受试者研究定量评估间接与直接通路中的运动命令传递及其在单侧脑损伤等神经系统疾病后的变化提供了理论基础。