Sinha Nirvik, Dewald Julius P A, Heckman Charles J, Yang Yuan
Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States.
School of Medical Science and Technology, Indian Institute of Technology, Kharagpur, India.
Front Syst Neurosci. 2020 Jan 14;13:86. doi: 10.3389/fnsys.2019.00086. eCollection 2019.
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.
神经振荡的耦合对于通过直接和间接下行运动通路将皮质运动指令传递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同频耦合，即皮质-肌肉相干性，它被认为反映了单突触皮质脊髓通路中的运动指令传递。与这条直接通路相比，间接皮质延髓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峰间期的调制，从而产生交叉频率耦合。本理论研究旨在使用计算机模拟评估下行通路中突触层数对脊髓上输入与运动神经元池累积输出之间交叉频率耦合表达的影响。我们使用霍奇金-赫胥黎风格的神经元模型，将下行通路模拟为具有终端运动神经元池的不同层中间神经元。使用一种新颖的广义相干度量，即n:m相干性，计算脊髓上输入与运动神经元池输出之间的交叉频率耦合和同频耦合。我们发现，同频耦合仅在单突触皮质脊髓束中占主导地位，而交叉频率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，相对于单突触直接通路，对多突触间接通路的依赖增加会增强脊髓上输入与运动神经元池输出之间交叉频率耦合的主导地位。这些结果为未来人类受试者研究提供了理论基础，该研究将定量评估间接与直接通路中的运动指令传递及其在诸如单侧脑损伤等神经系统疾病后的变化。
