Ye Hui, Hall Vincent, Hendee Jenna
Department of Biology, Loyola University Chicago, Chicago, IL, United States.
Front Comput Neurosci. 2023 Feb 2;17:1105505. doi: 10.3389/fncom.2023.1105505. eCollection 2023.
The novel micromagnetic stimulation (μMS) technology aims to provide high resolution on neuronal targets. However, consistency of neural activation could be compromised by a lack of surgical accuracy, biological variation, and human errors in operation. We have recently modeled the activation of an unmyelinated axon by a circular micro-coil. Although the coil could activate the axon, its performance sometimes lacked focality and consistency. The site of axonal activation could shift by several experimental factors, including the reversal of the coil current, displacement of the coil, and changes in the intensity of the stimulation. Current clinical practice with transcranial magnetic stimulation (TMS) has suggested that figure-eight coils could provide better performance in magnetic stimulation than circular coils. Here, we estimate the performance of μMS by a figure-eight micro-coil, by exploring the impact of the same experimental factors on its focality and consistency in axonal activation. We derived the analytical expression of the electric field and activating function generated by the figure-eight micro-coil, and estimated the location of axonal activation. Using NEURON modeling of an unmyelinated axon, we found two different types (A and B) of axon activation by the figure-eight micro-coil, mediated by coil currents of reversed direction. Type A activation is triggered by membrane hyperpolarization followed by depolarization; Type B activation is triggered by direct membrane depolarization. Consequently, the two types of stimulation are governed by distinct ion channel mechanisms. In comparison to the circular micro-coil, the figure-eight micro-coil requires significantly less current for axonal activation. Under figure-eight micro-coil stimulation, the site of axonal activation does not change with the reversal of the coil current, displacement of the coil, or changes in the intensity of the stimulation. Ultimately, the figure-eight micro-coil provides a more efficient and consistent site of activation than the circular micro-coil in μMS.
新型微磁刺激（μMS）技术旨在实现对神经元靶点的高分辨率刺激。然而，手术精度不足、生物变异以及操作中的人为误差可能会影响神经激活的一致性。我们最近对圆形微线圈刺激无髓鞘轴突的过程进行了建模。尽管该线圈能够激活轴突，但其性能有时缺乏聚焦性和一致性。轴突激活的位置可能会因多种实验因素而发生偏移，包括线圈电流的反转、线圈的位移以及刺激强度的变化。目前经颅磁刺激（TMS）的临床实践表明，在磁刺激方面，8字形线圈比圆形线圈具有更好的性能。在此，我们通过研究相同实验因素对8字形微线圈轴突激活聚焦性和一致性的影响，来评估μMS中8字形微线圈的性能。我们推导了8字形微线圈产生的电场和激活函数的解析表达式，并估算了轴突激活的位置。通过对无髓鞘轴突进行NEURON建模，我们发现8字形微线圈通过方向相反的线圈电流介导，可产生两种不同类型（A和B）的轴突激活。A类激活由膜超极化后再去极化触发；B类激活由直接的膜去极化触发。因此，这两种类型的刺激受不同的离子通道机制支配。与圆形微线圈相比，8字形微线圈激活轴突所需的电流显著更少。在8字形微线圈刺激下，轴突激活的位置不会随线圈电流的反转、线圈的位移或刺激强度的变化而改变。最终，在μMS中，8字形微线圈比圆形微线圈提供了更高效、更一致的激活位点。
