Amassian V E, Eberle L, Maccabee P J, Cracco R Q
Department of Physiology, State Univesity of New York, Brookly 11203.
Electroencephalogr Clin Neurophysiol. 1992 Oct;85(5):291-301. doi: 10.1016/0168-5597(92)90105-k.
To help elucidate some basic principles of magnetic coil (MC) excitation of cerebral cortex, a model system was devised in which mammalian phrenic nerve, or amphibian sciatic nerve with its branches was suspended in appropriate Ringer's solution in a human brain-shaped volume conductor, an inverted plastic skull. The nerve was recorded monophasically out of the volume conductor. The site of nerve excitation by the MC was identified by finding where along the nerve a bipolar electrical stimulus yielded a similar action potential latency. MC excitation of hand-related corticospinal (CT) neurons was modelled by giving the distal end of nerve attached to the lateral skull an initial radial (perpendicular) trajectory, with subsequent bends towards the base and posterior part of the skull; this nerve was optimally excited by a laterally placed figure 8 or round MC when the induced electric field led to outward membrane current at the initial bend. By contrast, nerve given a trajectory modelling CT neurons related to the foot was optimally excited when the coil windings were across the midline, but again when membrane current flowed outward at the first bend. Corticocortical fibers were modelled by placing the nerve in the anteroposterior axis lateral to the midline; with the round MC vertex-tangentially orientated, optimal excitation occurred at the bend nearest the interaural line, i.e., near the peak electric field. The findings emphasize the importance of orientation and direction of current in the MC and fiber bends in determining nerve excitation. The findings in the peripheral nerve-skull model help explain (1) why lateral and vertex-tangentially orientated MCs preferentially excite arm-related CT neurons directly and indirectly (through corticocortical fibers), respectively, and (2) why the MC orientations for optimally exciting directly arm and leg-related CT neurons differ.
为了帮助阐明大脑皮层磁线圈（MC）刺激的一些基本原理，设计了一个模型系统，其中将哺乳动物的膈神经或带有分支的两栖动物坐骨神经悬浮在人脑形状的容积导体（一个倒置的塑料颅骨）中的合适林格氏液中。从容积导体中以单相方式记录神经信号。通过找到沿神经双极电刺激产生相似动作电位潜伏期的位置来确定MC对神经的刺激部位。通过使附着于外侧颅骨的神经远端具有初始径向（垂直）轨迹，随后向颅骨底部和后部弯曲，来模拟与手部相关的皮质脊髓（CT）神经元的MC刺激；当感应电场在初始弯曲处导致外向膜电流时，横向放置的8字形或圆形MC能最佳地刺激该神经。相比之下，当线圈绕组穿过中线时，模拟与足部相关的CT神经元的神经轨迹能被最佳刺激，但同样是在第一个弯曲处膜电流向外流动时。通过将神经置于中线外侧的前后轴上来模拟皮质皮质纤维；当圆形MC的顶点与切线方向对齐时，在最靠近耳间线的弯曲处，即靠近电场峰值处，会出现最佳刺激。这些发现强调了MC中电流的方向和神经纤维弯曲在决定神经刺激方面的重要性。在周围神经 - 颅骨模型中的发现有助于解释：（1）为什么横向和顶点与切线方向对齐的MC分别优先直接和间接（通过皮质皮质纤维）刺激与手臂相关的CT神经元，以及（2）为什么最佳直接刺激与手臂和腿部相关的CT神经元的MC方向不同。
