Institute of Neurology, Campus Biomedico University Rome, Italy ; Fondazione Alberto Sordi - Research Institute for Ageing Rome, Italy.
Front Neural Circuits. 2013 Feb 13;7:18. doi: 10.3389/fncir.2013.00018. eCollection 2013.
Although transcranial magnetic stimulation (TMS) activates a number of different neuron types in the cortex, the final output elicited in corticospinal neurones is surprisingly stereotyped. A single TMS pulse evokes a series of descending corticospinal volleys that are separated from each other by about 1.5 ms (i.e., 670 Hz). This evoked descending corticospinal activity can be directly recorded by an epidural electrode placed over the high cervical cord. The earliest wave is thought to originate from the direct activation of the axons of fast-conducting pyramidal tract neurones (PTN) and is therefore termed "D" wave. The later waves are thought to originate from indirect, trans-synaptic activation of PTNs and are termed "I" waves. The anatomical and computational characteristics of a canonical microcircuit model of cerebral cortex composed of layer II and III and layer V excitatory pyramidal cells, inhibitory interneurons, and cortico-cortical and thalamo-cortical inputs can account for the main characteristics of the corticospinal activity evoked by TMS including its regular and rhythmic nature, the stimulus intensity-dependence and its pharmacological modulation. In this review we summarize present knowledge of the physiological basis of the effects of TMS of the human motor cortex describing possible interactions between TMS and simple canonical microcircuits of neocortex. According to the canonical model, a TMS pulse induces strong depolarization of the excitatory cells in the superficial layers of the circuit. This leads to highly synchronized recruitment of clusters of excitatory neurons, including layer V PTNs, and of inhibitory interneurons producing a high frequency (670 Hz) repetitive discharge of the corticospinal axons. The role of the inhibitory circuits is crucial to entrain the firing of the excitatory networks to produce a high-frequency discharge and to control the number and magnitude of evoked excitatory discharge in layer V PTNs. In summary, simple canonical microcircuits of neocortex can explain activation of corticospinal neurons in human motor cortex by TMS.
尽管经颅磁刺激 (TMS) 会激活皮质中的许多不同类型的神经元，但在皮质脊髓神经元中引发的最终输出却惊人地刻板。单个 TMS 脉冲会引发一连串彼此相隔约 1.5 毫秒（即约 670 Hz）的下行皮质脊髓冲动。这种诱发的下行皮质脊髓活动可以通过放置在高颈段脊髓上方的硬膜外电极直接记录。最早的波被认为起源于快速传导的锥体束神经元 (PTN) 的直接激活，因此称为“D”波。较晚的波被认为起源于 PTN 的间接、突触间激活，因此称为“I”波。由 II 层和 III 层以及 V 层兴奋性锥体细胞、抑制性中间神经元以及皮质-皮质和丘脑-皮质输入组成的经典大脑皮层微电路模型的解剖学和计算特性可以解释 TMS 诱发的皮质脊髓活动的主要特征，包括其规则性和节律性、刺激强度依赖性及其药理学调节。在这篇综述中，我们总结了目前关于 TMS 对人类运动皮层影响的生理基础的知识，描述了 TMS 与新皮层简单经典微电路之间可能的相互作用。根据经典模型，TMS 脉冲会导致电路浅层的兴奋性细胞强烈去极化。这导致兴奋性神经元簇的高度同步募集，包括 V 层的 PTN 和抑制性中间神经元，从而产生皮质脊髓轴突的高频 (~670 Hz) 重复放电。抑制性回路的作用对于使兴奋性网络的放电同步产生高频放电以及控制 V 层 PTN 中诱发的兴奋性放电的数量和幅度至关重要。总之，新皮层的简单经典微电路可以解释 TMS 对人类运动皮层中皮质脊髓神经元的激活。
