PDF(Pubmed)
Abstract：
Transcranial (electro)magnetic stimulation (TMS) is currently the method of choice to non-invasively induce neural activity in the human brain. A single transcranial stimulus induces a time-varying electric field in the brain that may evoke action potentials in cortical neurons. The spatial relationship between the locally induced electric field and the stimulated neurons determines axonal depolarization. The induced electric field is influenced by the conductive properties of the tissue compartments and is strongest in the superficial parts of the targeted cortical gyri and underlying white matter. TMS likely targets axons of both excitatory and inhibitory neurons. The propensity of individual axons to fire an action potential in response to TMS depends on their geometry, myelination and spatial relation to the imposed electric field and the physiological state of the neuron. The latter is determined by its transsynaptic dendritic and somatic inputs, intrinsic membrane potential and firing rate. Modeling work suggests that the primary target of TMS is axonal terminals in the crown top and lip regions of cortical gyri. The induced electric field may additionally excite bends of myelinated axons in the juxtacortical white matter below the gyral crown. Neuronal excitation spreads ortho- and antidromically along the stimulated axons and causes secondary excitation of connected neuronal populations within local intracortical microcircuits in the target area. Axonal and transsynaptic spread of excitation also occurs along cortico-cortical and cortico-subcortical connections, impacting on neuronal activity in the targeted network. Both local and remote neural excitation depend critically on the functional state of the stimulated target area and network. TMS also causes substantial direct co-stimulation of the peripheral nervous system. Peripheral co-excitation propagates centrally in auditory and somatosensory networks, but also produces brain responses in other networks subserving multisensory integration, orienting or arousal. The complexity of the response to TMS warrants cautious interpretation of its physiological and behavioural consequences, and a deeper understanding of the mechanistic underpinnings of TMS will be critical for advancing it as a scientific and therapeutic tool.
摘要：
经颅（电）磁刺激（TMS）是当前非侵入性诱导人脑神经活动的首选方法。单个经颅刺激会在大脑中诱导时变电场，该电场可能会在皮质神经元中引起动作电位。局部诱导电场与受激神经元之间的空间关系决定了轴突去极化。感应电场受组织区室的导电特性的影响，并且在目标皮质回和下面的白质的浅表部分中最强。TMS可能靶向兴奋性和抑制性神经元的轴突。个体轴突激发动作电位以响应TMS的倾向取决于它们的几何形状，髓鞘形成和与施加的电场和神经元生理状态的空间关系。后者是由其跨突触树突和体细胞输入决定的，固有膜电位和激发率。建模工作表明，TMS的主要目标是皮质回的冠顶和唇区域的轴突末端。感应的电场可能还会激发回旋冠下近皮层白质中有髓轴突的弯曲。神经元兴奋沿受刺激的轴突正交和反方向传播，并在目标区域的局部皮质内微回路中引起连接的神经元群体的二次兴奋。兴奋的轴突和跨突触扩散也沿着皮质-皮质和皮质-皮质下连接发生，影响目标网络中的神经元活动。局部和远程神经激发都关键取决于受激目标区域和网络的功能状态。TMS还引起外周神经系统的大量直接共刺激。外围共激在听觉和体感网络中集中传播，但也会在其他网络中产生大脑反应，服务于多感官整合，定向或唤醒。对TMS反应的复杂性保证了对其生理和行为后果的谨慎解释，更深入地了解TMS的机械基础对于将其作为科学和治疗工具进行改进至关重要。
