T019 models of neuronal response to suprathreshold electric fields

Abstract

Introduction Existing models of non-invasive brain stimulation focus almost exclusively on the calculation and analysis the electric field distribution. This information is necessary but insufficient to predict the effect of the stimulation, which also depends on the response of neurons and neuronal networks to the applied electric field. Objectives To determine the current status of models of neuronal response to suprathreshold electric fields induced by transcranial magnetic stimulation (TMS) and point out limitations and future perspectives. Materials & methods Presentation of a critical overview of studies that model the effect of TMS on neurons and neuronal networks. Results In neuronal stimulation, changes in membrane potential are driven by ionic currents through membrane channels. Action potentials are due to the response of voltage-dependent channels to suprathreshold stimuli. It is well established that only the component of the electric field tangent to the trajectory of a neuron, E s , can alter its membrane potential. In TMS, changes in membrane polarization are most pronounced at terminations and bends, and are proportional to E s and the membrane length constant, λ . These results are usually modelled in terms of Hodgkin–Huxley (HH) type models for the membrane, and the cable model for elongated neuronal structures. The few published studies modelling suprathreshold neuronal responses to TMS vary considerably in their approach. Studies trying to identify stimulation sites looked in more detail at the distribution of the induced electric field and at the neuronal paths within that field. Another study included a detailed description of the morphology and the electrophysiological parameters of a single neuron to investigate the characteristics of the neuronal response to TMS. In the best known study, a detailed model of the motor thalamocortical system implemented. This model could reproduced the generation of I-waves and paired pulse responses. Conclusion Investigating different features of neuronal responses to suprathreshold stimuli requires modelling at different scales, from single neuron to complex networks. The choice of an appropriate scale will depend on the question to be addressed.