Single neuron electrophysiology of transcranial magnetic stimulation. I. Passive responses

Transcranial magnetic stimulation (TMS) is a widely used noninvasive stimulation technique that induces neurons to discharge via a rapidly changing magnetic field directed through the scalp. However, the interaction between neural tissue and TMS stimulation is not well understood. Kamitani et al. [1] and Miyawaki et al. [2] have investigated the afterhyperpolarization that follows TMS evoked discharge in a single cortical neuron model, but they described only the discharge response and silent period evoked by a single TMS stimulus without details as to the orientation of the stimulating E-field. We have previously reported that spiking is induced only when the primary electric (E) field is oriented from the soma toward the apical dendrite (antidromic stimulus). In contrast, an E-field oriented in the opposite direction from the apical dendrite toward the soma (orthodromic stimulus) evokes an excitatory potential (EP) considerably larger (3 mV) than that of an excitatory postsynaptic potential (about 0.75 mV at -70 mV resting potential in this system) [4]. The EP is not sufficient to induce cell firing [4]. To better understand this behavior, we explored the passive membrane response of the model system to different stimulus magnitudes.