Factors affecting neural stimulation with magnetic fields

Abstract

High amplitude magnetic field pulses produced by coils external to the body have been used for medical diagnosis since the mid-1980s to stimulate motor neurons in the brain cortex and peripheral nerves. While successful applications have since blossomed, it has only been during the last three years that quantitative dosimetric data have become available. The factors affecting neural stimulation can be divided into three categories broadly related to the characteristics of (i) the stimulus, (ii) the neuron, and (iii) the induced electric field as related to the configuration of the stimulating coil. The stimulus, in the case of magnetic field stimulation, has the form of an exponentially decaying pulse with a small overshoot of the opposite polarity. Physical and electrical properties of a neuron affect its electrical stimulation. Dosimetric considerations are limited to the linear model describing the threshold phenomena, where passive electrical properties and the cable model provide a reasonable approximation of neuron behavior. The electromagnetic variable responsible for stimulation is the spatial derivative of the induced electric field along the neuron axis. This paper examines the factors involved in eliciting threshold excitation of motor neurons by magnetic fields. The description of various factors is largely based on published data except for the analysis of the electromagnetic stimuli induced by various coils.