Finding the Location of Axonal Activation by a Miniature Magnetic Coil

Abstract

Magnetic stimulation for neural activation is widely used in clinical and lab research. In comparison to electric stimulation using an implanted electrode, stimulation with a large magnetic coil is associated with poor spatial specificity and incapability to stimulate deep brain structures. Recent developments in micromagnetic stimulation (μMS) technology mitigates some of these shortcomings. The sub-millimeter coils can be covered with soft, biocompatible material, and chronically implanted. They can provide highly specific neural stimulation in the deep neural structure. Although the μMS technology is expected to provide a precise location of neural stimulation, the exact site of neural activation is difficult to determine. Furthermore, factors that could cause the shifting of the activation site during μMS have not been fully investigated. To estimate the location of axon activation in μMS, we first derived an analytical expression of the activating function, which predicts the location of membrane depolarization in an unmyelinated axon. Then, we developed a multi-compartment, Hodgkin-Huxley (H-H) type of NEURON model of an unmyelinated axon to test the impact of several important coil parameters on the location of axonal activation. The location of axonal activation was dependent on both the parameters of the stimulus and the biophysics properties of the targeted axon during μMS. The activating function analysis predicted that the location of membrane depolarization and activation could shift due to the reversal of the coil current and the change in the coil-axon distance. The NEURON modeling confirmed these predictions. Interestingly, the NEURON simulation further revealed that the intensity of stimulation played a significant role in the activation location. Moderate or strong coil currents activated the axon at different locations, mediated by two distinct ion channel mechanisms. This study reports several experimental factors that could cause a potential shift in the location of neural activation during μMS, which is essential for further development of this novel technology.

Short-pulsed micro-magnetic stimulation of the vagus nerve

“…Previous studies suggested that axonal activation is ineffective when the electric field component is perpendicular to the axon direction (Amassian et al, 1989;Basser and Roth, 2000;Golestanirad et al, 2018). Thus, the activating function (AF) is the electric field gradient along the direction of the axon, which is also known as the driving function for the activation of the axon (Rattay, 1989;Ye, 2022). The AF [V/m 2 ] when the nerve axon is parallel to the x-axis is as follows,…”

Improving focality and consistency in micromagnetic stimulation

“…This paper will examine the consistency of axonal activation by a figure-eight micro-coil, and investigate the underlying ion channel mechanisms using combined activating function analysis and NEURON modeling. We will compare these results from our recent analysis on a single, circular micro-coil ( Ye, 2022 ). This work provides valuable insights to the further development of μMS technology for focal and consistent neural stimulation.…”

“…This field gradient, or activating function, is directly related to the site of neural activation ( Rattay, 1986 ; Lee and Fried, 2017 ). Therefore, several research endeavors have computed the activating function for the single coil system to identify the site of neural activation ( Lee et al, 2016 ; Ye, 2022 ). We recently simulated axonal activation by a circular micro-coil using activating function analysis and NEURON simulation ( Ye, 2022 ).…”

“…Therefore, several research endeavors have computed the activating function for the single coil system to identify the site of neural activation ( Lee et al, 2016 ; Ye, 2022 ). We recently simulated axonal activation by a circular micro-coil using activating function analysis and NEURON simulation ( Ye, 2022 ). This work suggests that μMS stimulation with a circular micro-coil could be inconsistent, and that the location of axonal activation could shift by several experimental factors, including the reversal of the coil current, increases in the coil-axon distance, and changes in the intensity of stimulation.…”

“…This work suggests that μMS stimulation with a circular micro-coil could be inconsistent, and that the location of axonal activation could shift by several experimental factors, including the reversal of the coil current, increases in the coil-axon distance, and changes in the intensity of stimulation. When a micro-coil with a dimension of 1 mm was tested, the activation site on the axon could shift by several hundred micrometers ( Ye, 2022 ). So far, activating function analysis has not been performed to estimate the focality and consistency of stimulation in a figure-eight coil system for the activation of a straight axon.…”