To treat diseases associated with vagal nerve control of peripheral organs, it is necessary to selectively activate efferent and afferent fibers in the vagus. As a result of the nerve’s complex anatomy, fiber-specific activation proves challenging. Spatially selective neuromodulation using micromagnetic stimulation(
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Abstract μ MS) is showing incredible promise. This neuromodulation technique uses microcoils(μ coils) to generate magnetic fields by powering them with a time-varying current. Following the principles of Faraday’s law of induction, a highly directional electric field is induced in the nerve from the magnetic field. In this study on rodent cervical vagus, a solenoidalμ coil was oriented at an angle to left and right branches of the nerve. The aim of this study was to measure changes in the mean arterial pressure (MAP) and heart rate (HR) followingμ MS of the vagus. Theμ coils were powered by a single-cycle sinusoidal current varying in pulse widths(PW = 100, 500, and 1000μ sec) at a frequency of 20 Hz. Under the influence of isoflurane,μ MS of the left vagus at 1000μ sec PW led to an average drop in MAP of 16.75 mmHg(n = 7). In contrast,μ MS of the right vagus under isoflurane resulted in an average drop of 11.93 mmHg in the MAP(n = 7). Surprisingly, there were no changes in HR to either right or left vagalμ MS suggesting the drop in MAP associated with vagusμ MS was the result of stimulation of afferent, but not efferent fibers. In urethane anesthetized rats, no changes in either MAP or HR were observed uponμ MS of the right or left vagus(n = 3). These findings suggest the choice of anesthesia plays a key role in determining the efficacy ofμ MS on the vagal nerve. Absence of HR modulation uponμ MS could offer alternative treatment options using VNS with fewer heart-related side-effects. -
Abstract Objective. The objective of this study was to investigate the effects of micromagnetic stimuli strength and frequency from theMag neticPen (MagPen) on the rat right sciatic nerve. The nerve’s response was measured by recording muscle activity and movement of the right hind limb.Approach. The MagPen was custom-built to be stably held over the sciatic nerve. Rat leg muscle twitches were captured on video, and movements were extracted using image processing algorithms. EMG recordings were also used to measure muscle activity.Main results. The MagPen prototype, when driven by an alternating current, generates a time-varying magnetic field, which, according to Faraday’s law of electromagnetic induction, induces an electric field for neuromodulation. The orientation-dependent spatial contour maps of the induced electric field from the MagPen prototype have been numerically simulated. Furthermore, in thisin vivo work onµ MS, a dose-response relationship has been reported by experimentally studying how varying the amplitude (Range: 25 mV p-pthrough 6V p-p) and frequency (range: 100 Hz through 5 kHz) of the MagPen stimuli alters hind limb movement. The primary highlight of this dose-response relationship (repeated overn rats, wheren = 7) is that for aµ MS stimuli of higher frequency, significantly smaller amplitudes can trigger hind limb muscle twitch. This frequency-dependent activation can be justified by Faraday’s Law, which states that the magnitude of the induced electric field is directly proportional to the frequency.Significance. This work reports thatµ MS can successfully activate the sciatic nerve in a dose-dependent manner. The impact of this dose-response curve addresses the controversy in this research community about whether the stimulation from theseμ coils arise from a thermal effect or micromagnetic stimulation. MagPen probes do not have a direct electrochemical interface with tissue and therefore do not experience electrode degradation, biofouling, and irreversible redox reactions like traditional direct contact electrodes. Magnetic fields from theμ coils create more precise activation than electrodes because they apply more focused and localized stimulation. Finally, unique features ofµ MS, such as the orientation dependence, directionality, and spatial specificity, have been discussed.