Undulations and bending in peripheral nerves benefit coil positions projecting transverse fields

Area of Science:

• Neuroscience
• Biophysics
• Biomedical Engineering

Background:

• Peripheral magnetic stimulation (PMS) traditionally assumes parallel field components are key for nerve activation.
• Experimental data suggests transverse field components may also contribute significantly to PMS.
• Nerve curvature and undulations are potential factors influencing PMS efficacy, leading to uncertainty in optimal coil positioning.

Purpose of the Study:

• To investigate the impact of nerve undulations and bending on peripheral magnetic stimulation.
• To determine optimal coil placement and orientation for maximizing magnetic nerve stimulation.
• To resolve discrepancies between theoretical models and experimental findings in PMS.

Main Methods:

• Neuronal simulations with varying fascicle and fiber undulations using a figure-of-8 coil.
• Anatomical modeling of median nerve stimulation to incorporate forearm nerve bending.
• Experimental validation of simulated findings through median nerve stimulation on human subjects with manipulated wrist positions.

Main Results:

• Simulations indicated that nerve undulations enhance the efficacy of transverse field components over parallel ones.
• Coil position relative to the nerve's anatomical course was found to be more critical than coil orientation.
• Experimental results confirmed lower stimulation thresholds with transverse coil positions and suggested wrist position (influencing nerve undulation) affects thresholds.

Conclusions:

• Nerve undulations and bending significantly impact peripheral magnetic stimulation thresholds.
• Transverse coil positions and central placement above the nerve are associated with lower stimulation thresholds.
• Optimal coil placement for PMS involves considering the nerve's three-dimensional path and potential for undulation, with central positioning being most effective.