The Fascicle Undulation Effect on the Activating Function in Magnetic Stimulation of Peripheral Nerves with Transverse and Longitudinal Fields

Abstract

Analysis of activating function for a long, myelinated nerve fiber with undulating path in transverse and longitudinal induced electric fields was performed. The induced electric field was computed using a finite element model composed by a round coil beneath a bath with saline solution. Longitudinal and transverse components of the induced electric field were computed along two axes, one tangential and the other axial to the coil. The influence of a transverse field on the modified activating function was analyzed when the fiber path was determined by the fascicle undulation, and by the fascicle undulation plus the fiber undulation inside the fascicle. For the first path type, undulation wavelength of 40 to 90 mm and 0.8 mm amplitude determined a classic activating function with: (a) multiple virtual cathodes that could generate two or three stimulation sites for axially oriented coil, and (b) virtual cathode with distorted shape in amplitude up to 35% and location up to 67% for tangentially oriented coil. For axially oriented coil, the transverse field term of the modified activating function was comparable in amplitude with the classic activating function, however significant attenuation could occur due to perineurium. For the second path type with wavelength of tenths of millimeters and amplitudes of a quarter of the wavelength, the classic activating function had such a dramatic increase in the spatial frequency that could not predict the stimulation site with the usual interpretation of the virtual cathode. Similarity between the results obtained with the first path type and the ones obtained in a previous in-vitro experiment suggests that the undulating fascicles within the nerve trunk can be responsible for stimulation with transverse fields.