Theoretical and experimental aspects of magnetic nerve stimulation

The driving function for axons exposed to electric and magnetic fields is the sum of two terms, the first spatial derivative of the induced field inside the axon and the first spatial derivative of the extracellular conduction electric field. This extracellular field is generated by primary sources such as electrodes and secondary sources due to inhomogeneities in the volume conductors. The magnetic driving function must be modified for terminations and bending or diameter changes. For a termination the driving function is proportional to the magnitude of the electric field and can be significantly larger than the first spatial derivative of the same field. This is demonstrated numerically and experimentally.