Properties of peripheral nerve excitation in respect of location of coil for magnetic nerve stimulation

In the magnetic stimulation of the peripheral nerve fiber with the figure-of-eight coil, the nerve fiber beneath the figure-of-eight coil is considered to be stimulated with the lowest intensity when it is parallel to the junction of the figure-of-eight coil. However, some experimental studies with the magnetic peripheral stimulation showed that the large compound muscle action potential is elicited with the figure-of-eight coil oriented in the other directions. In the present study, we try to explain the cause of such a discrepancy with the analysis of the model of the magnetic nerve stimulation, and confirm the validity of the result obtained from the model analysis by the experimental study of the magnetic peripheral nerve stimulation. We show that the threshold for the nerve excitation become lowest not only when the junction of the figure-of-eight coil is parallel to the nerve fiber but also when that is perpendicular to the nerve fiber.