The effect of magnetic coil orientation on the excitation of the median nerve.

Abstract

We investigated the effect of magnetic coil orientation on the excitation of the median nerve in healthy subjects. An 8-shaped coil, 3.2 cm in outer diameter, was used. The median nerve was stimulated at the elbow while the compound muscle action potentials (CMAPs) of abductor pollicis brevis muscle were recorded at 4 different directions of the induced current: orthodromic, antidromic, medio-lateral and latero-medial. We found that the amplitude of the CMAP was the greatest in a medio-lateral (ML) direction. We also measured the induced electric field in the saline tank that mimicked the forearm. The induced electric field and its spatial gradient were the greatest in the ML direction among 4 directions. The fact that the forearm is a restrictive volume conductor may result in the different effects of coil orientation on the excitation of the median nerve at the elbow.