Visualization of a moving quadrupole with magnetic measurements of peripheral nerve action fields.

Magnetic compound action fields (CAFs) over the right arm were measured from 63 sensor positions with two 7-channel SQUID gradiometer systems following electrical stimulation of the median nerve at the wrist. The field mapping of the CAFs revealed a propagating quadrupolar pattern with the leading depolarization and trailing repolarization fronts. The average distribution of the CAFs in the longitudinal direction was 9.0 cm in length for the depolarization field and 7.3 cm for the repolarization field in good agreement with a theoretical prediction based on the duration (3 msec) of the CAFs and the conduction velocity of the nerve (50 m/sec). The distance between the maxima of the depolarization front and the minima of the repolarization front was 6.3 cm. This spatial separation of the leading and trailing dipole locations suggests in part mutual cancellation of the fields with opposite polarity at or near the depolarized segment of nerve fibers.