VISUALIZATION OF A MOVING QUADRUPOLE WITH MAGNETIC MEASUREMENTS OF PERIPHERAL-NERVE ACTION FIELDS

Magnetic compound action fields (CAFs) over the right arm were measure d from 63 sensor positions with two 7-channel SQUID gradiometer system s following electrical stimulation of the median nerve at the wrist. T he field mapping of the CAFs revealed a propagating quadrupolar patter n 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 repo larization field in good agreement with a theoretical prediction based on the duration (3 msec) of the CAFs and the conduction velocity of t he nerve (50 m/sec); The distance between the maxima of the depolariza tion front and the minima of the repolarization front was 6.3 cm. This spatial separation of the learning and trailing dipole locations sugg ests in part mutual cancellation of the fields with opposite polarity at or near the depolarized segment of nerve fibers.