Characteristics of the Magnetic Fields at the Neck Evoked by the Stimulation of Upper Limb Peripheral Nerves

Abstract

We have measured late magnetic responses at the neck following the electric stimulation of the ulnar nerve at the wrist and at the little finger using a SQUID. The amplitude of the response depended on the subject's posture, i. e., bending of the neck and position and bending of the stimulated arm. Distributions of the magnetic field over the neck showing a polarity reversal between right-left regions and upper-lower regions were observed. This indicates existence of two different current sources directing horizontally and vertically. Excitation of the cutaneous nerve was crucial to evoke the magnetic response. Moreover, the magnetic response was affected by the stimulation rate, where the waveform and the peak latency changed as the stimulation frequency increased above several Hz. Electromyograms of abductor digiti minimi were observed at latencies compatible with those of the magnetic responses at the neck. It was suggested that the long latency reflex is involved in the neural process generating the magnetic responses at the neck.