Evaluation of motor unit electrical current activity using magnetic field measurement

Abstract

Magnetic field measurement has potential to evaluate biological current activity with high spatial resolution. Using a SQUID magnetometer designed to detect three-directional components of magnetic field and a spatial filter method, we attempted to evaluate the electrical activity of a hand muscle whose surrounding conductivity is not uniform. The estimated electrical currents of single motor unit in abductor pollicis brevis (APB) evoked by very weak electrical stimulation for the median nerve at the elbow was visualized. In the area the APB muscle was thought to be located, inward currents almost perpendicular to the direction of the muscle fibers and electrical currents in the distal and proximal direction parallel to the muscle fibers were simultaneously observed. These currents are considered to be activity near the neuromuscular junction and could be a quantitative indicator of the single motor unit activity. Therefore, magnetomyography could be another method replacing needle electromyography.