Origin of far-field potentials in the ulnar compound muscle action potential.

INTRODUCTION

The compound muscle action potential (CMAP) following ulnar nerve stimulation receives a considerable contribution from far-field potentials (FFPs), although their origin is not entirely clear. We investigated this issue using voluntary contractions.

METHODS

In 7 control subjects, we placed multiple recording electrodes over the motor points of ulnar-innervated muscles. We asked the subjects to perform a weak movement corresponding to the action of each muscle, and identified the single motor unit potentials (MUPs) from that muscle. We summed the MUPs from each individual muscle and reconstructed the CMAPs.

RESULTS

The reconstructed CMAPs coincided well with the actual ones. The N1, P1, and early N2 components of the FFPs were generated primarily by palmar, but also by dorsal, interosseous muscles. The abductor digiti minimi muscle usually generated positive-negative biphasic FFPs, and the negative FFP generated the late N2 component.

CONCLUSIONS

These results should prompt a revision of the theory of FFP generation.