Far-field potentials from peripheral nerve: generated at sites of muscle mass change

Using a forearm-sternal referential recording montage, three peripheral nerve far-field potentials, P4, P7, and P9, were elicited by median nerve stimulation. The latencies correlated with the distances between the stimulating cathode at the wrist and the proximal forearm, mid-upper arm, and the shoulder, respectively. At these sites, the electrophysical characteristics of the surrounding volume conductor suddenly changes. Altered girth of muscle mass seemed to be a factor.