Far-field recording of the junctional potential generated by median nerve volleys at the wrist.

In 20 arms from 10 healthy subjects, the orthodromic median sensory volleys crossing the wrist gave rise to a stationary, biphasic potential rendering the forearm initially positive (Pw) and subsequently negative (Nw) compared with the hand. A pair of electrodes located on each side of the wrist best detected the junctional potential, Pw-Nw, that developed across the partition. Here the hand and the forearm each, in effect, acted as a lead, connecting any points within the respective compartment to the voltage source. Thus, nearly the same potential difference appeared across the wrist regardless of the inter-electrode distance, allowing the detection of the voltage step in far-field derivation. Our data verify that not only positive but also negative peaks of scalp-recorded evoked responses could represent junctional potentials generated along the course of the conduction medium in the absence of fixed neural discharges.