Median/ulnar premotor potential identification and localization

A small negative waveform is known to precede the median and ulnar compound muscle action potentials when recorded with surface or concentric needle electrodes. This investigation documents that there are two distinct waveforms preceding the median compound muscle action potential (CMAP) depending upon the type of recording electrodes used (concentric needle versus surface) and their respective locations. The negative waveform originally described with a concentric needle electrode positioned within the substance of the distal thenar eminence and having a restricted zone of detection is referred to as the intramuscular nerve action potential (INAP). This potential is shown to be distinct from the premotor potential (the small negative waveform preceding surface recorded ulnar and median CMAPs). Detection of the median and ulnar premotor potentials at multiple locations about the hand with the same respective onset/peak latencies and amplitudes substantiates that this potential is a far‐field potential. The median and ulnar premotor potentials most likely originate from a dipolar moment imbalance generated by digital sensory nerve action potentials as they cross the first and fifth metacarpophalangeal junctions, respectively. Applying far‐field principles permits the documentation of additional far‐field potentials as they are generated at the second through fourth metacarpophalangeal junctions following median nerve stimulation. Also, because the premotor potential is a far‐field potential, caution must be exercised with respect to its diagnostic utility as joint position and other unknown factors may affect amplitude and onset/peak latency. The INAP following median nerve excitation, however, is documented to be a near‐field potential distinct from the premotor potential arising from the recurrent branch of the median nerve. Therefore, although the median intramuscular nerve action potential and premotor potentials both precede the compound muscle action potential, they are different potentials with unique generator sites. © 1995 John Wiley & Sons, Inc.