Waveform complexity of unit activity recorded with concentric needle electrodes from human peripheral nerves

Using standardised concentric needle electrodes 170 single units were recorded from myelinated cutaneous afferents in the human median or ulnar nerves. The unitary waveforms were of four types: single-peaked monophasic potentials (type I), double-peaked monophasic potentials (type II), biphasic potentials (type III) and triphasic potentials (type IV). Type II and IV occurred more frequently than the other types. Units of different functional classes had similar waveforms and there was no specific type of waveform distribution in any particular unit category. In some recording situations there were changes in unitary waveforms from one type to another. There was a tendency for the complex type IV, type III and type II waveforms to change to the simple type I. Adjustment of the electrode often provoked such waveform changes. The waveform profiles and waveform changes observed during recordings with concentric needles were significantly different from those encountered with conventional tungsten electrodes, which might be due to differences in recording properties between the two electrodes. Possible neural mechanisms underlying the observed waveforms and waveform transitions are discussed. In particular, our data suggest that concentric needle electrodes record single-unit activity from myelinated fibres extracellularly.