CONSISTENCY OF UNITARY SHAPES IN DUAL LEAD RECORDINGS FROM MYELINATEDFIBERS IN HUMAN PERIPHERAL-NERVES - EVIDENCE FOR EXTRACELLULAR SINGLE-UNIT RECORDINGS IN MICRONEUROGRAPHY

Percutaneous microneurography is a powerful technique allowing studies of activity in single nerve fibres of conscious humans. However, the mechanisms by which single-unit recordings are achieved with this tech nique an not fully understood. To further elucidate these mechanisms, dual-lend recordings, using a modified concentric needle electrode wit h two separate recording surfaces at the tip. were performed in normal subjects. Sixty-two Single units supplied by large myelinated afferen ts were studied. The majority (90%) of the units were recorded simulta neously on both surfaces but with different action potential amplitude s. Four types of unitary waveforms were encountered. The potentials re corded on the two channels were of the same type, although occasionall y some details differed. Parallel waveform changes of the same units o ccurred simultaneously on the two surfaces. A displacement of a single fibre from one recording surface to the other with or without concomi tant waveform-type transitions was observed when the electrode was sli ghtly repositioned intraneurally. The results provided direct evidence to confirm that concentric needle electrodes record single-unit activ ity extracellularly from myelinated nerve fibres, probably at or close to a node of Ranvier. All the types of action potentials encountered with conventional tungsten electrodes were also identified in dual-cha nnel recordings with concentric electrodes. which casts doubt on the p revious explanation that single-unit activity recorded with tungsten e lectrodes is derived from intracellular sources. Some biological and t echnical aspects of the findings are discussed. especially concerning the applicability of in vivo measurements of the time course of the ac tion potentials in humans and ways to improve microneurography towards multichannel recordings.