Functional organisation and population behaviour of human peripheral nerve fibres : a microneurography study

With regard to the intraneural organisation of peripheral nerves findings obtained from previous anatomical and physiological studies appear to be controversial. The conventional concept seems to be that peripheral nerves lack a detailed topographical organisation. The aim of this study was to further explore the functional arrangement of peripheral nerve fibres in humans.

Microneurography was performed on healthy adult volunteers with one-surface or two-surface concentric needle electrodes. Neural activity was recorded from the limb nerves. The recording properties of the electrodes were studied by performing waveform analysis of the identified unit activity. The outcome of the analyses confirmed that this type of electrode records single unit activity from a myelinated fibre extracellularly, probably at a site where the recording surface of the electrode is close to a Ranvier node of the studied fibre. A novel interpretation to explain the occurrence of the various encountered unitary waveforms was proposed and reliable signal recognition established. Some advantages with the concentric electrodes were specified in particular for multichannel microneurography recordings. Four main classes of low-threshold mechanoreceptive A afferent fibres were studied with this technique. Their intrafascicular relationship was determined. All four classes of nerve fibres tended to be segregated by modality in the nerve trunk. Also, the clustered nerve fibres tended to innervate the same skin areas. In addition, population responses of mechanoreceptors belonging to the same modality were studied and demonstrated to occur when tested with restricted electrical or tactile skin stimulation. Different unit recruitment patterns to various stimuli were revealed.

Thus, earlier findings were confirmed in the present study which suggest that nerve fibres are orderly organised intrafascicularly in human peripheral nerves. Population encoding of sensory input within the nerve was commented upon in the light of the findings. In addition, a novel hypothesis concerning intraneural nerve organisation was proposed, namely population projection of peripheral nerve fibres. A group of nerve fibres of the same modality innervating the same shn area is suggested to project jointly to the brain thereby serving as a basic peripheral projection unit.