CLUSTERING OF SLOWLY ADAPTING TYPE-II MECHANORECEPTORS IN HUMAN PERIPHERAL-NERVE AND SKIN

The intrafascicular organization of human peripheral nerves was studie d with percutaneous microneurography in the median, radial and peronea l nerves with one-surface or two-surface thin diameter concentric need le electrodes. Data from 33 recording sites containing 47 slowly adapt ing type II (SAII) units were analysed. At many sites two, sometimes e vert three, neighbouring SAII units were recorded from the explored ne rve fascicle and they had adjacent or even overlapping cutaneous recep tive fields. Among pairs of SAII units found at the same site, one uni t often had ongoing discharge, whereas the other was silent under rest ing conditions. The neighbouring SAII units were optimally activated b y stretching the skin in different directions. Stretching the same ski n area in different directions produced different unit recruitment. Cl ustered SAII units were often found in sites where Pacinian afferents and skin sympathetic activity were also recorded. No significant diffe rence was observed in the degree of grouping of SAII units either betw een recordings obtained with one-surface versus two-surface electrodes or between glabrous and hairy skin. The data do not support the notio n that myelinated fibres are randomly organized in peripheral nerve fa scicles. Instead the findings suggest that SAII units tend to be clust ered in human peripheral nerves. Furthermore, the response of groups o f SAII units to skin stretch suggests that they play a role in proprio ception. Dual channel recordings with two-surface concentric needle el ectrodes may provide a novel approach to study fibre organization in h uman peripheral nerves and the behaviour of groups of nerve fibres.