Conduction velocity of low-threshold mechanoreceptive afferent fibers in the glabrous and hairy skin of human hands measured with microneurography and spike-triggered averaging.

The axonal conduction velocity (CV) of afferent fibers innervating low-threshold mechanoreceptors in the skin of the human hand was measured utilizing a spike-triggered averaging technique. Two tungsten microelectrodes were inserted into the median, or the ulnar, the superficial branch of the radial nerve, at two different points in the distal forearm. Unitary spike potentials were picked up with the proximal electrode. Unit-type SA (I and II), and FA (I and II), was determined from receptive field properties and response patterns to mechanical stimuli. Using these potentials as a trigger, the records from the distal electrode were averaged to reveal corresponding unitary potentials embedded in the background noise activities. CVs were calculated by dividing the interelectrode distance by the conduction time measured from the two neurograms. 122 mechanoreceptive afferent fibers in the glabrous and the hairy skin were recorded. The CVs of all sampled units were 36-73 m/s and the mean (SD) was 58.7 (7.4) m/s. The CVs did not differ between units in the three nerves, nor between units from the glabrous and the hairy skin. The mean CV of the FAI group was slower than the mean CV of the SA group by 4-5 m/s, but the overlap of the distributions was large.