Unmyelinated afferents constitute a second system coding tactile stimuli of the human hairy skin

Unmyelinated afferents constitute a second system coding tactile stimuli of the human hairy skin. J. Neurophysiol. 81: 2753-2763, 1999. Impulses were recorded from unmyelinated afferents innervating the forearm skin of human subjects using the technique of microneurography. Units responding to innoc uous skin deformation were selected. The sample (n = 38) was split into low -threshold units (n = 27) and high-threshold units (n = 11) on the basis of three distinctive features, i.e., thresholds to skin deformation, size of response to innocuous skin deformation, and differential response to sharp and blunt stimuli. The low-threshold units provisionally were denoted tacti le efferents on the basis of their response properties, which strongly sugg est that they are coding some feature of tactile stimuli. They exhibited, i n many respects, similar functional properties as described for low-thresho ld C-mechanoreceptive units in other mammals. However, a delayed accelerati on, not previously demonstrated, was observed in response to long-lasting i nnocuous indentations. It was concluded that human hairy skin is innervated by a system of highly sensitive mechanoreceptive units with unmyelinated a fferents akin to the system previously described in other mammals. The conf irmation that the system is present in the forearm skin and not only in the face area where it first was identified suggests a largely general distrib ution although there are indications that the tactile C afferents may be la cking in the very distal parts of the limbs. The functional role of the sys tem remains to be assessed although physiological properties of the sense o rgans invite to speculations that the slow tactile system might have closer relations to limbic functions than to cognitive and motor functions.