Sensual touch: A slow touch system revealed with microneurography

Abstract

Unmyelinated afferents responding to light touch were first described in furry animals in 1939. In humans, a similar type of afferents was identified about 50 years later (CT, C tactile) using the microneurography technique. CT afferents are present in hairy but not in the glabrous skin. Receptive fields are small (maximal 35 mm2) and patchy. Thresholds of CT and Aβ tactile afferents fall in the same range, whereas response properties differ in most other respects. CT afferents exhibit a pronounced and long-lasting fatigue. Maximal impulse rate is about ten times higher in Aβ as CT rate does not exceed 100 impulses s-1. CT afferents exhibit intermediate adaptation. A key difference emerges when the dynamic range is explored using light touch stimuli moving over the skin surface. In Aβ afferents, the impulse rate increases monotonously with speed of movement, whereas in CT afferents maximal rate occurs at a fairly low speed (about 1-3 cm s-1) which corresponds fairly well to human caressing movements. The CT response to a moving touch stimulus is dependent on the temperature of the moving object with the optimal response at a neutral temperature. A similar effect is not present in Aβ afferents. Psychoneural analyses indicate that subjects’ estimate of pleasantness during slowly moving skin touch matches the impulse rate of CT when the speed of movement or temperature of the moving object is varied. Aβ afferents do not exhibit a similar correspondence. Functional properties of CT afferents as well as psychoneural correlations are consistent with the interpretation that the CT system has a key role in physical contact with an amiable conspecific, that is, your parents, lover, kin, or friends. It seems that the social touch hypothesis is a reasonable interpretation of the survival value of a seemingly superfluous tactile system. On the other hand, the exact role of the CT system remains to be clarified.