Unmyelinated tactile afferents signal touch and project to insular cortex

Abstract

There is dual tactile innervation of the human hairy skin: in addition to fast-conducting myelinated afferent fibers, there is a system of slow-conducting unmyelinated (C) afferents that respond to light touch. In a unique patient lacking large myelinated afferents, we found that activation of C tactile (CT) afferents produced a faint sensation of pleasant touch. Functional magnetic resonance imaging (fMRI) analysis during CT stimulation showed activation of the insular region, but not of somatosensory areas S1 and S2. These findings identify CT as a system for limbic touch that may underlie emotional, hormonal and affiliative responses to caress-like, skin-to-skin contact between individuals.

More than skin deep: Body representation beyond primary somatosensory cortex

“…Most skin mechanoreceptors project via myelinated afferents to the thalamus and primary somatosensory cortex (Johnson & Hsiao, 1992;Mountcastle, 2005). However, a distinct subgroup of tactile mechanoreceptors project via unmyelinated afferents to the insula (Olausson et al, 2002;Vallbo, Olausson, & Wessberg, 1999). The unique contribution of these tactile C-fibres was elegantly shown by Olausson and colleagues (2002) in a study with an individual lacking myelinated afferents.…”

“…Here the focus has generally been on nociceptive inputs to the insula. While visceral inputs to the insula are strong and important (e.g., Critchley et al, 2004;Mehnert et al, 2008;Wang et al, 2008), most research has focussed on tactile and thermal inputs (e.g., Craig, Chen, Bandy, & Reiman, 2000;Olausson et al, 2002), which are methodologically easier to study. Craig (2009), reviewing a wide range of human studies, noted that anterior insular activation is repeatedly associated with awareness of one's own body in general, and with awareness of the emotional significance of bodily events in particular.…”

Haptic perception: A tutorial

“…In keeping with recent trends in other fields, the science of touch has also begun to focus on understanding affective aspects of this modality, such as pleasantness and emotional expression. For example, neuroscientists and psychophysicists have recently hypothesized that the rewarding, emotional aspects of touch may be subserved by a class of unmyelinated peripheral nerve fibers known as CT (or C tactile) afferents that are found in hairy, but not glabrous (hairless), skin (Löken, Wessberg, Morrison, McGlone, & Olausson, 2009;McGlone, Vallbo, Olausson, Löken, & Wessberg, 2007;Olausson et al, 2002).…”

The brain effects of laser acupuncture in depressed individuals: an fMRI investigation

“…The concept of acupoint specificity per condition has been tested previously. [45][46][47][48][49] Classically, any acupuncture treatment has primary acupoints (essential for the condition) and secondary acupoints (helpful but not essential for the condition). Little biological evidence has been recorded in support of the above.…”