Feeling good: on the role of C fiber mediated touch in interoception

Abstract

The human skin is innervated by a network of thin, slow-conducting afferent (C and Aδ) fibers, transmitting a diverse range of information. Classically, these fibers are described as thermo-, noci- or chemoreceptive, whereas mechanoreception is attributed exclusively to thick, fast-conducting (Aβ) afferents. A growing body of evidence, however, supports the notion that C tactile afferents comprise a second anatomically and functionally distinct system signaling touch in humans. This review discusses established as well as recent findings which highlight fundamental differences in peripheral and central information coding and processing between Aβ and C mechanoreception. We conclude that from the skin through the brain, C touch shares more characteristics with interoceptive modalities (e.g. pain, temperature, and itch) than exteroceptive Aβ touch, vision or hearing. In this light, we discuss the motivational-affective role of C touch as an integral part of a thin-fiber afferent homeostatic network for the maintenance of physical and social well-being.

Vicarious Responses to Social Touch in Posterior Insular Cortex Are Tuned to Pleasant Caressing Speeds

“…Whereas fast-conducting, myelinated A␤ afferents follow a pathway to first and second somatosensory cortex, CT afferents and other slowerconducting, thinly myelinated and unmyelinated fibers follow a distinct pathway to posterior insular cortex (Craig, 2002(Craig, , 2008. Since this pathway is similar to that of visceral afferents, CT afferents have more in common with interoceptive systems conveying information about body state regulation than with classical exteroceptive systems conveying discriminative aspects of touch (Björnsdotter et al, 2010). The CT afferent pathway projection to insular cortex in humans is supported by neuroimaging studies of both patients and healthy controls, demonstrating that brushing stimulation likely to elicit CT discharge results in activation of contralateral mid-and posterior insular cortex (Olausson et al, 2002(Olausson et al, , 2008aBjörnsdotter et al, 2009Björnsdotter et al, , 2010.…”

“…Since this pathway is similar to that of visceral afferents, CT afferents have more in common with interoceptive systems conveying information about body state regulation than with classical exteroceptive systems conveying discriminative aspects of touch (Björnsdotter et al, 2010). The CT afferent pathway projection to insular cortex in humans is supported by neuroimaging studies of both patients and healthy controls, demonstrating that brushing stimulation likely to elicit CT discharge results in activation of contralateral mid-and posterior insular cortex (Olausson et al, 2002(Olausson et al, , 2008aBjörnsdotter et al, 2009Björnsdotter et al, , 2010. In the present study, the felt and seen velocity-specific activations in experiments 1 and 2 fell in granular insular cortex (Ig2 of Kurth et al, 2010b) on or near the long gyri and within the insular central sulcus, sometimes extending onto the posterior short gyrus.…”

“…This region is activated by a broad range of visceral and somatosensory stimulation in humans (Kurth et al, 2010a), including human touch (Lovero et al, 2009), and is highly functionally connected with sensorimotor cortices (Deen et al, 2010;Cauda et al, 2011). It also exhibits distinct somatotopic responses for stroking on the arm and thigh within the range of CT afferents' preferred speed (Björnsdotter et al, 2009(Björnsdotter et al, , 2010. Functional and spatial specialization between visceral (interoceptive) and somatic stimulation is low in the insula (Eickhoff et al, 2006), the weak separation implying that this region plays an integrative role among somatosensory and interoceptive inputs.…”

“…This velocity-sensitive CT response is tightly correlated with subjective ratings of how pleasant the touch feels . Human CT pathways ultimately target the insular cortex (Olausson et al, 2002(Olausson et al, , 2008aBjörnsdotter et al, 2009Björnsdotter et al, , 2010, a region associated with emotion and homeostatic balance (Craig, 2002(Craig, , 2008(Craig, , 2009Paulus, 2007).…”

C-tactile afferents: Cutaneous mediators of oxytocin release during affiliative tactile interactions?

“…Here, in common with other c-fibre mediated thermal, painful and pruritic inputs, they are thought to contribute to the central representation of the physical condition of the body (Björnsdotter et al, 2010).…”