The touch landscape

Abstract

Somatic sensation comprises four main modalities relaying tactile, thermal, painful, or pruritic (itch) information to the central nervous system. These input channels can be further classified as sub-serving sensory functions, such as spatial and temporal discrimination, and the provision of essential information for controlling and guiding exploratory manual behaviours, or affective functions that include the provision of the subjective experience of affective or emotional pleasurable touch. Signalling in fast-conducting myelinated peripheral nerve fibres (Aβ afferents) is important for the discriminative properties of tactile sensations, whereas signalling in unmyelinated peripheral nerve fibres, C-tactile (CT) afferents seems to be important for the rewarding, emotional properties of touch. CT afferents have specific biophysical, electrophysiological, neurobiological and anatomical properties to drive the temporally delayed affective somatic system. This chapter explores step by step the differences between the discriminative and affective touch systems, from the first stage of encoding touch in the skin to the neural pathways in the brain. The below quote from Bentley (Am J Psychol 11:405-425, 1900) reiterates the complexity of the skin and the wonder in the phenomenon of somatosensation: ‘The skin is burdened with offices. One of the surprises of physiology is the revelation of the multitude of functions performed by this apparently simple organ. As a rind it is not only the container, but the warder-off, and also the go-between for the organism and its world; tegument, buckler, interagent. It is small wonder that its work is represented in mental process; that many of our most worn and useful perceptions are made up of cutaneous sensations.'.