The cutaneous sensory system.

The cutaneous senses are traditionally thought to comprise four recognized submodalities that relay tactile, thermal, painful and pruritic (itch) information to the central nervous system, but there is growing evidence for the presence of a fifth modality that conveys positive affective (pleasant) properties of touch. Cutaneous sensory channels can be further classified as serving predominantly either discriminative or affective functions. The former provides information about the spatial and temporal localisation of events on the body surface, e.g., the presence of an insect or the temperature of a cold wind; and the latter, although widely recognised as providing the afferent neural input driving the negative emotional experience of pain, is here posited to provide the afferent neural input driving the positive emotional experience of affiliative touch as well. A distinction is made between the properties of fast conducting myelinated afferents and those of slowly conducting unmyelinated afferents, with the former subserving a sensory-discriminative role, and the latter an affective-motivational one. Here we review the basic elements of the somatosensory system and outline evidence for the inclusion of the 'fifth' sub-modality, conveyed by low-threshold C-fiber mechanoreceptors as the counterpart of high-threshold C-fiber nociceptors with both C-fiber systems serving opposing aspects of affective touch, yet underpining a common mechanism for the preservation of self and species.