[Alternative tactile system: C-fibers coding the affective aspect]

Abstract

It has been accepted that human tactile sensation is mediated exclusively by large myelinated (Abeta) fibres. Nevertheless, recent studies indicated a dual mechanoceptive innervation of the skin in various mammals. Besides the known A fibers, the skin is also innervated by slow-conducting, low-threshold, small unmyelinated (C) afferents. These unmyelinated fibers respond vigorously to innocuous skin deformation, but poorly to rapid skin movement. They project to outer lamina II of spinal cord, and form synapse with the secondary sensory neurons. The latter then project to insular cortex via spinothalamic tracts. Functional magnetic resonance imaging (fMRI) studies showed that a slowly moving tactile stimulus along hairy skin produced a strong activation of the insular cortex. Pleasant touch has also been demonstrated to activate orbitofrontal cortex adjacent to areas responding to pleasant taste and smell. Overall, the response characteristics and activated brain regions suggest that they are related with the limbic system and affective aspect rather than tactile discriminative function.