[The peripheral mechanism of physiological pain].

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or it is described in terms of such damage. Additionally, activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not considered pain, which is always a psychological state (IASP). Aδ- and C-fibers play important roles in conducting pain to the central nervous system. The former conducts fast pain and the latter slow pain. Intraneural microstimulation by microneurography causes mainly dull pain in glabrous skin and burning pain in hairy skin. Most of the burning pain is conducted by C-fibers, which are receptive to heating (CH) and are mechano- and heat-insensitive (CMiHi). The characters of C-fibers are easily influenced by chemical stimulants. For example, capsaicin induces sensitization to CMiHi and desensitization in the C-fibers that are mechano- and heat-responsive (CMH). Secondary hyperalgesia and allodynia have been shown to be the changes in the central network of pain, but maintenance of pain seems to require the excitation of nociceptors. Sensitization to heat is observed much more prominently in Aδ-fibers than in C-fibers. Such function of Aδ-fibers is closely related to the maintenance of pain and spontaneous pain in the case of burn injury. Physiological pain has significant importance as a warning sign that ensures human safety.