Cutaneous polymodal receptors: characteristics and plasticity.

The cutaneous sensory units labeled C-fiber polymodal nociceptors have a broadly coherent set of responsive characteristics. These include; (a) elevated thresholds to mechanical stimulation and to heat; (b) excitation by irritant and algesic chemicals; and (c) sensitization by injury or algesic substances. These characteristics and the match between the signals produced by C-polymodal nociceptors to pain-causing stimuli and human reports of pain indicate a probable causal connection. Nevertheless, there are indications that this population of sensory units may contain functionally-distinct subtypes. Some human C-polymodal nociceptors have been reported to be excited by histamine at low concentrations, whereas much of the population lacks such responsiveness. Further, in vitro studies of the effects of non-steroidal anti-inflammatory agents and low pH on sensitization suggest distinctions in the responsiveness of different elements whose general characteristics place them into the C-polymodal category. The enhanced responsiveness of C-polymodal nociceptors after heat stimulation or exposure to acidity has a probable relationship to the primary hyperalgesia produced after injury to hairy skin or in the presence of inflammation. Furthermore, the alterations of C-polymodal nociceptor characteristics after partial nerve injury and sympathectomy imply a change in phenotype of neurons spared by denervation and are suggestive of a possible relationship to sympathetically related pain and post-sympathetic neuralgias. These evidences of plasticity in responsiveness of a set of sense organs putatively associated with cutaneous pain represent lessons in the adaptability of biological mechanisms, and clues to the pathophysiology of pain.