Peripheral Pain Perception

When tissue damage occurs in the periphery, then both inflammatory and neuronal responses occur which are intimately linked with the inflammatory process profoundly altering the physical and chemical response of the sensory fibers. Pain is perceived in the periphery by nociceptive nerve endings. Very few nociceptors respond entirely to noxious stimuli, but also respond to other sensory modalities, such as heat. Pain impulses are carried by the slowly conducting unmyelinated C fibers and the relatively faster-conducting, lightly myelinated A-delta fibers; classification of fibers is shown in Table 33.1.

The information about tissue damage, however, is not only conveyed by nerve impulses in sensory fibers, but also by the slow transport of chemicals along axons, e.g. neurotrophins to dorsal root ganglion cells. The unmyelinated C fibers are particularly involved with this type of transmission, responding to chemicals, which then change the metabolism and chemistry of the cell cytoplasm and membrane.

Inflammatory Mediators The algogenic or pain-producing substances include H+, K+, prostaglandins and leuko-trienes in tissues, bradykinin in plasma and substance P in nerve terminals. These chemicals profoundly alter the physical and chemical

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