Pain is a specific form of sensation associated with an unpleasant feeling, suffering, and, potentially, capable of causing pathological changes in the body. Pain is usually associated with injury of tissues and ensuing release of nociceptive stimuli.
The pain receptors in the skin and other tissues are all free nerve endings. They are widespread in the superficial layers of the skin as well as in certain internal tissues, such as the periosteum, the arterial walls, the joint surfaces, the falx and tentorium of the cranial vault, the parietal peritoneal membrane, the mesentery, the parietal pleura, the capsule of the liver or spleen.
Impulses that produce sensation of pain are conducted by two types of primary afferent nociceptors.
Unmyelinated axons (C-fibers) that are characterized by a low rate of conduction (<1 m/s) and are present in nerves to the skin and to deep somatic and visceral structures. Stimulation of these nerves yields a delayed, dull, diffuse, burning, long-lasting pain sensation.
Small-diameter myelinated A-delta (A5) axons with a higher conduction rate (12-30 m/s). Their stimulation gives rise to a fast sharp pain that may be accurately localized and rapidly subsides.
Most A5 and C-fibers respond maximally only to intense (painful) stimuli. The ability to detect painful stimuli is completely abolished when these axons are blocked. Individual primary afferent nociceptors respond to several types of noxious stimuli: heating, intense mechanical effect (e.g. pinch), irritating chemicals.
Chemicals that excite the chemical type of pain are bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, and proteolytic enzymes. In addition, prostaglandins and substance P enhance the sensitivity of pain endings but do not directly excite them.