Nothing is known about how these stimuli activate nociceptors. Mechanical and heat stimuli are usually brief, whereas chemical stimuli are usually long lasting. Three types of stimuli can activate pain receptors in peripheral tissues: mechanical (pressure, pinch), heat, and chemical. To improve our understanding and treatment of pain we will need better animal models of human pain and better tools for studying clinical pain. Not only do they provide little information about the muscles, joints, and tendons that are most often affected by chronically painful conditions, but they do not address the vast array of psychosocial factors that influence the pain experience profoundly. Thus, while experimental studies provide fairly good models for acute pain, they are poor models for clinical syndromes of chronic pain. However, most of what is known about the anatomy and physiology of pain is from studies of experimentally induced cutaneous (skin) pain, while most clinical pain arises from deep tissues. Research into basic mechanisms underlying pain is an increasingly exciting and promising area. However, in recent years, experimental studies of human subjects using physiological, pharmacological, and psychophysical methods indicate that much of what has been learned in animals is applicable to humans (National Academy of Sciences, 1985). For obvious reasons, most of this information comes from animal experiments.
We also discuss some of the physiological processes that modify the pain experience and that may contribute to the development of chronicity. In this chapter we review the anatomy and physiology of pain pathways. It is thought that the processes underlying pain perception involve primarily the thalamus and cortex. The second-order cells relay the message through well-defined pathways to higher centers, including the brain stem reticular formation, thalamus, somatosensory cortex, and limbic system.
The primary afferent nociceptor contacts second-order pain-transmission neurons in the spinal cord. The sensitive nerve ending in the tissue and the nerve attached to it together form a unit called the primary afferent nociceptor. Second, the messages initiated by these noxious stimuli are transmitted by specific, identified nerves to the spinal cord. These are nerve endings, present in most body tissues, that only respond to damaging or potentially damaging stimuli. First, there are specific pain receptors. Pain has much in common with other sensory modalities (National Academy of Sciences, 1985).
Pain is a subjective experience with two complementary aspects: one is a localized sensation in a particular body part the other is an unpleasant quality of varying severity commonly associated with behaviors directed at relieving or terminating the experience.