Disability and distress related to pain are among the most common reasons that lead people to seek rehabilitation. Pain is a somatosensory modality—along with thermoreception, touch, and proprioception—that is defined as the unpleasant sensory experience associated with actual or potential tissue damage.1 Pain serves sensory, emotional, and cognitive functions.2,3 Pain's sensory-discriminative function allows for self-preservation. When a hand is placed on a hot stove, the sensory-discriminative function of pain compels the withdrawing of the hand and inspecting for tissue damage. Tissue damage results when the sensory-discriminative function of pain is impaired. One example of this phenomenon is the skin ulceration often seen on the plantar surfaces of the feet in people with diabetic polyneuropathy. The affective function of pain provides emotional unpleasantness to pain sensations. This causes people to avoid additional pain and the tissue damage that pain represents. The cognitive-evaluative function serves to encourage learning and foster behavioral adaptation. Disorder involving the affective and cognitive-evaluative functions of pain may result in maladaptive behavioral responses to pain, such as a disabling avoidance of work, family, and recreational activities.
The same basic anatomical and physiological processes that generate and conduct pain sensations are found in all people. However, there are vast differences in how people respond to pain sensations. This causes a great variation in how people will demonstrate pain in the clinic. Individual differences in how pain sensation is processed often make it difficult to interpret for both patients and clinicians in the diagnostic process. This chapter discusses pain as it relates to the diagnostic process. The chapter first describes the processes that all people have in common, and then describes anatomical and physiological reasons for individual differences in pain perception.
All People Share Common Nociceptive Processes
The association between pain and actual tissue damage suggests that all people share common anatomy and physiology that mediate pain perception. Nociception occurs when pain is perceived through the activation of peripheral pain receptors due to tissue damage.
Nociceptors Respond to Various Forms of Tissue Injury
Pain receptors are free nerve endings called nociceptors. Nociceptors are broadly categorized by their location. Nociceptors located throughout the skin and musculoskeletal structures are called somatic nociceptors. Somatic nociceptors are primarily classified by their adequate stimuli—stimuli to which they respond. Mechanical nociceptors are activated by intense pressure.4 Thermal nociceptors respond to temperatures less than 5°C and greater than 45°C. Mechanical and thermal nociceptors transmit action potentials along Aδ axons when activated. The Aδ axons are thinly myelinated and conduct at velocities of 5 to 30 m/s.4 Polymodal nociceptors are activated by mechanical, thermal, and chemical stimuli, sending action potentials along C fibers to the spinal cord when activated.4 The C fibers are ...