Physiology and Pathophysiology of Chronic Pain

This chapter presents an overview of the anatomy and physiology of the pain system and the neurobiological changes that occur in the establishment and maintenance of chronic pain states. Nociceptive pain reflects ongoing tissue damage, inflammation, and noxious stimulation in intact tissues, whereas neuropathic pain emanates from an anatomic region not subject to noxious stimulation, even if the physiologic changes sustaining it are not located in that area. This type of pain reflects damage to and improper functioning of neural tissue. The neuropathic pain does not respond well to narcotic medications, or may respond to higher doses and is often described by patients as “burning” or “searing.” Injured nerves not only develop abnormal hyperexcitability at their terminals, but also at previously inactive sites along the axon. Nerve injury results in the sprouting of new terminals as part of the normal process of peripheral nerve regeneration. However, these new extensions may be hyperexcitable and exhibit ectopic electrical discharges. Ongoing tissue damage or inflammation as well as nerve injury can also result in both short and long-term changes in the central nervous system.