Ronald Melzack, Ph.D.
The neuromatrix theory of pain proposes that pain is a multidimensional experience produced by characteristic “neurosignature” patterns of nerve impulses generated by a widely distributed neural network—the “body-self neuromatrix”—in the brain. These neurosignature patterns may be triggered by sensory inputs, but they may also be generated independently of them. Acute pains evoked by brief noxious inputs have been meticulously investigated by neuroscientists, and their sensory transmission mechanisms are generally well understood. In contrast, chronic pain syndromes, which are often characterized by severe pain associated with little or no discernible injury or pathology, remain a mystery. Furthermore, chronic psychological or physical stress is often associated with chronic pain, but the relationship is poorly understood. The neuromatrix theory of pain provides a new conceptual framework to examine these problems. It proposes that the output patterns of the body-self neuromatrix activate perceptual, homeostatic, and behavioral programs after injury, pathology, or chronic stress. Pain, then, is produced by the output of a widely distributed neural network in the brain rather than directly by sensory input evoked by injury, inflammation, or other pathology. The neuromatrix, which is genetically determined and modified by sensory experience, is the primary mechanism that generates the neural pattern that produces pain. Its output pattern is determined by multiple influences, of which the somatic sensory input is only a part, that converge on the neuromatrix.
Pain has many valuable functions. It often signals injury or disease and produces a wide range of actions to stop it and treat its causes. Toothache, for example, is usually a signal of caries, and forces us to seek dental help. Memories of earlier pain and suffering also warn us to avoid potentially dangerous situations. Yet another effect of pain, especially after serious injury or disease, is to make us rest, thereby promoting the body’s healing processes. All of these actions induced by pain—to seek help, avoid, or rest—have obvious value for survival. Yet despite these valuable features of pain, there are negative aspects that challenge our attempts to understand the puzzle of pain. What is the value of persistent phantom limb pain to amputees whose stump has healed completely? The pain, not the physical disability, prevents them from leading normal lives. Similarly, most backaches, headaches, muscle pains, nerve pains, pelvic pains, and facial pains serve no discernible purpose, are difficult to treat, and are a disaster for the people who suffer them.
Pain may be the warning signal that saves the lives of some people, but it destroys the lives of countless others. Chronic pains, clearly, are not a warning to prevent physical injury or disease. They are the disease—the result of neural mechanisms gone awry. The neuromatrix concept suggests brain mechanisms that may underlie some kinds of chronic pain and points to new forms of treatment.