Pain: Neuromatrix Theory

The neuromatrix theory proposes that the anatomical substrate of the body-self is a widespread network of neurons that consists of recurrent loops between the thalamus and the cerebral cortex. The traditional theory of pain, which evolved during the early 20th century, holds that pain sensation is produced by a direct-line spinal cord pathway from “pain receptors” in the body to a “pain center” in the cerebral cortex. Research based on this theory focuses on acute pain evoked by noxious stimulation and has revealed complex physiological mechanisms at every level of the pathway from receptors to the cortex. However, it has failed to explain types of chronic pain, such as relentless backaches in the absence of any pathology or phantom limb pain that persists for decades after the amputation of a limb.

Phantom limb pain after amputation of a limb reveals the powerful role of the brain in chronic pain. A high-level cordectomy—total removal of several segments of spinal cord so that sensory information from the pelvis and legs is unable to arrive at the brain—does not stop intense pain in the phantom half of the body. The extraordinary reality of painful phantom limbs indicates that the brain does more than detect and analyze sensory inputs; it generates perceptual experience even in the absence of external inputs. We do not need a body to feel pain or a physical injury to elicit pain. The brain can generate both experiences. These conclusions are the basis of a new theory of pain: the neuromatrix theory, which is described in this entry.

The spatial distribution and synaptic links of the body-self neuromatrix are initially determined genetically and are later sculpted by sensory inputs. The loops diverge to permit parallel processing in different components of the neuromatrix and converge in the thalamus to permit interactions among the output products of processing. The repeated cyclical analysis and synthesis of nerve impulses through the neuromatrix imparts a characteristic pattern: the neurosignature. The neurosignature of the neuromatrix is imparted on all nerve impulse patterns that flow through it as a result of the patterns of synaptic connections in the entire neuro-matrix. All inputs from the body undergo cyclical analysis and synthesis so that characteristic firing patterns are impressed on them in the neuromatrix. Portions of the neuromatrix are specialized to process information related to major sensory events—such as injury, temperature change, and stimulation of erogenous tissue—and are labeled as neuromodules, which impress subsignatures on the larger neurosignature.

At any instant in time, millions of nerve impulses arrive at the brain from all the body's sensory systems, including the proprioceptive and vestibular systems. How can all this be integrated to a unity of experience? As a result of the constant, spontaneous neural activity in the brain, the body-self neuromatrix produces a continuous message that represents the whole body in which details are differentiated within the whole as inputs come into it. The neuromatrix, then, is a template of the whole, which provides the characteristic neurosignature pattern for the whole body as well as subsets of signature patterns from neu-romodules that relate to events at (or in) different parts of the body.

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