The Neuromatrix approach to chronic pain
Once the danger signals reach the brain they are processed by the neuromatrix. This is a term that Ronald Melzack used to describe the combination of cortical mechanisms involved in processing sensory information. Imaging studies have demonstrated that there is no single pain centre; instead multiple brain areas are excited during pain. The pain neuromatrix includes the anterior cingulate cortex, insular cortex, thalamus, sensorimotor cortex, hypothalamus and limbic areas and is determined both genetically and based on past experience. This is a very important point to highlight and is beautifully summed up in Louis Gifford’s Mature Organism Model (MOM).
The MOM was developed to give a broader understanding of pain and to appreciate it in a biological and psychological context (Gifford, 1998) It describes pain as a survival threat such that the body will be programmed to avoid it. In biological terms the brain / CNS is viewed as the central scrutinising area, that continually samples its environment including sensory information, thoughts, beliefs, memories, expectations, preferences and values. The brain / CNS will then respond on what it finds to be to the best advantage for its body and the vital genes it contains (Gifford, 1998).
Consider a man that has hurt his back while lifting, the most appropriate response to this sensory information may be fear, depression, avoidance and the perception of pain thus the organism will successfully avoid the threat of lifting. This type of behaviour may well be appropriate in the acute stage of an injury to allow healing. In the long term Vlaeyen et al (1995) in the Cognitive behaviour model of fear of movement and re-injury in back pain describe two behavioural patterns that a person can adopt in such a situation. A person can confront their problem, take control and actively participate in their recovery, or can think the worst (catasrophise), fear movement and re-injury and become stuck in a loop of disability, disuse and depression.
This processing and formulation of an output is what Butler and Moseley (Explain pain) have termed a “neurotag”. If this neurotag gets played over and over again within the CNS it becomes like an orchestra playing a piece of music. The more they practice the tune, the more efficient they become at producing it. This neurotag is the threat specific multi system output that Lorimar Moseley describes in his definition of pain.
A fine example to use to demonstrate that pain can be produced by the neuromatrix in the absence of tissue pathology is phantom limb pain. The process that underpins phantom limb pain also occurs in the neuromatrix of non-amputees. Inside the brain there are virtual bodies that are areas of brain tissue devoted to various body tissues. These virtual bodies are both genetically determined and dependent on past experience. The neuromatrix draws upon these virtual bodies when deciding which neurotag is appropriate to output in response to the incoming sensory information. The most well known virtual body is the homunculus in the sensorimotor cortex, which is a strip of brain tissue above your ear. This homunculus is constantly changing and adapting to the environment (remember neuroplasticity) and under faulty processing can “smudge”. Virtual areas can get larger and take over areas of adjacent or distant virtual tissues. Consider the virtual hand of a guitarist or violinist, the homuncular representation for this hand will be much greater than that of the other hand or the hand of a non musician. An injury to this hand may then be more painful to a musician than to a non-musician.