Pain messages travel along the peripheral nervous system until they reach the spinal cord. The gate control theory proposes that there are "gates" on the bundle of nerve fibers in the spinal cord between the peripheral nerves and the brain. These spinal nerve gates control the flow of pain messages from the peripheral nerves to the brain.
Many factors determine how the spinal nerve gates will manage the pain signal. These factors include the intensity of the pain message, competition from other incoming nerve messages (such as touch, vibration, heat, etc), and signals from the brain telling the spinal cord to increase or decrease the priority of the pain signal. Depending on how the gate processes the signal, the message can be handled in any of the following ways:
- Allowed to pass directly to the brain
- Altered prior to being forwarded to the brain (for instance, influenced by expectations)
- Prevented from reaching the brain (for instance, by hypnosis-induced anesthesia)
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The complexity of this process is illustrated by the "phantom limb" phenomenon described earlier in this article, in which pain signals can seem to arise from amputated limbs. The gate control theory provides a framework to explain this by the complex interaction of the structures of the nervous system - and the role of the most complex structure known.
Once a pain signal reaches the brain, a number of things can happen. Certain parts of the brain stem (which connects the brain to the spinal cord) can inhibit or muffle incoming pain signals by the production of endorphins, which are morphine-like substances that occur naturally in the human body. Stress, excitement, and vigorous exercise are among the factors that may stimulate the production of endorphins. The impact of endorphins is why athletes may not notice the pain of a fairly serious injury until the "big" game is over. It is also why regular low-impact aerobic exercise (e.g. a riding stationary bike) can be an excellent method to help control chronic back pain.
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Pain messages may also be directed along a variety of pathways in the brain.
- For instance, a "fast" pain message (A-delta fiber) is relayed by the spinal cord to specific locations in the brain, namely the thalamus and cerebral cortex. The cortex is the portion of the brain where higher thinking takes place. A fast pain message reaches the cortex quickly and prompts immediate action to reduce the pain or threat of injury.
- In contrast, chronic pain tends to move along a "slow" pathway (C-fiber). Slow pain tends to be perceived as dull, aching, burning, and cramping. Initially, the slow pain messages travel along the same pathways as the fast pain signals through the spinal cord. Once they reach the brain, however, the slow pain messages take a pathway to the hypothalamus and limbic system. The hypothalamus is responsible for the release of certain stress hormones in the body, while the limbic system is responsible for processing emotions. This is one reason why chronic back pain is often associated with stress, depression, and anxiety. The slow pain signals are actually passing through brain areas that control these experiences and emotions.
The brain also controls pain messages by attaching meaning to the personal and social context in which the pain is experienced. This occurs in the cortex. As we have seen previously, soldiers who are wounded in combat may display much less pain than similarly wounded civilians involved in accidents. The meaning attached to the situation seems to be the important difference.