The Nervous System, Pain Messages, Pain Gates and Chronic Pain Signaling
The human nervous system has a tremendous capacity to constantly relay vital messages throughout the body. If this complex system is damaged, nerve signals can go awry, causing intense pain.
The spinal cord is the main transportation hub of the body's central nervous system, carrying signals from the brain to nerves throughout the body. Nerves coming from and leading to all parts of the body enter and exit the spinal cord along its entire length.
Thirty-one pairs of spinal nerves exit the spinal cord through openings between the vertebrae. The point at which a nerve exits the spinal cord is called a nerve root. The nerve root then branches out into many smaller peripheral nerves that control different parts of the body.
The spinal cord starts at the base of the brain, runs throughout the cervical and thoracic spine, and typically ends at the lower part of the thoracic spine.
The spinal cord does not run through the lumbar spine (lower back). After the spinal cord stops in the lower thoracic spine, the nerve roots from the lumbar and sacral levels come off the bottom of the cord like a "horse's tail" (named the cauda equina) and exit the spine.
Because the lumbar spine has no spinal cord and has a large amount of space for the nerve roots, even serious conditions—such as a large disc herniation—do not typically cause paraplegia (loss of motor function in the legs).
Damage to any part of the central nervous system or peripheral nerves can cause neuropathic pain.
The spinal cord can be divided into segments according to the nerve roots that branch off it. Nerves along the cord consists of:
8 cervical nerves
12 thoracic nerves
5 lumbar nerves
5 sacral nerves
1 coccygeal nerve
For most spinal segments, the nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine.
Cervical spine nerve roots. In the neck, the nerve root is named for the lower segment that it runs between (e.g. C6 nerve root at C5-C6 segment).
Lumbar spine nerve roots. In the lower back, the nerve is named for the upper segment that it runs between (e.g. L4 nerve root at L4-L5 segment).
A nerve that exits the lower back has peripheral branches that extend all the way down to the toes. Peripheral nerves comprise the peripheral nervous system. The peripheral nerves include both motor nerves and sensory nerves:
Sensory nerves are nerves that receive sensory stimuli, telling us how something feels—whether it is hot, cold, or painful. These nerves are made up of nerve fibers, called sensory fibers (mechanoreceptor fibers sense body movement and pressure against the body, and nociceptor fibers sense tissue injury).
Motor nerves lead to the muscles and stimulate movement. They are made up of nerve fibers called motor fibers.
Pain messages travel along the peripheral nervous system until they reach the spinal cord. There is a theory 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)
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.
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.
In times of anxiety or stress, descending messages from the brain may actually amplify the pain signal at the nerve gate as it moves up the spinal cord. Alternatively, impulses from the brain can "close" the nerve gate, preventing the pain signal from reaching the brain and being experienced as pain.
Some other factors that can open or close the pain gates as messages move up and down the spinal cord. These can be roughly divided into sensory (physical being and activities), cognitive (thoughts), or emotional (feelings) areas, although of course there is substantial overlap between these areas in practice.
The events and conditions that may open the pain gates and cause more suffering include:
Sensory factors, such as injury, inactivity, long-term narcotic use, poor body mechanics, and poor pacing of activities
Cognitive factors, such as focusing on the chronic pain, having no outside interests or distractions, worrying about the pain, and other negative thoughts
Emotional factors, such as depression, anger, anxiety, stress, frustration, hopelessness, and helplessness.
Alternatively, influences that can close the pain gates and reduce suffering include:
Sensory factors, such as increasing activities, short-term use of pain medication, relaxation training and meditation.
Cognitive factors, including outside interests, thoughts that help the patient cope with the pain, and distracting oneself from the chronic pain.
Emotional factors, such as having a positive attitude, overcoming depression, feeling reassured that the pain is not harmful, taking control of one’s chronic pain and life, and stress management.
Finding the right pain specialist to decipher these factors, and the signals sent through your pain gate, is key to building the best approach to and treating your chronic pain.