What Is Pain?
Everyone knows what pain feels like. Most people medicate their pain without ever really understanding what it is, but when pain is persistent this is like muzzling a crying baby without bothering to figure out why they are crying. Fortunately most pain goes away within a few days or weeks because of the bodies natural ability to heal itself. But in some cases the pain persists beyond a reasonable period, even after there has been plenty of time for the body to heal. We have to ask, "Why does pain persist even when there is no recent injury?"
The International Association for the Study of Pain defines pain as: “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described by the patient in terms of such damage”. This definition is misleading because a great deal of common pain persists when there is no apparent tissue damage or inflammation.
At one time it was thought that pain was merely an intensification of normal sensation like pressure or temperature. But this is not the case. Pain is its own sensation unto itself. There are specific receptors (called nociceptors) in the body whose entire purpose is specifically to communicate pain to the brain and spinal cord.
How Pain Works
The nervous system functions as an information system in the body. Information includes how fast and hard for the heart to beat, how much to dilate or constrict blood vessels, the activation and relaxation of muscles, the sensations of touch, pressure, temperature and stretching on the skin, and the list goes on and on. These nerves all travel from the body back to the spinal cord and then to the brain. Pain receptors communicate through slow nerve fibers (C-Fibers). Other sensory information such as touch, pressure, temperature, etc. travels along much fast nerve fibers (A-Fibers). At the spinal cord all of this information comes together, both painful and non-painful information, to the Transmission cell. The Transmission Cell passes this incoming information up the spinal cord toward the brain
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If there is more non-painful information coming in (like touch, pressure, muscle stretching), then the Transmission cells can't handle the information overload and are not able to effectively convey as much of the pain information to the brain. In pain research this is know as the “Gate Theory” (Melzack and Wall). This explains why rubbing on a painful area or applying some lotion that irritates the skin seems to reduce the pain.
As this neurochemical transmission of pain makes its way up the spinal cord past each nerve root level the pain stimulus can spill over to other areas of the spinal cord including areas that control blood flow (vasomotor), sweating (sudomotor) and “goose bumps" (pilomotor). The pain stimulus can spread to other areas of the body, even to the other side of the body where it is might be felt as referred pain at some unrelated location.
The pain system is wired into a primitive part of our brain called the limbic system. The limbic system contains the emotional centers and it also regulates the function of all the internal organ and glands through the autonomic nervous system. The stimulation of the limbic system can create a feeling of stress and anxiety and can lead to many emotional disturbances.
The touch and other sensory routes are more advanced development in humans. They are faster and more efficient. For example, when you sit on a pin you can feel the pain and you can also localize it. The peripheral threshold of pain does not vary from one person to another. However, the brain's interpretation of that pain can vary greatly from person to person. One person's pin-prick may be felt as stab pain by another. People who have a “high threshold for pain” are coping with it mentally (emotionally). But the actual sensory input from the body is the same for everyone.
Tissue Injury and Inflammatory Pain
If an injury creates tissue damage there is a release of chemicals from the damaged cells (histamine, serotonin, bradykinin and hydrogen ions) that lead to inflammation. There is redness, swelling, warmth and pain. The sensation of pain is transmitted back to the spinal cord via the slow Type-C fibers we talked about above.
As time passes the inflammation will subside and the injury will heal. But in some cases the pain continues and becomes “chronic” pain. At this point the pain, not the injury, become debilitating. Patient's are likely to self-medicate, become anxious or depressed and have trouble sleeping or working. Many will jump from doctor to doctor and from drug to drug trying to find relief. Standard physical examination, blood tests, x-rays and MRI's reveal nothing abnormal. At some point doctors might consider you to be malingering (faking), especially if you're getting compensation for and injury.
Neuropathic Pain
With chronic pain there is typically no discernible injury and little or no inflammation. With the absence of these conditions doctors (and insurance companies) are likely to question the veracity of a claim. Patient's might even begin to question whether there is something wrong “in their head.”
Malfunctioning or neuropathic nerves and muscle can become overly sensitive and generate abnormal impulses -- the technical term for overly sensitive structures is “Supersensitivity.” This type of pain is known as neuropathic pain and it is related to three factors: neuropathy, prespondylosis, and Cannon’s Law.
1. Neuropathy
Most degenerative changes in our body proceed slowly, often with no symptoms. But optimal health is not merely the absence of symptoms; the absence of symptoms, does not indicate true optimal health. For example, what does diabetes feel like? Or heart disease, hypertension or high cholesterol for that matter? If we are guided to care for our health only by our symptoms then we can be in big trouble.
Cells of the skin, bones, liver and other organs can repair and replicate themselves. But nerve cells lose virtually all their cellular replication ability during youth. Healing in damaged nerves is slow and unpredictable. However, a nerve, even without damage, can have altered function. This is a common condition is Neuropathy. Neuropathy that occurs at the nerve root is called Radiculopathy.
2. Prespondylosis
The degeneration of the intervertebral disc and surrounding tissues is called spondylosis. Spondylosis produces pathology in the spine and it can also cause radiculopathy leading to pathology in structures innervated by the spinal nerves (which is just about every organ and every part of the body). In other words, many of the common health disorders that people suffer from (stomach, heart, digestion, respiratory, skin problems) may be due to nerve pressure from the spine.
Like many serious health problems, spondylosis and radiculopathy can often exist without pain. This is called “prespondylosis”. Prespondylosis and spondylosis are the most common causes of nerve impingement and neuropathy. But there are other causes of neuropathy as well. There are several hundred causes of peripheral neuropathy. The more common ones are peripheral trauma or injury, infection, metabolic diseases like diabetes and thyroid disease, vitamin and trace element deficiencies, chemical and radiation pollutants and even old age.
3. Cannon's Law
Cannon’s law states that tissues become "supersensitive" when their nerve supply is affected. All tissues crave a nerve supply. Without a normal, constant signal the affected tissues become irritable or supersensitive.
Nerve pressure in the lower back affects all the tissues in your leg and back in some abnormal way. The general response will be electrical supersensitivity, meaning that the altered nerve function will produce overly sensitive, spastic and shortened muscles. Remember again: Muscles tighten when their nerve is interrupted and that these tight muscles will then be susceptible to scarring and even more injury.
“Supersensitivity” increases sensitivity in target tissues not just by a little, but sometimes by over 1000 times. Supersensitivity can occur in practically any part of the body including skeletal muscle, smooth muscle (the muscle of internal organs), spinal neurons, adrenal glands, sweat glands, and even brain cells. Supersensitive pressure receptors are the reason that neuropathic muscles are tender. Some individuals with musculoskeletal pain even feel temperature changes and barometric pressure changes in the weather. The motor points (where the motor neuron attaches to the muscle) have the greatest most abundance of pressure receptors which is why tender motor points in the muscle are one of the of key indicators of neuropathy. The result is that the entire muscle responds abnormally to an arriving signal from the motor neuron.
When nerve supply to a muscle is interrupted, the nearby surviving nerve axons send sprouts to invade the deprived area (like weeds on an unhealthy lawn). This sprouting distorts the normal pattern of muscle contraction and intensifies it even with a small from the motor neuron. This intensification of muscle action can be seen on surface electromyography.
Another reason that neuropathic muscles become “twitchy” is the reduction in acetylcholine esterase, an enzyme that normally breaks down the neurotransmitter acetylcholine. In neuropathy a small stimulus from the nerve is greatly exaggerated into spasms and contractures.
All these abnormal changes wreak havoc on the body. Skeletal muscle cannot respond normal to instructions issued by the brain. Different fibers will contract individually and without coordination. These changes are gradual and creep up on you possibly with little clues like twitches, cramps and spasms. The condition ultimately becomes a contracture when the muscle assumes a permanently shortened state. This muscle will feel tender and ropey. Uncorrected, contracture can lead to fibrous changes and gets labeled as “fibrositis.”
Even though you cannot see neuropathic pain on an x-ray, CT scan or a blood test you can detect it by the proper physical examination methods. The distribution of neuropathy usually correlates with specific affected nerve roots of the spine involving any or all components of the nerve: motor, sensory and autonomic. The importance of the spine and nervous system to our overall health underlines why regular chiropractic care is important, even in the absence of symptoms. The silent and potentially health damaging effects of prespondylosis, neuropathy and radiculopathy do not always warn of with something we can sense or feel, even as they silently damage the health of both adults and children.
Also refer to my article “Three Types of Pain.”
Dr. Philip Greenwood, D.C.
Murrieta, California
