Arthritis and Other Pain Syndromes

Chapter 1

The Physiology of Pain Production

A pain must be defined in order to understand the origin of that pain.  A pain is perceived in the brain as an electrical impulse.  Therefore where does that impulse come from?  Pain cannot be imagined, because it is the result of a noxious or painful stimulus causing a sensory nerve to fire off, creating an electrical impulse which travels from the source of that pain via the nerve to the spinal cord and then to the sensory areas of the brain.  Pain is caused by an injured structure.  We know this is true because there is always an antecedent event such as a fall or twisting which caused the symptom—pain does not begin out of the blue.  The tissue injury starts an inflammatory change, which is the body’s method of repair and begins after any injury.  Simply put, injured tissue releases chemicals in the body that promote inflammation, cause nerves to signal pain, and attract repair cells to the injured area.  For musculoskeletal pains like back and neck pains, there are only 5 structures in the body to cause a pain other than a fracture or tumor, which are entirely different than the usual causes and will not be considered in this text.

The first structure is a muscle.  Muscle pain is perceived a day after a strain injury occurs, such as lifting weights in the gym, raking leaves, or lifting the baby, or falling or tripping.  In other words, a trauma or accident is the cause.  Muscle pain typically lasts for 3-4 days maximally.  There is a general soreness where the strain took place, which is in the substance of the muscle tissue.  Palpation or squeezing the muscle will produce a typical soreness.  There will be no referral of pain to the leg or arm or tingling.  Muscle pain will resolve itself in the expected 3-4 days.  Aspirin is the best drug for this pain because it is an anti-inflammatory medication.  The injury to the muscle may not be clear, but the time for improvement is clear.

The second pain producer is nerve.  Any nerve has a beginning and an end point.  There are two components of any peripheral nerve, which is a nerve away from the spine.  These components are the sensory, which allows you to feel, and the motor, which produces movement.  If the sensory component is affected there will be a sensory change, like a tingling and mild loss of touch sensation.  If the sensory component is severely affected there may be complete numbness in the areas of the body that the nerve serves, which are well known to your physician.  If the motor component is affected there will be weakness in the muscles that the nerve goes to.  Pure motor nerve diseases, such as polio and amyotrophic lateral sclerosis, or Lou Gehrig’s disease, are never painful.  Those afflicted have weakness in a specific distribution of the nerve.  Another well known example is when someone strikes the back of the elbow hitting the “funny bone”, which is actually the ulnar nerve.  They will notice a tingling sensation, but it will not be described as a pain usually. 

If you have a pain in the leg or the arm without the above sensory or motor changes, it is likely that there is no “pinched nerve”, but there are other structures causing that pain.  A nerve does not usually cause a pain but when compressed, it will produce a tingling sensation in a specified area that is well known.  A typical nerve from the low back such as L₅ will cause tingling to the ankle, top of the foot, and big toe, but very little pain.  The surrounding structures causing the nerve to malfunction is actually causing the pain.  A nerve should never be touched, or injected, but the surrounding structures must be treated to relieve the pain.  The success of treatment is noted very quickly as the tingling is resolved.  The principle to understand here is that a pain down the leg is not necessarily a pinched nerve as some will espouse.  There are other structures that will cause pain in the arm or leg.  Without the tingling and sensory loss in specified areas, there is probably no nerve damage.  Peripheral nerves do heal after injury, albeit slowly over time.  The cause and severity of any nerve abnormality must be taken into consideration, for many nerve diseases have irreversible changes.  Prognosis for each condition should really be discussed with your physician.  These abnormalities will be discussed further in this book in other chapters.