Pain Is a Warning
To understand neuropathic pain, it is necessary to understand “normal” pain, that is, what pain is and how it functions in normal circumstances. Pain is a warning. Its unpleasantness gets one’s attention in order for one to learn what can injure or damage the physical body. It is of significant evolutionary importance.
All pain is perceived in the brain; before that, it is just a signal from a point of injury. At the brain, this signal is used to locate where the pain is, how severe the injury is, and how painfully it should be perceived. Nociceptors are the nerve pain receptors that set off these signals when provoked by trauma, burns, inflammation, or infection.
From the nociceptors, the signal travels up that area’s nerve to the spinal cord, and where it enters is met by some altering, competitive signals. One of these is the brain’s inhibitory stimulus. (Once warned by pain, the brain makes an individual functional by dampening this pain so that he or she can get away from a source of injury.) Another alteration at the site is a connecting nerve called an “interneuron,” which also diminishes the pain as the signal crosses to the other side of the spine to rise up through spinal nerve tracts to the brain, the whole circuit giving feedback between the pain stimulation and its inhibition.
As an example, hitting one’s thumb results in an immediate pain signal (from an evolutionary standpoint, to stop hitting one’s thumb). Immediately after, the pain is not quite as severe. This is not because the thumb is any less injured; it is because of the dampening processes in action.
When Acute Pain Becomes Chronic Pain
If repeated signals continue to bombard the spinal areas where the pain is dampened, this system can become overwhelmed and recruit other previously inactive pain receptors in the spine. As this happens, the inhibitory interneuron changes into an “amplification” interneuron and pain signals continue to be generated and sent toward the brain at a more powerful level, even after the original source of the pain has long healed. Pain gets worse. Thus, the most common cause of chronic pain is inadequately treated acute pain.
When pain is due to nerve nociceptors firing off their pain signals by themselves, this is what is called neuropathic pain. Neuropathic pain will present as what the nociceptors involved in pain generation were designed to do–initiate sensations of itching, tingling, stabbing, aching, or devastating pain, but instead of initiating these sensations due to some injury, they have begun initiating them without provocation.
The Psychological Toll of Neuropathic Pain
Acute pain causes anger (the example of hitting the thumb with a hammer). Chronic pain is associated with depression. Overactive areas of the brain, as with neuropathic pain, “recruit” other areas from emotional centers, which is why a third of those with chronic pain have depression. Depression makes the perception of pain worse, and worse pain makes the depression worse. It becomes a vicious cycle that can only be stopped when both pain and its psychological effects are treated together.