Different parts of the body vary in their sensitivity to painful stimuli, according to the number and distribution of the receptors. For example, the cornea is a hundred times more sensitive to pain than the soles of the feet.

Pain is Complicated

Until recently, it was thought that pain was a simple message by which neurons sent messages from the site of injury directly to the brain. Now we know that it is a more complex process. Nerve impulses from injury sites that last for hours or days or longer lead to changes in the nervous system that result in an amplification and increased duration of the pain. These changes involve dozens of chemical messengers and receptors. In many aspects, persistent pain is a disease of the nervous system and not just a symptom of a disease.

The Pain Causers

The sensory fibers that respond to pain stimuli are called nocireceptors, special receptors that respond to tissue-damaging stimuli. Furthermore, injury can cause the release of numerous chemicals at the site of damage and inflammation. One such family of chemicals includes the prostaglandins, which enhance the sensitivity of the receptors to tissue damage and can ultimately induce more intense pain sensations. They also contribute to the clinical condition where the sensitivity of the nocireceptors is reduced significantly, with the result that normal stimuli (such as a touch) can produce pain.

The Pain Transmitters

Pain is transmitted to the spinal cord by small myelinated fibers and C fibers (very small unmyelinated fibers). The small myelinated fibers produce fast, sharp pain, while the C fibers induce pain that is slower in onset, dull and more diffuse. The reason for this is that myelin increases transmission speed. Pain can be influenced by two systems:

• The ascending system: Impulses are relayed from the spinal cord to several brain structures that are involved in pain becoming a more conscious experience. The emotional aspect of the experience is a major contributor to the overall pain.
• The descending system: Pain is suppressed by a system of neurons originating in the gray matter of the brain. This descending system sends messages to the dorsal horn of the spinal cord, where it suppresses the transmission of pain messages to the higher brain centers. The chemicals used by the system are quite similar to opioids. Knowledge of this system has led to new treatments for chronic pain.

Further Research

Studies using modern imaging tools to monitor brain activity when pain is experienced, have provided many new insights. One finding is that there is no single brain area where pain is generated, rather there are both emotional and sensory components.

As such techniques for brain study develop, it should be possible to monitor the changes in the brain that occur in people with persistent pain and to better evaluate the analgesic drugs that are being developed.