Pain

Pain is experienced by humans and animals in response to excessive pres­sure, heat, or chemicals. Although humans often view pain as undesirable, pain helps protect them from injury by alerting them to its presence. The need for pain is revealed by diseases in which pain is absent or suppressed, as in leprosy. People with such disorders, unaware of injuries or infections, often die prematurely or require limb amputations because unfelt injuries may be neglected until they lead to massive infection and tissue death.

Although skin and other tissues contain cells that are activated by heat and pressure, pain arises from specialized cells that only respond to exces­sive stimuli that have the potential to cause damage. Light pressure or warm­ing has no effect on these cells. Once activated, these cells transmit pain signals to the spinal cord and brain where pain sensations occur.

Unlike other senses such as sight, pain is highly variable. For example, pain sensitivity decreases in animals pursued or caught by predators, in women giving birth, and in patients taking pain-killing drugs like morphine. Although the ability of morphine to decrease pain sensation has been known for thousands of years, it was not until the 1970s that specific receptors were found in the brain that mediate the effect of the drug. Shortly thereafter, it was discovered that the human body manufactures its own morphine like chemicals, known as endorphins, that provide the body with its own built- in pain-regulatory system. This internal pain-suppression system is activated during stress such as fleeing lions or delivering babies.

Injury to skin or nerves can also cause a long-lasting increase in pain sensitivity, known as hyperalgesia, that can persist for days or even years. Sunburn is a common example; the skin burn causes normally nonpainful touch to become painful for days. Changes in both the skin and the cen­tral nervous system appear responsible for hyperalgesia. In the skin, chem­icals released by inflammation, the process that also causes redness and swelling, sensitize the pain nerve endings to touch. In the central nervous system, neural circuits are permanently altered in much the same way that memories are stored.

Pain arises not only from skin and muscle, but also from internal or­gans such as the heart and kidney. Interestingly, pain from internal organs is often not perceived as arising from the internal organ, but instead from nearby areas of skin or muscle. This is known as referred pain. For exam­ple, heart attacks commonly produce pain perceived to arise from the left shoulder and not the heart.

Neurological injury following trauma or caused by diseases such as di­abetes sometimes leads to severe, unrelenting pain. Following amputation, the cut nerve in the limb continues to send pain signals to the brain even though the limb has been severed. Consequently, the brain perceives the pain as arising from the amputated limb. This is known as phantom pain.

Given how commonly humans experience pain, it is not surprising that many medical treatments have been developed to suppress it. For example, aspirin blocks the inflammation in skin that leads to hyperalgesia. Acupunc­ture, an ancient treatment in which the skin is punctured by a pattern of fine needles, activates the internal morphine system and reduces pain in much the same way as taking morphine.

References

Silverthorn, Dee. Human Physiology: An Integrated Approach, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Wall, Patrick. Pain: The Science of Suffering. London: Weidenfeld & Nicolson, 1999.

Wall, Patrick, and Ronald Melzack. Textbook of Pain. Edinburgh: Churchill Liv­ingston, 2000.