Section 6. Brain, Spinal Cord, and Nerve Disorders

Chapter 78. Pain

Topics: Introduction | Evaluation of Pain | Types of Pain | Treatment of Pain

Introduction

Pain is an unpleasant sensation signaling actual or possible injury.

Pain may be sharp or dull, intermittent or constant, or throbbing or steady. Sometimes pain is very difficult to describe. Pain may be felt at a single site or over a large area. The intensity of pain can vary from minor to intolerable.

People differ remarkably in their ability to tolerate pain. One person cannot tolerate the pain of a small cut or bruise, but another person can tolerate pain caused by a major accident or knife wound with little complaint. The ability to withstand pain varies according to mood, personality, and circumstance. In a moment of excitement during an athletic match, an athlete may not notice a severe bruise but is likely to be very aware of the pain after the match, particularly if the team lost.

The ability to tolerate pain may change with age. As people age, they complain less of pain, perhaps because changes in the body decrease the sensation of pain. On the other hand, older people may simply be more stoic than younger people.

Pain Pathways: Pain due to injury begins at special pain receptors scattered throughout the body. These pain receptors transmit messages or signals as electrical impulses along nerves to the spinal cord and then upward to the brain. Sometimes the signal evokes a reflex response (see Section 6, Chapter 77). When the signal reaches the spinal cord, a signal is immediately sent back along motor nerves to the original site of the pain, triggering the muscles to contract without involving the brain. For example, when people inadvertently touch something very hot, they immediately pull away. This reflex reaction helps prevent permanent damage. The pain signal is also sent to the brain. Only when the brain processes the signal and interprets it as pain do people become conscious of the pain.

Pain receptors and their nerve pathways differ in different parts of the body. For this reason, pain sensation varies with the type and location of injury. For example, pain receptors in the skin are plentiful and capable of transmitting precise information, including where an injury is located and whether the source was sharp, such as a knife wound, or dull, such as pressure, heat, or cold. In contrast, pain receptors in the intestine are limited and imprecise. The intestine can be pinched, cut, or burned without generating a pain signal. However, stretching and pressure can cause severe intestinal pain, even from something as relatively harmless as a trapped gas bubble. The brain cannot identify the precise source of intestinal pain; the pain is difficult to locate and is likely to be felt over a large area.

See the figure What Is Referred Pain?

Sometimes pain felt in one area of the body does not accurately represent where the problem is, because the pain is referred there from another area. Pain can be referred because signals from several areas of the body often travel through the same nerve pathways going to the spinal cord and brain. For example, pain from a heart attack may be felt in the neck, jaws, arms, or abdomen. Pain from a gallbladder attack may be felt in the back of the shoulder.