Pain is an unpleasant sensation signaling actual or possible injury.
Pain is the most common reason people visit their doctor. 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 mild 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.
Acute Versus Chronic Pain:
Pain may be acute or chronic. Acute pain begins suddenly and usually does not last long. Chronic pain lasts for weeks or months. Usually, pain is considered chronic if it does one of the following:
When severe, acute pain may cause anxiety, a rapid heart rate, an increased breathing rate, elevated blood pressure, sweating, and dilated pupils. Usually, chronic pain does not have these effects, but it may result in other problems, such as depression, disturbed sleep, decreased energy, a poor appetite, weight loss, decreased sex drive, and loss of interest in activities.
During treatment for chronic pain, many people experience a brief, often severe flare-up of pain. It is called breakthrough pain because it breaks through in spite of regularly scheduled pain treatment. Typically, breakthrough pain begins suddenly, lasts up to 1 hour, and feels much like the original chronic pain except it is more severe. Breakthrough pain may differ from person to person and is often unpredictable.
Chronic pain can make the nervous system more sensitive to pain. For example, chronic pain repeatedly stimulates the nerve fibers and cells that detect, send, and receive pain signals. Repeated stimulation can change the structure of nerve fibers and cells or make them more active and can thus increase pain transmission to the spinal cord and brain. As a result, pain may result from stimulation that might not ordinarily be painful, or painful stimuli may be felt as more severe.
When pain occurs repeatedly, people may anticipate it by becoming fearful and anxious. These emotions can stimulate the body to produce substances that make pain feel more intense. An example is prostaglandins, which make nerve cells more likely to respond to pain signals. Fear and anxiety can also reduce the production of substances that reduce the sensitivity of nerve cells to pain. An example is endorphins, the body's natural pain relievers. Fatigue can have the same effects on pain as fear and anxiety.
These changes in pain sensitivity partly account for pain that persists after its cause resolves and for pain that feels more severe than expected.
Pain due to injury begins at special pain receptors scattered throughout the body. These pain receptors transmit signals as electrical impulses along nerves to the spinal cord and then upward to the brain. Sometimes the signal evokes a reflex response (see Fig. 1: Reflex Arc: A No-Brainer). 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 internal organs, such as 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, which is difficult to locate and is likely to be felt over a large area.
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 in 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.
Last full review/revision August 2007 by Russell K. Portenoy, MD