Overview of Pain
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.
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 figure 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 aware 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, cold, or itching. 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.
Reflex Arc: A No-Brainer
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.
What Is Referred Pain?
Pain may be acute or chronic. Acute pain begins suddenly and usually does not last long (days, weeks, or sometimes a few months). Chronic pain lasts for many months or years.
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.
Different types of pain have different causes.
Nociceptive pain results from stimulation of pain receptors. It is caused by an injury to body tissues. Most pain, particularly acute pain, is nociceptive pain.
Neuropathic pain results from damage to or dysfunction of the brain or spinal cord (central nervous system) or the nerves outside the brain and spinal cord (peripheral nervous system). It may occur when
In diabetes, nerves outside the brain and spinal cord (peripheral nerves) are damaged. Symptoms include numbness, tingling, and pain in the toes, feet, and sometimes hands.
In postherpetic neuralgia, the area where the rash first occurred becomes painful and tender to the touch.
Nociceptive or neuropathic pain or both may be involved in acute or chronic pain. For example, chronic low back pain and most cancer pain are caused mainly by ongoing stimulation of pain receptors (nociceptive pain). But in these disorders, pain can also result from nerve damage (neuropathic pain).
Psychologic factors can also contribute to pain. Psychologic factors often affect how people feel pain and how intense it seems, but these factors are rarely the only cause of pain.