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 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
A reflex arc is the pathway that a nerve reflex, such as the knee jerk reflex, follows.
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 felt in one area of the body does not always represent where the problem is because the pain may be referred there from another area. For example, pain produced by a heart attack may feel as if it is coming from the arm because sensory information from the heart and the arm converge on the same nerve pathways in the spinal cord.
Acute versus chronic 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 Chronic Pain Chronic pain is pain that lasts or recurs for months or years. Usually, pain is considered chronic if it does one of the following: Lasts for more than 3 months Lasts for more than 1 month after... read more 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.
Causes of Pain
Different types of pain have different causes.
Nociceptive pain Nociceptive Pain Nociceptive pain is caused by an injury to body tissues. (See also Overview of Pain.) Most pain is nociceptive pain. It results from stimulation of pain receptors for tissue injury (nociceptors)... read more 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 Neuropathic Pain Neuropathic pain is caused by damage to or dysfunction of the nerves, spinal cord, or brain. (See also Overview of Pain.) Neuropathic pain may result from Compression of a nerve—for example... read more 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 Overview of the Peripheral Nervous System The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord. Thus, the peripheral nervous... read more ). It may occur when
Pressure is put on a specific nerve—for example, by a tumor or a ruptured disk Herniated Disk A herniated disk occurs when the tough covering of a disk in the spine tears or ruptures. The soft, jelly-like interior of the disk may then bulge out (herniate) through the covering. Aging... read more in the spine, causing low back pain Low Back Pain Low back pain and neck pain are among the most common reasons for health care visits. The pain usually results from problems with the musculoskeletal system—most notably the spine, including... read more and/or pain radiating down the leg. Pressure on a nerve in the wrist can cause carpal tunnel syndrome Carpal Tunnel Syndrome Carpal tunnel syndrome is a painful compression (pinching) of the median nerve as it passes through the carpal tunnel in the wrist. The cause of most cases of carpal tunnel syndrome is unknown... read more .
Nerves are damaged, as occurs in diabetes mellitus Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more or postherpetic neuralgia Postherpetic Neuralgia Postherpetic neuralgia is chronic pain in areas of skin supplied by nerves infected with herpes zoster (shingles). Shingles is a painful rash of fluid-filled blisters that is caused by reactivation... read more (pain after shingles) Shingles Shingles is a painful skin rash caused by a viral infection that results from reactivation of the varicella-zoster virus, the virus that causes chickenpox. What causes the virus to reactive... read more .
The brain and spinal cord do not process pain signals normally or something disrupts this processing, as occurs in phantom limb pain Phantom limb pain Neuropathic pain is caused by damage to or dysfunction of the nerves, spinal cord, or brain. (See also Overview of Pain.) Neuropathic pain may result from Compression of a nerve—for example... read more , postherpetic neuralgia (pain after shingles), and complex regional pain syndrome Complex Regional Pain Syndrome Complex regional pain syndrome is characterized by persistent burning or aching pain plus certain abnormalities that occur in the same area as the pain. Abnormalities include increased or decreased... read more .
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 Pain At first, cancer, as a tiny mass of cells, causes no symptoms whatsoever (see also Overview of Cancer). As a cancer grows, its physical presence can affect nearby tissues (see also Warning Signs... read more 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 Psychologic Factors That Contribute to Pain Psychologic factors that commonly contribute to pain, particularly chronic pain, may include anxiety, depression, and insomnia. (See also Overview of Pain.) Psychologic factors can strongly... read more 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.