Pain is defined as an unpleasant sensory and emotional experience that is associated with actual or potential tissue damage. It is the body’s warning system, signaling that something is wrong. There may not be evident or actual tissue damage for the pain to occur, which is especially true with chronic pain.

The American Pain Foundation describes pain as the fifth vital sign after blood pressure, pulse, respiration and temperature. Vital signs are clues that physicians use to diagnose and treat illnesses. Pain is a symptom of a wide variety of injuries, conditions and diseases.

Pain is a function of the nervous system, which is made up of nerve cells (neurons) and fibers that send and receive messages in the form of electrical currents and chemical interactions.

There are three major categories of nerves:

• Autonomic nerves. Maintain unconscious body processes, such as breathing, heart rate, blood pressure and perspiration.
  
  
• Motor nerves. Control muscle movement.
  
  
• Sensory nerves. Govern sensation. For instance, these nerves allow a person to feel an object while touching it.

Pain results from an interaction between several components of the nervous system, including:

• Peripheral nerves. Nerves that extend from the spinal cord to the skin, muscles and internal organs. Some peripheral nerves are designed to sense temperature, touch, pressure and vibration. Others are designed to detect potential or actual damage. These types of nerves are called (nociceptors). They are located throughout the skin, bones, joints and muscles but are concentrated in areas more prone to injury, such as fingers and toes.
  
  
• Spinal cord. Nerve fibers that carry messages enter the spinal cord in an area called the dorsal horn (a part of the spinal cord that processes messages). The nerve fibers release neurotransmitters (chemicals) that activate nerve cells in the spinal cord. These nerve cells process the information and send it to the brain.
  

• Brain. Nerve messages enter the area of the brain called the thalamus (located in the center of the brain, it is formed of gray matter and serves as a relay station for nerve impulses). The thalamus sends the messages simultaneously to three areas of the brain – the physical sensation region (somatosensory cortex), the emotional feeling region (limbic system) and the thinking region (frontal cortex).

The body may respond to pain messages in several ways. Chemicals released by spinal nerve cells may increase the strength of the pain signal that reaches the brain. This process is called wind-up or sensitization. The brain may block pain messages from traveling to higher parts of the brain, or it may send nerve signals that release natural painkillers (called endorphins or enkephalins) that diminish pain messages.

Other substances in the body cause the opposite reaction. A protein called substance P stimulates nerve endings at the site of the injury and spinal cord. This increases pain messages and worsens the pain sensation.

Sometimes people experience pain from a stimulus that normally causes no discomfort, such as light touch or a breeze. This condition, allodynia, results when neurons in the central nervous system become unusually excitable. Allodynia is a common feature of disorders including migraines and fibromyalgia.

Patients may also experience hyperalgesia (extreme sensitivity to pain) or referred pain, which originates in another part of the body.

Sleep disturbances and fatigue often accompany pain. These symptoms can in turn cause additional disorders such as headaches and muscle pain (myalgia). The many pain conditions that involve fatigue include arthritis, fibromyalgia, lupus, chronic fatigue syndrome, polymyalgia rheumatica, myositis, sarcoidosis, sickle cell anemia and Lyme disease.

In recent years, the medical community has recognized that pain is not simply a physical response to an unpleasant external stimulus (noxious stimulus). Rather, there are a number of biological, social and psychological factors that interact in complex ways to influence the experience of pain.

In the 1960s psychologist Ronald Melzack and anatomist Patrick Wall developed the gate control theory of pain. Under the theory, pain is “gated” or framed by past experience.

Gender and age also appear to play a role in pain. Research indicates that men and women respond differently to pain. Women are believed to seek help more quickly, are more likely to use a variety of support resources to deal with pain and recover more quickly from pain. The sex hormones estrogen (in women) and testosterone (in men) help account for the difference.

Individuals’ experiences of pain vary greatly. The greatest level of pain a person can endure is called the pain tolerance level. The lowest intensity of stimulation at which a person experiences pain is known as the threshold. People often reveal their response to pain through pain behaviors, such as grimacing, guarding the affected area, limping or avoiding activity.

According to the American Pain Foundation, pain is the number one complaint of older Americans, and one in five older Americans regularly use painkillers. Elderly patients are often treated using a special set of guidelines developed by the American Geriatrics Society. Children are also treated using special guidelines, in part because they often lack the verbal skills necessary to describe the pain they are experiencing.