Pain, unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage. This definition, formulated in 1980 by the International Association for the Study of Pain, emphasises the psychological contribution to the experience of pain. (The same group has also used the term nociception-from the word noxious-for the experiencing of a stimulus that is tissue damaging.) Thus, the definition is primarily applicable to human beings. Animals exhibit behaviour that can be labelled pain, and such modes of behaviour have been studied intensively in research on the mechanisms of pain and in trials of potential pain-relieving drugs; but whether animals have the strong psychological component to pain that human beings do is doubtful. Pain is the single most common complaint for which people visit doctors.
Acute pain starts with the stimulation of one or more of the many special sense receptors, called nociceptors, in the skin or internal organs. These receptors receive information about intense heat, extreme pressure, sharp pricks or cuts, or other events that can cause body damage. Two types of nerve fibres carry this information from the nociceptors to the spinal cord: A-delta fibres, which transmit information quickly and appear to be responsible for the acute sense of pain; and C-type fibres, which transmit impulses more slowly and may cause the nagging sense of pain.
At the spinal cord, messages from nociceptors may be modulated by other spinal nerves that enhance or, more frequently, diminish the intensity of the pain stimulus. The impulse then travels to several parts of the brain. Some brain areas determine where the pain is and what is causing it, while other areas integrate the sensory information with the total state of the organism and produce the emotional sensation called pain. These same brain centres can activate long nerve fibres that descend to the place in the spinal cord where the pain signal originates and decrease the signal.
In the mid-1970s, researchers showed that many fibres that inhibit pain messages in the spinal cord release a neurotransmitter called enkephalin. Some areas of the brain that process pain messages secrete a related chemical called endorphin. Although the exact roles of these two substances in pain perception is not yet clear, scientists hope that studies of these chemicals may eventually give rise to better modes of pain treatment.
The complex nature of pain is illustrated by anecdotes about soldiers who are severely wounded and do not complain of pain, or of athletes who are injured but do not experience pain until the contest is over. In some cultures, an operation called trepanning is performed on the skull without anaesthetic. On the other hand, scientists have recently shown that the expectation of pain can actually intensify the experience, perhaps by inducing anxiety. The emotional component of pain is also illustrated by the words frequently used to describe its nature, such as "vicious", "nauseating", and "nagging".
Acute pain-such as that produced by physical trauma or burns, or following surgery-is most often treated with analgesic drugs, which can range from aspirin to morphine. In the terminal stages of cancer, combinations of painkilling drugs may be used, including psychotropic medication such as a tranquilliser or an antidepressant. In some patients who have had surgery, pain is effectively relieved by a nerve block: the injection of an anaesthetic into the regional nerve centre through which the nerves from the surgery site pass. With certain types of back pain, surgery can correct the problem causing the pain.
Beginning in about 1965, doctors came to appreciate the unique nature of the condition called chronic pain. In this syndrome, patients may complain of pain for years, without having any apparent organic injury as the cause. Researchers suggest that chronic pain is a behaviour state, initiated by a real injury, in which the pain has lasted so long that it has itself become the disease. Many such patients are dependent on strong painkilling medicines, and they have usually fallen into a cycle of pain, depression, and inactivity.
A number of special clinics have been formed to treat people who suffer from chronic pain. Such clinics emphasise reduction of drug dosages, along with exercise, activity therapy, and relaxation techniques such as hypnosis and biofeedback. Some include psychological counselling, and many attempt to change learned pain behaviour patterns by enlisting the help of the patient's family. In other cases, patients are helped by an electronic device, called a transcutaneous electronic nerve stimulator, which can be activated to send an electronic current up the spinal cord. The reason for the effectiveness of this device is not known, but it may stimulate the brain to send pain-inhibiting impulses down the spine.