Pain, in the sense of physical pain, is a typical sensory experience that may be described as the unpleasant awareness of a noxious stimulus or bodily harm. Individuals experience pain by various daily hurts and aches, and occasionally through more serious injuries or illnesses. For scientific and clinical purposes, pain is defined by the International Association for the Study of Pain (IASP) as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".
Pain is highly subjective to the individual experiencing it. A definition that is widely used in nursing was first given as early as 1968 by Margo McCaffery: "'Pain is whatever the experiencing person says it is, existing whenever he says it does".
Pain of any type is the most frequent reason for physician consultation in the United States, prompting half of all Americans to seek medical care annually. It is a major symptom in many medical conditions, significantly interfering with a person's quality of life and general functioning. Diagnosis is based on characterizing pain in various ways, according to duration, intensity, type (dull, burning or stabbing), source, or location in body. Usually pain stops without treatment or responds to simple measures such as resting or taking an analgesic, and it is then called ‘acute’ pain. But it may also become intractable and develop into a condition called chronic pain, in which pain is no longer considered a symptom but an illness by itself. The study of pain has in recent years attracted many different fields such as pharmacology, neurobiology, nursing sciences, dentistry, physiotherapy, and psychology. Pain medicine is a separate subspecialty figuring under some medical specialties like anesthesiology, physiatry, neurology, psychiatry.
Pain is part of the body's defense system, triggering a reflex reaction to retract from a painful stimulus, and helps adjust behaviour to increase avoidance of that particular harmful situation in the future. Given its significance, physical pain is also linked to various cultural, religious, philosophical, or social issues.
The terms pain and suffering are often used together in different senses which can become confusing, for example:
To avoid confusion: this article is about physical pain in the narrow sense of a typical sensory experience associated with actual or potential tissue damage. This excludes pain in the broad sense of any unpleasant experience, which is covered in detail by the article Suffering.
Damage to the nervous system itself, due to disease or trauma, may cause neuropathic (or neurogenic) pain. Neuropathic pain may refer to peripheral neuropathic pain, which is caused by damage to nerves, or to central pain, which is caused by damage to the brain, brainstem, or spinal cord.
Nociceptive pain and neuropathic pain are the two main kinds of pain when the primary mechanism of production is considered. A third kind may be mentioned: see below psychogenic pain.
Nociceptive pain may be classified further in three types that have distinct organic origins and felt qualities.
Nociception is the unconscious afferent activity produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue. It should not be confused with pain, which is a conscious experience. It is initiated by nociceptors that can detect mechanical, thermal or chemical changes above a certain threshold. All nociceptors are free nerve endings of fast-conducting myelinated A delta fibers or slow-conducting unmyelinated C fibers, respectively responsible for fast, localized, sharp pain and slow, poorly-localized, dull pain. Once stimulated, they transmit signals that travel along the spinal cord and within the brain. Nociception, even in the absence of pain, may trigger withdrawal reflexes and a variety of autonomic responses such as pallor, diaphoresis, bradycardia, hypotension, lightheadedness, nausea and fainting.
Brain areas that are particularly studied in relation with pain include the somatosensory cortex which mostly accounts for the sensory discriminative dimension of pain, and the limbic system, of which the thalamus and the anterior cingulate cortex are said to be especially involved in the affective dimension.
The gate control theory of pain describes how the perception of pain is not a direct result of activation of nociceptors, but instead is modulated by interaction between different neurons, both pain-transmitting and non-pain-transmitting. In other words, the theory asserts that activation, at the spine level or even by higher cognitive brain processes, of nerves or neurons that do not transmit pain signals can interfere with signals from pain fibers and inhibit or modulate an individual's experience of pain.
Pain may be experienced differently depending on genotype; as an example individuals with red hair may be more susceptible to pain caused by heat, but redheads with a non-functional melanocortin 1 receptor (MC1R) gene are less sensitive to pain from electric shock. Gene Nav1.7 has been identified as a major factor in the development of the pain-perception systems within the body. A rare genetic mutation in this area causes non-functional development of certain sodium channels in the nervous system, which prevents the brain from receiving messages of physical damage, resulting in congenital insensitivity to pain. The same gene also appears to mediate a form of pain hyper-sensitivity, while other mutations may be the root of paroxysmal extreme pain disorder.
Despite its unpleasantness, pain is an important part of the existence of humans and other animals; in fact, it is vital to healthy survival (see below Insensitivity to pain). Pain encourages an organism to disengage from the noxious stimulus associated with the pain. Preliminary pain can serve to indicate that an injury is imminent, such as the ache from a soon-to-be-broken bone. Pain may also promote the healing process, since most organisms will protect an injured region in order to avoid further pain.
Interestingly, the brain itself is devoid of nociceptive tissue, and hence cannot experience pain. Thus, a headache is not due to stimulation of pain fibers in the brain itself. Rather, the membrane surrounding the brain and spinal cord, called the dura mater, is innervated with pain receptors, and stimulation of these dural nociceptors is thought to be involved to some extent in producing headache pain. The vasoconstriction of pain-innervated blood vessels in the head is another common cause. Some evolutionary biologists have speculated that this lack of nociceptive tissue in the brain might be because any injury of sufficient magnitude to cause pain in the brain has a sufficiently high probability of being fatal that development of nociceptive tissue therein would have little to no survival benefit.
Chronic pain, in which the pain becomes pathological rather than beneficial, may be an exception to the idea that pain is helpful to survival, although some specialists believe that psychogenic chronic pain exists as a protective distraction to keep dangerous repressed emotions such as anger or rage unconscious. It is not clear what the survival benefit of some extreme forms of pain (e.g. toothache) might be; and the intensity of some forms of pain (for example as a result of injury to fingernails or toenails) seem to be out of all proportion to any survival benefits.
By using the gestalt of these characteristics, the source or cause of the pain can often be established. A complete diagnosis of pain will require also to look at the patient's general condition, symptoms, and history of illness or surgery. The physician may order blood tests, X-rays, scans, EMG, etc. Pain clinics may investigate the person's psychosocial history and situation.
Pain assessment also uses the concepts of pain threshold, the least experience of pain which a subject can recognize, and pain tolerance, the greatest level of pain which a subject is prepared to tolerate. Among the most frequent technical terms for referring to abnormal perturbations in pain experience, there are:
Medical management of pain has given rise to a distinction between acute pain and chronic pain. Acute pain is 'normal' pain, it is felt when hurting a toe, breaking a bone, having a toothache, or walking after an extensive surgical operation. Chronic pain is a 'pain illness', it is felt day after day, month after month, and seems impossible to heal.
In general, physicians are more comfortable treating acute pain, which usually is caused by soft tissue damage, infection and/or inflammation among other causes. It is usually treated simultaneously with pharmaceuticals, commonly analgesics, or appropriate techniques for removing the cause and for controlling the pain sensation. The failure to treat acute pain properly may lead to chronic pain in some cases.
General physicians have only elementary training in chronic pain management. Often, patients suffering from it are referred to various medical specialists. Though usually caused by an injury, an operation, or an obvious illness, chronic pain may as well have no apparent cause, or may be caused by a developing illness or imbalance. This disorder can trigger multiple psychological problems that confound both patient and health care providers, leading to various differential diagnoses and to patient's feelings of helplessness and hopelessness. Multidisciplinary pain clinics are growing in number since a few decades.
Analgesia is an alteration of the sense of pain without loss of consciousness. The body possesses an endogenous analgesia system, which can be supplemented with painkillers or analgesic drugs to regulate nociception and pain. Analgesia may occur in the central nervous system or in peripheral nerves and nociceptors. The perception of pain can also be modified by the body according to the gate control theory of pain.
The endogenous central analgesia system is mediated by 3 major components : the periaquaductal grey matter, the nucleus raphe magnus and the nociception inhibitory neurons within the dorsal horns of the spinal cord, which act to inhibit nociception-transmitting neurons also located in the spinal dorsal horn. The peripheral regulation consists of several different types of opioid receptors that are activated in response to the binding of the body's endorphins. These receptors, which exist in a variety of areas in the body, inhibit firing of neurons that would otherwise be stimulated to do so by nociceptors.
The gate control theory of pain postulates that nociception is "gated" by non-noxious stimuli such as vibration. Thus, rubbing a bumped knee seems to relieve pain by preventing its transmission to the brain. Pain is also "gated" by signals that descend from the brain to the spinal cord to suppress (and in other cases enhance) incoming nociceptive information.
Traditional Chinese medicine views pain as a 'blocked' qi, akin to electrical resistance, with treatments such as acupuncture claimed as more effective for nontraumatic pain than traumatic pain. Although the mechanism is not fully understood, acupuncture may stimulate the release of large quantities of endogenous opioids.
Hypnosis as well as diverse perceptional techniques provoking altered states of consciousness have proven to be of important help in the management of all types of pain.
Some kinds of physical manipulation or exercise are showing interesting results as well.
Phantom pain is the sensation of pain from a limb or organ that has been lost or from which a person no longer receives physical signals. Phantom limb pain is an experience almost universally reported by amputees and quadriplegics. Phantom pain is a neuropathic pain.
Philosophy of pain is a branch of philosophy of mind that deals essentially with physical pain. Identity theorists assert that the mental state of pain is completely identical with some physical state caused by various physiological causes. Functionalists consider pain to be defined completely by its causal role and nothing else.
Religious or secular traditions usually define the nature or meaning of physical pain in every society. Sometimes, extreme practices are highly regarded: mortification of the flesh, painful rites of passage, walking on hot coals, etc.
Variations in pain threshold or in pain tolerance occur between individuals because of genetics, but also according to cultural, ethnical, or gender background.
Physical pain is an important political topic in relation to various issues, including resources distribution for pain management, drug control, animal rights, torture, pain compliance (see also pain beam, pain maker, pain ray). Corporal punishment is the deliberate infliction of pain intended to punish a person or change his/her behavior. Historically speaking, most punishments, whether in judicial, domestic, or educational settings, were corporal in basis.
More generally, it is rather as a part of pain in the broad sense, i.e. suffering, that physical pain is dealt with in cultural, religious, philosophical, or social issues.
Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.