Pain is the unpleasant and aversive feeling common to such experiences as a stubbed toe, a headache, a burnt finger, and salt in a wound. Typically, pain is characterized by its intensity, location and duration. It is initiated by stimulation of nociceptors in the peripheral nervous system, or by damage to or malfunction of the peripheral or central nervous systems. Cultural values, hypnotic suggestion, and cognitive activities such as distraction or appraisal can all significantly modulate pain's intensity and unpleasantness. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".

This often quoted definition was first published in 1979 by IASP in 'Vol 6 of the journal Pain, page 250. It is derived from a definition of pain given earlier by Harold Merskey: "An unpleasant experience that we primarily associate with tissue damage or describe in terms of tissue damage or both." Merskey, H. (1964) An Investigation of Pain in Psychological Illness, DM Thesis, Oxford. A definition widely employed in nursing, emphasizing the subjective nature of pain and the importance of believing patient reports, was introduced by Margo McCaffery in 1968: "Pain is whatever the experiencing person says it is, existing whenever he says it does". '

Pain is a major symptom in many medical conditions, and can significantly interfere with a person's quality of life and general functioning. It is the single most frequent reason for physician consultation in the United States. 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, dentistry, physiotherapy, and psychology. Pain medicine is a separate subspecialty figuring under some medical specialties like anesthesiology, physiatry, neurology, and psychiatry.

Physical pain is linked to various cultural, religious, philosophical, or social issues.

Terminology

• Pain, used without a modifier, usually refers to physical pain, but it may also refer to pain in the broad sense, i.e. suffering. Care should be taken to make the right distinction when required between the two meanings. For instance, 'philosophy of pain' is essentially about physical pain, while a philosophical outlook on pain is rather about suffering. Or, as another quite different instance, nausea or itch are not 'physical pain', but they are unpleasant sensory or bodily experience, and a person 'suffering' from severe or prolonged nausea or itch may be said to be 'in pain'. More generally, the terms pain and suffering are often used together in different senses which can become confusing: they may be used as synonyms; they may be used in 'contradistinction' to one another: e.g. "pain is inevitable, suffering is optional", or "pain is physical, suffering is mental"; they may be used to define each other: e.g. "pain is physical suffering", or "suffering is severe physical or mental pain". To avoid confusion, this article is about physical pain in the narrow sense of a sensory experience typically 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.

• Qualifiers, such as physical, mental, emotional, psychological, and spiritual, are often used for referring to more specific types of pain or suffering. In particular, 'physical pain' may be used along with 'mental pain' for distinguishing between two wide categories of pain. A first caveat concerning such a distinction is that it uses 'physical pain' in a broad sense that includes not only physical pain in the narrow sense but also other unpleasant bodily experiences such as itch or nausea. A second caveat is that the terms physical or mental should not be taken too literally: physical pain, as a matter of fact, happens through conscious minds and involves emotional aspects, while mental pain happens through physical brains and, being an emotion, it involves important bodily physiological aspects.

• Nociception, the unconscious activity induced by a harmful stimulus in sense receptors, peripheral nerves, spinal column and brain, should not be confused with physical pain, which is a conscious experience. Nociception or noxious stimuli usually cause pain, but not always, and sometimes pain occurs without them.

• Unpleasantness commonly means painfulness in a broad sense. It is also used in (physical) pain science for referring to the affective dimension of pain, usually in contrast with the sensory dimension. For instance: “Pain-unpleasantness is often, though not always, closely linked to both the intensity and unique qualities of the painful sensation.” Pain science acknowledges, in a puzzling challenge to IASP definition, that pain may be experienced as a sensation devoid of any unpleasantness: see below pain asymbolia.

Mechanism

Stimulation of a nociceptor, due to a chemical, thermal, or mechanical event that has the potential to damage body tissue, may cause nociceptive pain.

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.

1. Superficial somatic pain (or cutaneous pain) is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin and, due to the high concentration of nerve endings, produce a sharp, well-defined, localized pain of short duration. Examples of injuries that produce cutaneous pain include minor wounds, and minor (first degree) burns.
2. Deep somatic pain originates from ligaments, tendons, bones, blood vessels, fasciae, and muscles. It is detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull, aching, poorly-localized pain of longer duration than cutaneous pain; examples include sprains, broken bones, and myofascial pain.
3. Visceral pain originates from the viscera, or organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces pain that is usually more aching or cramping and of a longer duration than somatic pain. Visceral pain may be well-localized, but often it is extremely difficult to localize, and several injuries to visceral tissue exhibit "referred" pain, where the sensation is localized to an area completely unrelated to the site of injury.

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 slow-conducting, thinly myelinated A delta fibers or even slower-conducting, unmyelinated C fibers, respectively responsible for fast, localized, sharp pain and slow, poorly-localized, dull pain. Once stimulated, they transmit signals that travel to the spinal cord and up to 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. The brain contains no nociceptors, and hence cannot sense pain inside itself.

Pain may be experienced differently depending on genotype. For example, individuals with red hair may be more susceptible to pain caused by heat, but redheads with a non-functional melanocortin 1 receptor 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 hypersensitivity, while other mutations may be the root of paroxysmal extreme pain disorder.

Evolution of the Theory

Specificity Theory

his 1664 Treatise of Man, René Descartes traced a pain pathway. "Particles of heat" (A) activate a spot of skin (B) attached by a fine thread (cc) to a valve in the brain (de) where this activity opens the valve, allowing the animal spirits to flow from a cavity (F) into the muscles that then flinch from the stimulus, turn the head and eyes toward the affected body part, and move the hand and turn the body protectively. The underlying premise of this model - that pain is the direct product of a noxious stimulus activating a dedicated pain pathway, from a receptor in the skin, along a thread or chain of nerve fibers to the pain center in the brain, to a mechanical behavioral response - remained the dominant perspective on pain until the mid-nineteen sixties; though by 1965 the pathway had evolved to: free nerve endings are pain receptors and generate electrical-chemical pulses that travel along thin (A delta and C) "pain" fibers to the spinal cord and up the spinothalamic tract to the pain center in the thalamus.

Pattern Theory

This specificity theory (dedicated pain receptor and pathway) has been challenged by the theory, proposed initially in 1874 by Wilhelm Erb, that a pain signal can be generated by stimulation of any sensory receptor, provided the stimulation is intense enough: the pattern of stimulation (intensity over time and area), not the receptor type, determines whether nociception occurs. Alfred Goldscheider (1894) proposed that stimulation might accumulate ("summate") in the dorsal horns of the spinal cord and begin to signal pain once a certain threshold of accumulated stimulation has been crossed. William K. Livingston (1943) proposed that a "reverberatory" circuit in the dorsal horns might be the mechanism of this summation. Willem Noordenbos (1953) proposed that a signal carried from the area of injury along large diameter (thought to be "touch") fibers may inhibit the signal carried by the thinner "pain" fibers - the ratio of large diameter fiber signal to thin fiber signal determining pain intensity; hence, we rub a smack.

Gate Control Theory

This all set the scene for Melzack and Wall's classic 1965 Science article "Pain Mechanisms: A New Theory". Here the authors proposed that the large diameter and thin fibers meet at two places in the dorsal horn: the "transmission" cells (now called "projection neurons"), and their "inhibitory" cells. Both large diameter fiber signals and thin fiber signals excite the transmission (T) cells, and when the output of the T cells exceeds a critical level, pain begins. The job of the inhibitory cells is to inhibit activation of the T cells. The T cells are the gate on pain, and inhibitory cells can shut the gate. If your large diameter ("touch") and thin ("pain") fibers have been activated by a noxious event, they will be exciting T cells (opening the pain gate). At the same time, the large diameter fibers will be activating the inhibitory cells (tending to close the gate), while the thin fibers will be impeding the inhibitory cells (tending to leave the gate open). So, the more large diameter fiber activity relative to thin fiber activity at the inhibitory cells, the less pain you will feel. They had conceived a neural "circuit diagram" to explain why we rub a smack.

The authors then added to the model a pain modulating signal coming down from the brain to the dorsal horn. They pictured the large diameter fiber signals traveling, not only from the site of injury to the inhibitory and T cells in the dorsal horn, but also up to the brain where, depending on the state of the brain, they may trigger a signal back down to the dorsal horn to further modulate T cell activity and so pain intensity. This model provided a neuroscientific rationale for taking seriously the effect of motivation and cognition on pain.

The Dimensions of Pain

In 1968 Melzack and Casey described pain in terms of its three “dimensions”:

:*"Sensory-discriminative" - sense of the intensity, location, quality and duration of the pain,

:*"Affective-motivational" - unpleasantness and urge to escape the unpleasantness,

:*"Cognitive-evaluative" - cognitions such as appraisal, cultural values, distraction and hypnotic suggestion.

They theorized that the degree of unpleasantness a person feels (the affective-motivational dimension) is not solely determined by the intensity of the painful sensation (the sensory discriminative dimension), and that “higher” cognitive activities (the cognitive-evaluative dimension) can influence both unpleasantness and perceived intensity. Cognitive activities "may affect both sensory and affective experience or they may modify primarily the affective-motivational dimension. Thus, excitement in games or war appears to block both dimensions of pain, while suggestion and placebos may modulate the affective-motivational dimension and leave the sensory-discriminative dimension relatively undisturbed." (p. 432) The paper ended with a call to action:

:"Pain can be treated not only by trying to cut down the sensory input by anesthetic block, surgical intervention and the like, but also by influencing the motivational-affective and cognitive factors as well." P. 435.

Over the last forty years this paper's clearly drawn model of pain has firmly framed theory and guided research.

Evolutionary and behavioral role

Pain is part of the body's defense system, producing a reflexive retraction from the painful stimulus, and tendencies to protect the affected body part while it heals, and avoid that harmful situation in the future. Despite its unpleasantness, pain is an important part of animal life; in fact, it is vital to healthy survival and people with congenital insensitivity to pain have greatly reduced life expectancy.Pain is classified as acute or chronic.

• Acute pain can last a few seconds or a few months but not more than six months. It lets the body know damage has occurred and something needs to be done to make it better. As healing of the injured areas occurs, the pain will normally decrease and eventually go away.
• Chronic Pain is pain that lasts for more than six months. It is also known as persistent pain. It can be malignant, getting worse as the source of the pain worsens (as a tumor grows in cancer). It can also be non-malignant as in a chronic illness such as arthritis. It can fluctuate over time with periods of severe pain followed by periods of no pain at all. Pain that does not stop when an injury is healed is also called chronic pain. Pain is not a normal part of aging and should be considered abnormal, and treated and ameliorated at any age.

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 John Sarno argues 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) seems to be out of all proportion to any survival benefits.

Pain in health care

Pain of any type is the most common 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, throbbing 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 of itself. The study of pain has in recent years attracted many different fields such as pharmacology, neurobiology, nursing, dentistry, physiotherapy, and psychology. Pain medicine is a separate subspecialty figuring under some medical specialties like anesthesiology, physiatry, neurology, and psychiatry.

Diagnosis and assessment

To establish an understanding of an individual's pain, health-care practitioners will typically try to establish certain characteristics of the pain: site (localization), onset and offset, character, radiation, associated symptoms, time pattern, exacerbating and ameliorating factors, and severity. According to its duration, pain may be categorized as acute (short term), subacute (medium term), or chronic (long term).

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 may also draw upon 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:

• allodynia, pain due to a stimulus which does not normally provoke pain,
• hyperalgesia, an increased response to a stimulus which is normally painful,
• hypoalgesia, diminished pain in response to a normally painful stimulus.

Verbal characterization

A key characteristic of pain is its quality. Typical descriptions of pain quality include sharp, stabbing, tearing, squeezing, cramping, burning, lancinating (electric-shock like), or heaviness. It may be experienced as throbbing, dull, nauseating, shooting or a combination of these. Indeed, individuals who are clearly in extreme distress such as from a myocardial infarction may not describe the sensation as pain, but instead as an extreme heaviness on the chest. Another individual with pain in the same region and with the same intensity may describe the pain as tearing which would lead the practitioner to consider aortic dissection. Inflammatory pain is commonly associated with some degree of itch sensation, leading to a chronic urge to rub or otherwise stimulate the affected area. The difference between these diagnoses and many others rests on the quality of the pain. The McGill Pain Questionnaire is an instrument often used for verbal assessment of pain.

Non-Verbal Characterization

When a patient is non-verbal and cannot self report pain, observation becomes critical, and specific behaviors can be monitored as pain indicators. Behaviors such as facial grimacing and guarding indicate pain, as well as an increase in or decreased vocalizations, changes in routine behavior patterns and mental status changes. Patients experiencing pain may exhibit withdrawn social behavior and possibly experience a decreased appetite and decreased nutritional intake. A change in condition that deviates from baseline such as moaning with movement or when manipulating a body part, and limited range of motion are also potential pain indicators. In patients that are vocal but incapable of expressing themselves effectively, such as those with a dementia related diagnosis, an increase in confusion or display of aggressive behaviors, including agitation, may signal that discomfort exists, and further assessment is necessary.

Intensity

Pain may range in intensity from slight through severe to agonizing and can appear as constant or intermittent. The threshold of pain varies widely between individuals. Many attempts have been made to create a pain scale that can be used to quantify pain, for instance on a numeric scale that ranges from 0 to 10 points. In this scale, zero would be no pain at all and ten would be the worst pain imaginable. The purpose of these scales is to monitor an individual's pain over time, allowing care-givers to see how a patient responds to therapy for example. Accurate quantification can also allow researchers to compare results between groups of patients.

Localization

Pains are usually called according to their subjective localization in a specific area or region of the body: headache, toothache, shoulder pain, abdominal pain, back pain, joint pain, myalgia, etc. Localization is not always accurate in defining the problematic area, although it will often help narrow the diagnostic possibilities. Some pain sensations may be diffuse (radiating) or referred. Radiation of pain occurs in neuralgia when stimulation of a nociceptor at one site is perceived as pain in the sensory distribution of that nerve. Sciatica, for instance, involves pain running down the back of the buttock, leg and bottom of foot that results from compression of a nerve root in the lumbar spine. Referred pain usually happens when sensory fibers from the viscera enter the same segment of the spinal cord as somatic nerves, i.e., those from superficial tissues. The sensory nerve from the viscera stimulates the nearby somatic nerve so that the pain localization in the brain is confused. A well-known example is when the pain of a heart attack is felt in the left arm rather than in the chest.

Barriers to Reporting Pain

There are many factors that can interfere with the individuals ability to recognize and report pain. Age, gender and culture play a major role in the expression and recognition of pain.

• Infants can feel pain. Pre-term babies are more sensitive to painful stimuli than full term babies. They communicate pain by crying. They lack the verbal skills needed to report pain. A non-verbal pain assessment should be conducted with the infant. It should include the parents who may have noted changes in the infant not obvious to the health care provider.
• An Aging Adult may not respond to pain in the way that a younger person would. Their ability to recognize pain may be blunted by illness or the use of multiple prescription drugs. Depression may also keep the older adult from reporting they are in pain. Cognitive impairment, associated with dementia or other neurological problems, may make the person unable to tell others they are in pain. They may display behavior problems instead. The older adult may also quit doing activities they love because it hurts too much. Decline in self-care activities (dressing, grooming, walking, etc.) may also be indicators that the older adult is experiencing pain. The older adult may refrain from reporting pain because they are afraid they will have to have surgery or will be put on a drug they become addicted to. They may not want others to see them as weak, or may feel there is something impolite or shameful in complaining about pain, or they may feel the pain is deserved punishment for past transgressions.
• Cultural Barriers can also keep a person from telling someone they are in pain. Religious beliefs may prevent the individual from seeking help. They may feel certain pain treatment is against their religion. They may not report pain because they feel it is a sign that death is near. Many people fear the stigma of addiction and avoid pain treatment so as not to be prescribed addicting drugs. Studies have shown that pain is under-treated in minorities compared to whites. Many Asians do not want to lose respect in society by admitting they are in pain and need help. They believe the pain should be born in silence. Other cultures feel they should report pain right away and get immediate relief.

The healthcare provider's cultural background may affect their response to the individual's complaint of pain. This may interfere with treatment of pain.

• Gender can also be a factor in reporting pain. Gender differences are usually the result of social and cultural expectations. Women are expected to be emotional and show pain. Men are supposed to be stoic and keep pain to themselves. Some studies show that hormones play a major part in pain respose for women.

Management

Acute pain is usually treated with drugs, including anesthetics, analgesics, and (occasionally) anxiolytics, along with techniques aimed at healing the body. Informed clinicians and researchers now take acute pain very seriously. One hypothesis with strong support from animal studies and growing support from clinical trials proposes that the failure to adequately treat acute pain can lead to longer lasting and more intense posttraumatic/postoperative pain, and that timely, adequate acute pain management may prevent the development of chronic pain.

Though usually caused by an injury, an operation, or an obvious illness, chronic pain may also have no apparent cause. Chronic pain can trigger multiple psychological problems, and can severely impair affectionate social relations. Consequently, patients suffering from chronic pain may be referred to a variety of specialists, depending on the presumed cause of the pain and the effect it has had on the patient's emotional state and social connections. Multidisciplinary pain clinics have been growing in number over the last few decades and their approach is to treat the whole person: physical, psychological and social.

Anesthesia

Anesthesia is the condition of having the feeling of pain and other sensations blocked by drugs. It may involve complete unawareness or reduced awareness of the entire body (i.e., general anesthesia), or a total or partial lack of awareness of a part of the body (i.e., regional or local anesthesia).

Analgesia

Analgesia is a reduction in the unpleasantness or intensity of pain without loss of either consciousness or the sense of touch. The body possesses an endogenous analgesia system, which can be supplemented with analgesic drugs to regulate nociception and pain. Analgesia may occur in the central nervous system or in peripheral nerves and nociceptors.

The endogenous central analgesia system is mediated by three major components: the periaqueductal 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. Peripheral regulation consists of several different types of opioid receptor 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.

Complementary and alternative medicine

A survey of American adults found pain was the most common reason that people use complementary and alternative medicine.

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.

Pain treatment may be sought through the use of nutritional supplements such as curcumin, glucosamine, chondroitin, bromelain and omega-3 fatty acids. There is interest in the relationship between vitamin D and pain, but the evidence for its relationship to pain, other than in osteomalacia, from controlled trials appears unconvincing.

Hypnosis as well as diverse perceptional techniques provoking altered states of consciousness have proven to be of important help in the management of some types of pain in some people.

Some kinds of physical manipulation or exercise are showing interesting results in some pain conditions.]

Special cases

Phantom pain

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 quadriplegic. Phantom pain is a type of neuropathic pain.

Pain asymbolia

Pain science acknowledges, in a puzzling challenge to IASP definition,See IASP Pain Terminology. The whole entry on the term pain itself reads like this:

Pain. An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Note: The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life. Biologists recognize that those stimuli which cause pain are liable to damage tissue. Accordingly, pain is that experience we associate with actual or potential tissue damage. It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience. Experiences which resemble pain but are not unpleasant, e.g., pricking, should not be called pain. Unpleasant abnormal experiences (dysesthesias) may also be pain but are not necessarily so because, subjectively, they may not have the usual sensory qualities of pain. Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually this happens for psychological reasons. There is usually no way to distinguish their experience from that due to tissue damage if we take the subjective report. If they regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain. This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.

that pain may be experienced as a sensation devoid of any unpleasantness: this happens in a syndrome called pain asymbolia or pain dissociation, caused by conditions like lobotomy, cingulotomy or morphine analgesia. Typically, such patients report that they have pain but are not bothered by it, they recognize the sensation of pain but are mostly or completely immune to suffering from it.

Insensitivity to pain

The ability to experience pain is essential for protection from injury, and recognition of the presence of injury. Insensitivity to pain may occur in special circumstances, such as for an athlete in the heat of the action, or for an injured soldier happy to leave the battleground. This phenomenon is now explained by the gate control theory. However, insensitivity to pain may also be an acquired impairment following conditions such as spinal cord injury, diabetes mellitus, or more rarely Hansen's Disease (leprosy). A few people can also suffer from congenital insensitivity to pain, or congenital analgesia, a rare genetic defect that puts these individuals at constant risk from the consequences of unrecognized injury or illness. Children with this condition suffer carelessly repeated damages to their tongue, eyes, bones, skin, muscles. They may attain adulthood, but they have a shortened life expectancy.

Psychogenic pain

Psychogenic pain, also called psychalgia or somatoform pain, is physical pain that is caused, increased, or prolonged by mental, emotional, or behavioral factors. Psychogenic pain commonly manifests as headache, back pain, or stomach pain. Sufferers are often stigmatized, because both medical professionals and the general public tend to think that pain from a psychological source is not "real". However, specialists consider that it is no less actual or hurtful than pain from other sources.

Pain as pleasure

Society and culture

Physical pain has been diversely understood or defined from antiquity to modern times.

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 for various reasons including genetics, cultural background, ethnicity and sex.

Physical pain is an important political topic in relation to various issues, including distribution of resources 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 behavior. 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.

In other than human beings

As explained above, the most reliable method for assessing pain in most humans is by asking a question: a person may report pain that cannot be detected by any known physiological measure. However, non-human animals, like infants (Latin infans meaning "unable to speak"), cannot answer questions about whether they feel pain; thus the defining criterion for pain in humans cannot be applied to them. Philosophers and scientists have responded to this difficulty in a variety of ways. René Descartes for example argued that animals lack consciousness and therefore do not experience pain and suffering in the way that humans do. Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and that veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain. In his interactions with scientists and other veterinarians, he was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain. Carbone writes that the view that animals feel pain differently is now a minority view. Academic reviews of the topic are more equivocal, noting that although the argument that animals have at least simple conscious thoughts and feelings has strong support, some critics continue to question how reliably animal mental states can be determined. The ability of invertebrate species of animals, such as insects, to feel pain and suffering is also unclear.

Currently the most prevalent attitude is that the presence of pain in an animal, or another human for that matter, cannot be known for certain, but it can be inferred through physical and behavioral reactions. Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too. As for other animals, or for plants, computers, or other entities, their ability to feel physical pain is at present a question beyond scientific reach, since no mechanism is known by which they could have such a feeling. In particular, there are no known nociceptors in groups such as plants, fungi, and most insects, except for instance in fruit flies.

In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opiate receptors. Opioid peptides and opiate receptors occur naturally in crustaceans, and although “at present no certain conclusion can be drawn,” some have interpreted their presence as an indication that lobsters may be able to experience pain. The aforementioned Scottish paper holds that lobsters' opioids may "mediate pain in the same way" as in vertebrates. Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.

References

External links

• Acute Pain Medicine: Scientific Evidence (2nd ed) (2007 updated version)
• Pain (Stanford Encyclopedia of Philosophy)