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Pain ICD-10 ICD-9 DiseasesDB MedlinePlus MeSH R52 338 9503 002164 D010146
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 sometimes 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". In medicine, pain is considered as highly subjective. 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 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 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. Pain is part of the body’s defense system, triggering a reflex reaction to retract from a painful stimulus, and helps adjust behavior 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. Etymology : "Pain (n.) 1297, "punishment," especially for a crime; also (c.1300) "condition one feels when hurt, opposite of pleasure," from O.Fr. peine, from L. poena "punishment, penalty" (in L.L. also "torment, hardship, suffering"), from Gk. poine "punishment," from PIE *kwei- "to pay, atone, compensate" (...)." —Online Etymology Dictionary
Clarification on the use of certain pain-related terms
• "Pain" used without a modifier usually refers to physical pain, but it may also refer to pain in the broad sense, i.e., suffering. The latter includes physical pain and mental pain, or any unpleasant feeling, sensation, and emotion. It may be described as a private feeling of unpleasantness and aversion associated with harm or threat of harm in an individual. 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 pain in the broad sense. Or, as another quite different instance, nausea or itch are not ’physical pains’, but they are unpleasant sensory or bodily experience, and a person ’suffering’ from severe or prolonged nausea or itch may be said ’in pain’. • Nociception, the unconscious activity induced by a harmful stimulus in sense receptors, peripheral nerves, spinal column and brain, should not be confused
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with physical pain, which is a conscious experience. Nociception or noxious stimuli usually cause pain, but not always, and sometimes pain occurs without them. • Qualifiers, such as mental, emotional, psychological, and spiritual, are often used for referring to more specific types of pain or suffering. In particular, ’mental pain’ may be used along with ’physical 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. • The term unpleasant or unpleasantness commonly means painful or 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. • Suffering is sometimes used in the specific narrow sense of physical pain, but more often it refers to mental pain, or more often yet to pain in the broad sense. Suffering is described as an individual’s basic affective experience of unpleasantness and aversion associated with harm or threat of harm. The terms pain and suffering are often used together in different senses which can become confusing, for example: • being used as synonyms; • being used in contradistinction to one another: e.g. "pain is inevitable, suffering is optional", or "pain is physical, suffering is mental";
• being 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 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.
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 neuropathic 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 body’s viscera, or organs. Visceral nociceptors are located within body organs and internal cavities. The even greater scarcity
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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 welllocalized, 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 fastconducting myelinated A delta fibers or slowconducting 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 nonpain-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.
Evolutionary and behavioral role
Pain is part of the body’s defense system, triggering mental and physical behavior to end the painful experience. It promotes learning so that repetition of the painful situation will be less likely. 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 has no nociceptive tissue, and hence cannot sense pain inside itself. 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 paininnervated 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
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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. However, it is likely that the significant pain levels experienced in these situations are related to the high sensitivity of nerves in these parts of the body. For instance, the nerves in the roots of teeth need to be particularly sensitive in order for the subject to be aware of the sensation of eating, since teeth move very little during this process.
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.
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.
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.
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 stimulus of a nerve at one site is perceived as
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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.
throughout the body (i.e., general anesthesia), or a lack of awareness in a part of the body (i.e., regional or local anesthesia).
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 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. 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.
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. This disorder can trigger multiple psychological problems that confound both patient and health care providers, leading to various differential diagnoses and to patients’ feelings of helplessness and hopelessness. Multidisciplinary pain clinics have been growing in number over the last few decades.
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
Anesthesia is the condition of having the feeling of pain and other sensations blocked by drugs that induces a lack of awareness. It may be a total or a minimal lack of awareness
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curcumin, glucosamine, chondroitin, bromelain and omega-3 fatty acids. There is interest in the relationship between vitamin D and pain, but the evidence is unconvincing. Severe vitamin D deficiency causes pain due to osteomalacia, but there is no clear mechanism for its relationship to other pain. As reported by Straube et al., of five small double-blind randomized controlled trials of vitamin D, only one found a statistically significant reduction in pain; uncontrolled case-series have showed reductions in pain, but there is nevertheless no persuasive evidence of a relationship between vitamin D status in those suffering from pain compared to controls. 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.
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, 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.
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.
Pain as pleasure
See also: algolagnia and sadomasochism
Pain science acknowledges, in a puzzling challenge to IASP definition, 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.
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.
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
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Variations in pain threshold or in pain tolerance occur between individuals because of genetics, but also according to cultural background, ethnicity and sex. 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.
known for sure, 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, plants, 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.
In other species
The idea that animals might not feel pain as human beings feel it traces back to the 17thcentury French philosopher, René Descartes, who argued that animals do not experience pain and suffering because they lack consciousness. Animals bred for research then killed as surplus, animals used for breeding purposes, and animals not yet weaned (which most laboratories do not count) 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. The presence of pain in an animal, or another human for that matter, cannot be
Pain in fish
Animal protection advocates have raised concerns about the possible suffering of fish caused by angling. In light of recent research, some countries, like Germany, have banned specific types of fishing and the British RSPCA now formally prosecutes individuals who are cruel to fish. Experiments done by William Tavolga provide evidence that fish have pain and fear responses. For instance, in Tavolga’s experiments, toadfish grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode. Additional tests conducted at both the University of Edinburgh and the Roslin Institute, in which bee venom and acetic acid was injected into the lips of rainbow trout, resulted in fish rubbing their lips along the sides and floors of their tanks, which the researchers believe was an effort to relieve themselves of pain. One researcher argues about the definition of pain used in the studies.
Pain in crustaceans
The question of whether or not lobsters can experience pain is unresolved. Because of the ambiguous nature of suffering, most people
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who contend that lobsters do have this capacity approach the issue using ’argument by analogy’ — that is, they hold that certain similarities between lobsters’ and humans’ biology or behavior warrant an assumption that lobsters can feel pain. In February 2005, a review of the literature by the Norwegian Scientific Committee for Food Safety tentatively concluded that "it is unlikely that [lobsters] can feel pain," though they note that "there is apparently a paucity of exact knowledge on sentience in crustaceans, and more research is needed." This conclusion is based on the lobster’s simple nervous system. The report assumes that the violent reaction of lobsters to boiling water is a reflex to noxious stimuli. However, review by the Scottish animal rights group Advocate for Animals released in the same year reported that "scientific evidence ... strongly suggests that there is a potential for [lobsters] to experience pain and suffering," primarily because lobsters (and other decapod crustaceans) "have opioid receptors and respond to opioids (analgesics such as morphine) in a similar way to vertebrates," indicating that lobsters’ reaction to injury changes when painkillers are applied. The similarities in lobsters’ and vertebrates’ stress systems and behavioral responses to noxious stimuli were given as additional evidence for their capacity for pain. A 2007 study at Queen’s University, Belfast, suggested that crustaceans do feel pain. In the experiment, when the antennae of prawns were rubbed with sodium hydroxide or acetic acid, the animals showed increased grooming of the afflicted area and rubbed it more against the side of the tank. Moreover, this reaction was inhibited by a local anesthetic, even though control prawns treated with only anesthetic did not show reduced activity. Professor Robert Elwood, who headed the study, argues that sensing pain is crucial to prawn survival, because it encourages them to avoid damaging behaviors. Some scientists responded, saying the rubbing may reflect an attempt to clean the affected area. In a subsequent 2009 study, Prof. Elwood and Mirjam Appel showed that hermit crabs make motivational tradeoffs between shocks and the quality of the shells they inhabit. In particular, as crabs are shocked more intensely, they become increasingly willing to leave their current shells for new shells, and
they spend less time deciding whether to enter those new shells. Moreover, because the researchers did not offer the new shells until after the electrical stimulation had ended, the change in motivational behavior was the result of memory of the noxious event, not an immediate reflex. Morphine, an analgesic, and naloxone, an opioid receptor antagonist, may affect a related species of crustacean (Chasmagnathus granulatus) in much the same way they affect vertebrates: injections of morphine into crabs produced a dose-dependent reduction of their defensive response to an electric shock. (However, the attenuated defensive response could originate from either the analgesic or sedative properties of morphine, or both) These findings have been replicated for other invertebrate species, but similar data is not yet available for lobsters.
 See section Clarification on the use of certain pain-related terms.  This often quoted definition was first published in 1979 by IASP in Pain journal, number 6, 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.  ^ See IASP Pain Terminology. The complete entry on the term ’pain’ reads: 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
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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.  Answer.com has this entry under subjective: "Medicine. Of, relating to, or designating a symptom or condition perceived by the patient and not by the examiner."  McCaffery M. Nursing practice theories related to cognition, bodily pain, and man-environment interactions. LosAngeles: UCLA Students Store. 1968.  More recently, McCaffery defined pain as "whatever the experiencing person says it is, existing whenever the experiencing person says it does.”
Pasero, Chris; McCaffery, Margo (1999). Pain: clinical manual. St. Louis: Mosby. ISBN 0-8151-5609-X. .  National Pain Education Council  From the American Board of Medical Specialties website: "Pain Medicine is the medical discipline concerned with the diagnosis and treatment of the entire range of painful disorders. (...) Due to the vast scope of the field, Pain Medicine is a multidisciplinary subspecialty (...)."  ^ "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." Source: IASP Pain Terminology.  Donald D. Price, Central Neural Mechanisms that Interrelate Sensory and Affective Dimensions of Pain, ‘’Molecular Interventions’’ 2:392-403 (2002)  ^ Nikola Grahek, Feeling pain and being in pain, Oldenburg, 2001. ISBN 3-8142-0780-7.  Compare definitions at IASP Pain Terminology: "Neurophathic pain —– Pain initiated or caused by a primary lesion or dysfunction in the nervous system." and "Neurogenic pain — Pain initiated or caused by a primary lesion, dysfunction, or transitory perturbation in the peripheral or central nervous system."  Pain Physiology  Feinstein B, J Langton, R Jameson, F Schiller. Experiments on pain referred from deep somatic tissues. J Bone Joint Surg 1954;36-A(5):981-97.  Liem EB, Joiner TV, Tsueda K, Sessler DI (2005). "Increased sensitivity to thermal pain and reduced subcutaneous lidocaine efficacy in redheads". Anesthesiology 102 (3): 509–14. doi:10.1097/ 00000542-200503000-00006. PMID 15731586.  Mogil JS, Ritchie J, Smith SB, et al (2005). "Melanocortin-1 receptor gene variants affect pain and mu-opioid analgesia in mice and humans". J. Med. Genet. 42 (7): 583–7. doi:10.1136/ jmg.2004.027698. PMID 15994880.  ^ Fertleman CR, Baker MD, Parker KA, et al (2006). "SCN9A mutations in paroxysmal extreme pain disorder: allelic variants underlie distinct channel defects
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• Acute Pain Medicine: Scientific Evidence (2nd ed) (2007 updated version) • Pain at the Open Directory Project