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Pain is an unpleasant feeling which may be associated with actual or potential tissue damage and which may have physical and emotional components. According to the International Association for the Study of Pain (IASP), one should distinguish between pain and nociception. The word "pain" comes from the meaning punishment, a fine, a penalty. The term "pain" is a subjective experience that typically accompanies nociception, but can also arise without any stimulus, and thus includes the emotional response. Nociception, on the other hand, is a neurophysiological term and denotes specific activity in nerve pathways. It is the transmission mechanism for physiological pain, and does not describe psychological pain. These pathways transmit the nominally "painful" signals, though they are not always perceived as painful. Although pain can be associated with tissue damage or inflammation, this is often not the case. Despite its causing suffering, pain is a critical component of the body's defense system. It is part of a rapid warning relay instructing the central nervous system to initiate motor neurons in order to minimize detected physical harm. Lack of the ability to experience pain, as in the rare condition Congenital insensitivity to pain or Congenital Analgesia, can cause various health problems. The two most common forms of pain reported in the U.S. are headache and back pain. Pain is also a term specifically used to denote a painful uterine contraction occurring in childbirth. Types of pain Pain can be classified as acute or chronic. The experience of physiological pain can be grouped according to the source and related nociceptors (pain detecting neurons). Head and neck Thorax Abdomen Limbs Joints Pain receptors All pain receptors are free nerve endings. There are mechanical, thermal and chemical pain receptors. They are found in skin and on internal surfaces such as periosteum and joint surfaces. Deep internal surfaces are only weakly supplied with pain receptors and will propagate sensations of chronic, aching pain if tissue damage in these areas is experienced. Pain receptors do not adapt to stimulus. In some conditions, excitation of pain fibres becomes greater as the pain stimulus continues, leading to a condition called hyperalgesia. Nociceptors are the free nerve endings of neurons that have their cell bodies outside the spinal column in the dorsal root ganglion and are named based upon their appearance at their sensory ends. These sensory endings look like the branches of small bushes. Two main types of nociceptor, Aδ and C fibres, mediate fast and slow pain respectively. Thinly myelinated type Aδ fibres, which transmit signals at rates of between 6 to 30 metres per second mediate fast pain. This type of pain is felt within a tenth of a second of application of the pain stimulus. It can be described as sharp, acute, pricking pain and includes mechanical and thermal pain. Slow pain, mediated by slower, unmyelinated ("bare") type C pain fibres that send signals at rates between 0.5 and 2 metres per second, is an aching, throbbing, burning pain. Chemical pain is an example of slow pain. Transmission of pain signals in the central nervous system The perception of pain occurs when the nociceptors are stimulated and transmit signals through sensory neurons in the spinal cord. These neurons release glutamate, a major exicitory neurotransmitter that relays signals from one neuron to another. The signals are sent to the thalamus, in which pain perception occurs. From the thalamus, the signal travels to the somatosensory cortex in the cerebrum, where the pain is localised, and the individual becomes fully aware of the pain. There are 2 pathways for transmission of pain in the central nervous system. These are the neospinothalamic tract (for fast pain) and the paleospinothalamic tract (for slow pain). Analgesia The gate control theory of pain, proposed by Patrick Wall and Ron Melzack, postulates that pain is "gated" by non-painful 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 pain information. The analgesia system is mediated by 3 major components the periaquaductal grey matter (in the midbrain), the nucleus raphe magnus (in the medulla), and the pain inhibitory neurons within the dorsal horns of the spinal cord, which act to inhibit pain-transmitting neurons also located in the spinal dorsal horn. The body has 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. Survival benefit Despite its unpleasantness, pain is an important part of the existence of humans and other animals; in fact, it is vital to survival. 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. People born with congenital insensitivity to pain usually have short life spans, and suffer numerous ailments such as broken bones, bed sores, and chronic infection. The study of pain has in recent years diverged into many different fields from pharmacology to psychology and neurobiology. It was even proposed that fruit flies may be used as an animal model for pharmacological pain research *. Pain is also of interest in the search for the neural correlates of consciousness, as pain has many subjective psychological aspects besides the physiological nociception. 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 (pain receptors) is thought to be involved to some extent in producing headache pain. 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. Since pain is defined as a signal of present or impending tissue damage affected by a harmful stimulus, the ability to experience pain or irritation is observable in most multicellular organisms. Even some plants have the ability to retract from a noxious stimulus. Whether this sensation of pain is equivalent to the human experience is debatable. Chronic pain, in which the pain becomes pathological rather than beneficial, is an exception to the idea that pain is helpful to survival. Furthermore, it is not clear what the survival benefit of sometimes 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. Children and pain Children have been proven to be markedly more sensitive to pain, but this fact is commonly dismissed as a fear reaction or a lack of coping abilities. Research has been carried out on how children can cope with pain due to increased sensitivity and it has been established that strategies that remove pain can help prevent long-term increases in sensitivity as the nervous system is still developing. Ethnicity and pain Pain may be experienced differently depending on ethnicity. A study by Liem et. al. suggests that redheads are more susceptible to thermal pain. Pain and alternative medicine A recent survey by NCCAM found pain was the most common reason that people use complementary and alternative medicine (CAM). Among American adults who used CAM in 2002, 16.8% used CAM to treat back pain; 6.6% for neck pain; 4.9% for arthritis; 4.9% for joint pain; 3.1% for headache; and 2.4% used CAM to treat recurring pain. (Some survey respondents may have used CAM to treat more than one of these pain conditions.) One such alternative, traditional Chinese medicine, views pain as a qi "blockage" equivalent to electrical resistance, or as "stagnation of blood" – theorized as dehydration inhibiting metabolism. Traditional Chinese treatments such as acupuncture are said to be more effective for nontraumatic pain than traumatic pain. These claims have not been scientifically established. Philosophy of pain The concept of pain has played an important part in the study of philosophy, particularly in the philosophy of mind. The question of what pain actually consists in is, dependent upon what subject one approaches the question from, an open one. Many identity theorists would assert that the mental state of pain is completely identical with some physical state caused by various physiological causes. Many functionalists believe pain to be defined completely by its causal role (ie in the role it has in bringing about various effects) and nothing else. Theologians have also had much to say about the nature of pain and its various religious consequences. See also | |||||||
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