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Pain is defined in medicine as the physical sensation of discomfort or distress caused by injury or illness. This assumes that there is a cause recognizable in biology for every pain - which is an effect. This definition has problems, and modern theories of pain control challenge it, for instance, the gate control theory of pain, which focuses on different pain states at the brain, rather than at the site where the brain perceives the pain to be.
Chronic pain is also poorly explained by the traditional biomedical model. There are some theories, such as the traditional Chinese medicine approach of Chi, which is said to be a "blockage" (some say this is equivalent to electrical resistance and indeed have measured this at pain sites) or "stagnation of blood" (theorized as being dehydration which inhibits metabolism). Such approaches as acupuncture are often reported as being more effective with types of pain that have no associated trauma.
Pain is ultimately a perception, and not an objective state of a body.
Nociception, one of the physiological senses, is the term commonly used to refer to the perception of physiological pain. Pain in this context can be defined as a harmful stimulus which signals current (or impending) tissue damage. As a result and despite its unpleasantness, pain is nonetheless a critical component of the body's defence system. The term nociception is not used to describe psychological pain.
Under the definition given above, the ability to experience pain or irritation has been observed in most multi-cellular organisms. Whether the actual sensation of pain corresponds even remotely to the human experience is (of course) highly debatable, but even plants can demonstrate the ability to retract from a noxious stimulus. However, the remainder of this article only examines nociception in organisms possessing a central nervous system of some description - up to and including a brain.
The very unpleasantness of pain encourages an organism to use any means at its disposal to disengage from the noxious stimuli that it assumes cause the pain. It can of course be wrong about this. Preliminary pain can serve to indicate that an injury is impending, such as the ache from a "soon-to-be-broken" bone. After an initial insult to an organism, pain can prevent further damage from occurring. Finally, pain may promote the healing process as most organisms will instinctively take great care to minimise the experience of more pain, hence protecting an injured region from further damage. However, there is much evidence that pain can retard healing in the hominoid: it may well be an evolutionary artifact that does us little good any more. It also may well be that the human species would not long survive without the biological tools it inherited from its animal ancestors.
The interpretation of pain occurs in the brain, primarily in the thalamus. Interestingly, the brain itself is devoid of nociceptive tissue, and hence cannot experience pain (thus headache is not pain in the brain itself). Some evolutionary biologists have speculated that this lack of nociceptive tissue might be due to the fact that any injury of sufficient magnitude to cause pain in the brain will incapacitate the organism and prevent it from taking appropriate action, which is the actual purpose of pain.
Acute pain is roughly defined as short-term pain or pain with an easily identifiable cause. Acute pain is the body's warning of current damage to tissue or disease. It is often fast and sharp followed by aching pain. Acute pain is centralized in one area before becoming somewhat spread out. This type of pain responds well to medications.
Chronic pain is roughly defined as long-term pain or pain that is not necessarily associated with any form of injury or disease. This constant or intermittent pain has no known purpose, as it does not help the body to prevent injury. It often does not respond well to medications. Expert knowledge and/or skills may be necessary to treat chronic pain adequately. When analgesics are used indiscriminately, addictions to narcotics may occur.
The experience of physiological pain can be grouped into four categories according to the source and related nociceptors (pain detecting nerves). 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 according to their point of termination.
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 well-defined, localised pain of short duration. Example injuries that produce cutaneous pain include paper cuts, minor (first degree) burns and lacerations.
Somatic pain originates from ligaments, tendons, bones, blood vessels, and even nerves themselves, and are detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull, poorly-localised pain of longer duration than cutaneous pain; examples include sprained ankle and broken bones.
Visceral pain originates from body organs visceral nociceptors are located within body organs and internal cavities. The even greater scarcity of nociceptors in these areas produces a pain usually more aching and of a longer duration than somatic pain. Visceral pain is extremely difficult to localise, and several injuries to visceral tissue exhibit "referred" pain, where the sensation is localised to an area completely unrelated to the site of injury. Myocardial ischaemia (the loss of blood flow to a part of the heart muscle tissue) is possibly the best known example of referred pain; the sensation can occur in the upper chest as a restricted feeling, or as an ache in the left shoulder, arm or even hand.
Phantom limb pain is the sensation of pain from a limb that one no longer has or no longer gets physical signals from - an experience almost universally reported by amputees and quadriplegics.
Finally neuropathic pain ("neuralgia") can occur as a result of injury or disease to the nerve tissue itself. This can disrupt the ability of the sensory nerves to transmit correct information to the thalamus, and hence the brain interprets painful stimuli even though there is no obvious or documented physiologic cause for the pain.
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