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 [1]. 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 effected 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.
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Pain