Pain is defined in medicine as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Pain is the sensation that warns us that damage to our bodies is occurring. It functions as a quick-acting system that tells the motor systems of the brain that they must act to minimize or eliminate this damage.
According to the International Association for the Study of Pain (IASP), one should distinguish between pain and nociception.
Nociception is a neurophysiologic term and denotes the activity in the nerve pathways that underlies the perception of physiological pain. These pathways transmit the pain signals. Nociception does not describe psychological pain.
Nociceptors or pain receptors 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. All pain receptors are free nerve endings. There are mechanical, thermal and chemical pain receptors. They are found in skin, 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 hyperalgesia, or increased sensitivity to pain.
Pain is a subjective experience that accompanies nociception, but can also arise without any stimuli. It includes the emotional response. Pain is ultimately a perception, and not an objective state of a body. The interpretation of pain occurs when the nociceptors are stimulated and subsequently transmit signals through sensory neurons in the spinal cord, which releases glutamate, a major exicitory neurotransmitter that relays signals from one neuron to another and ultimately to the thalamus, in which pain perception occurs. From the thalamus, the signal travels to the cerebrum, at which point the individual becomes fully aware of the pain.
Interestingly, the brain itself is devoid of nociceptive tissue, and hence cannot experience pain (thus a 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 has a sufficiently high probability of being fatal that development of nociceptive tissue in the brain would have little to no survival benefit.
If pain is defined as a signal of present or impending tissue damage effected by a harmful stimulus then the ability to experience pain or irritation is observable in most multi-cellular organisms. Even some plants have the ability to retract from a noxious stimulus. Whether this sensation of pain is even remotely equivalent to the human experience is highly debatable.
Despite its unpleasantness, pain is a critical component of the body's defense system. It is part of a rapid warning and defense relay instructing the motor neurons of the central nervous system to minimize detected physical harm. The 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 may, of course, have incorrectly determined the cause. 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 as most organisms will protect an injured region from further damage in order to avoid further pain. Despite its unpleasantness, pain is an important part of the existence of humans and other animals.
The study of pain has in recent years diverged into many different fields from pharmacology to psychology and neurobiology. Pain has many subjective psychological aspects besides the physiological nociception. The gate control theory of pain is a theory concerning how cognitive and emotional factors might dramatically influence painful sensations. It focuses on different pain states at the brain, rather than at the perceived site of injury.
A recent survey by NCCAM found pain was the most common reason to 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.)
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