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Pain is both a sensory and emotional experience, generally associated tissue damage, or inflammation. Pain is ultimately a perception, and not an objective bodily state.

Despite its unpleasantness, pain is a critical component of the body's defense system. It is part of a rapid warning and defence relay instructing the motor neurons of the central nervous system to minimize detected physical harm.

The gate control theory of pain, offers insight into how cognitive and emotional factors might dramatically influence painful sensations. Developed by Ronald Melzak and Pat Wall, it focuses on different pain states at the brain, rather than at the perceived site of injury.

Table of contents
1 Nociception
2 Interpretation of pain
3 Types of pain
4 Chronic pain treatment
5 Pain and alternative medicine
6 Related topics
7 External links


Nociception, is the physiological sense for perception of physiological pain. Nociception does not describe psychological pain.

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.

The interpretation of pain occurs when the nociceptors are stimulated and subsequently transmit signals through sensory nuerons 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 thalumus, 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 therein 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 equivalent to the human experience is debatable.

Interpretation of pain

The unpleasantness of pain encourages organisms 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. After an initial insult to an organism, pain can prevent further damage from occurring. Pain may also promote the healing process as most organisms will protect an injured region from further damage in order to avoid further pain. However, there is also evidence that pain may retard healing.

Despite its unpleasantness, pain remains an important part of human existence.

Types 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 present 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. This constant or intermittent pain has often outlived its purpose, as it does not help the body to prevent injury. It is often more difficult to treat with medication. Expert care is generally necessary to treat any pain that has become chronic. When opioid analgesics are used indiscriminately for prolonged periods, tolerance, dependence and even addiction may occur.

The experience of physiological pain can be grouped into four categories according to the source and related nociceptors (pain detecting nerves).

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.

Chronic pain treatment

In modern societies, medical care is highly effective at treating most causes of pain. The inability to provide adequate relief for the remaining group causes dissatisfaction for both professionals and patients. The small group of people that suffer from chronic pain, have, by definition, been poorly served by the biomedical model and often seek alternative therapy to assist their pain control.

Recently, scientific findings show the first evidence that sunlight can affect the perception of pain.

Pain and alternative medicine

A recent survey by NCCAM found pain was the most common reason to use complementary and alternative medicine (CAM) and that 38.7% of adult Americans used CAM for pain in 2002 ([1] p 9). The most common indication was back pain (16.8%), followed by neck pain (6.6%), arthritis (4.9%), joint pain (4.9%), headache (3.1%), and recurring pain (2.4%).

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 relatively more effective for nontraumatic pain than with traumatic pain.

Related topics

External links

Nervous system - Sensory system Edit
Visual system - Auditory system - Olfactory system - Gustatory system - Somatosensory system