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 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.
The very unpleasantness of pain encourages an organism to use any means at its disposal to disengage from the noxious stimuli that cause pain. 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 promotes 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.
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. 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.
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 produc 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.
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.