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The experience of physiological pain can be grouped according to the
source and related nociceptors (pain-detecting neurons). - 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, localized pain of short duration. Examples
of injuries that produce cutaneous pain include paper cuts, minor cuts, minor (first degree) burns and lacerations.
- Somatic
pain originates from ligaments, tendons, bones, blood vessels, and even nerves themselves. It is detected with somatic nociceptors. The scarcity of pain receptors in these areas produces a dull,
poorly-localized pain of longer duration than cutaneous pain; examples include sprains
and broken bones. Myofascial pain usually is caused by trigger points in muscles,
tendons and fascia, and may be local or referred.
- Visceral pain originates
from body's viscera, or organs. Visceral nociceptors are located within body
organs and internal cavities. The even greater scarcity of nociceptors in these areas produces pain that is usually more aching
and of a longer duration than somatic pain. Visceral pain is extremely difficult to localize, and several injuries to visceral
tissue exhibit "referred" pain, where the sensation is localized 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. The popularized term "brain freeze" is another example of referred pain, in which the
vagus nerve is cooled by cold inside the throat. Referred pain can be explained by the findings that pain receptors in the
viscera also excite spinal cord neurons that are excited by cutaneous tissue.
Since the brain normally associates firing of these spinal cord neurons with stimulation of somatic tissues in skin or muscle,
pain signals arising from the viscera are interpreted by the brain as originating from the skin. The theory that visceral
and somatic pain receptors converge and form synapses on the same spinal cord
pain-transmitting neurons is called "Ruch's Hypothesis".
- Phantom
limb pain, a type of referred pain, is the sensation of pain from a limb that has been lost
or from which a person no longer receives physical signals. It is an experience almost universally reported by amputees and quadriplegics.
- Neuropathic pain, 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 known physiologic cause for the
pain. Neuropathic pain is, as stated above, the disease of pain. It is not the sole definition for chronic pain, but does
meet its criteria.
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