Scientists have classified all itches into four basic categories [source: Potenzieri]:
- pruriceptive (from insect bites and inflammatory skin disorders like eczema)
- neuropathic (chronic itching as a result of nerve damage)
- neurogenic (the central nervous system gets activated to itch without any stimulation of nerve fibers)
- psychogenic (itching as a result of mental illness)
Pruriceptive is the most common type of itching. For the itch sensation to be triggered here, something mechanical, thermal or chemical has to stimulate the itch-sensing nerve endings on the skin, known as pruriceptors. These guys are super-sensitive. They can pick up an itchy sensation more than 3 inches away [source: Gawande].
Scientists have learned that for most types of pruriceptive itches, sensory nerve fibers called C-fibers get stimulated on the skin. They then send signals to the spinal cord and on to the brain, which generates a rubbing response reflexively from the person [source: Andrews].
The fibers are not quick to transmit the information, which is why itchiness can take so long to build up and subside. About 5 percent of the total C-fibers in human skin are connected to the itch mechanism, while many of the others are associated with pain (see sidebar on next page) [source: Andrews].
While that takes care of most itching, there are some more unusual types. Brachioradial pruritus, persistent outer-arm itching, is caused by a crimped nerve in the neck, and worsens in sunlight. Aquagenic pruritus, on the other hand, is recurrent and intense itching upon getting out of a shower. (That one is a symptom of a rare condition in which the body produces too many red blood cells.) [source: Gawande]
There is also itching that results from psychosis. People may have delusions that their skin is infested with parasites or crawling with bugs. So they scratch themselves all over.
