How Itches Work

Getting Under Your Skin

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.

More to Explore