Nerves send us the nasty, prickly feelings. While some nerve fibers focus on feeling pain and touch, we have others dedicated to the itch sensation.Scientific American:
Histamine, a protein released during an allergic reaction, commands some of our itch nerves to transmit information to the spinal cord where it is processed and zipped off to the brain. The sites activated in the brain when we itch are very similar to those switched on when we're in pain. Antihistamine drugs work by disabling the protein's signaling powers.
However, histamines aren't the only chemicals in the body that cause annoying tingles.
Fibers sensitive to itch were first discovered almost a decade ago by Schmelz. Recently, he's found the existence of new fibers that stimulate itchy feelings in a different way than the histamine-sensitive nerves he previously identified.
"It is evident that there are more than just one type of nerve fiber involved in the itch sensation," Schmelz said. "A lot of information has been generated related to histamine-sensitive itch fibers and currently new types of itch fibers are being investigated, which might help to explain the chronic itch condition more completely."
Along with several other itch researchers, Schmelz presented his findings at the Third International Workshop for the Study of Itch in Germany. The meeting's notes are published in the September issue of the Journal of Investigative Dermatology.
An itch, also known as pruritus, is a general sensation arising from the irritation of skin cells or nerve cells associated with the skin. While it can be a nuisance, pruritus serves as an important sensory and self-protective mechanism, as do other skin sensations such as touch, pain, vibration, cold and heat. It can alert us to harmful external agents, but can become unbearable if not treated.
Pruritus is a dominant symptom of many skin diseases and also occurs in some diseases that affect the entire body. An itching sensation of the skin arises due to stimulation of pruriceptors—itch-sensing nerve endings—by mechanical, thermal or chemical mediators. These include:
Chemicals for immune response (histamines) and pain relief (opiods) Neuropeptides, which include pain-regulating messengers released within the brain, such as endorphins The neurotransmitters acetylcholine and serotonin Prostaglandins, which are lipids that, among other functions, create the sensation of pain in spinal nerve cells Stimulation by any of these agents is typically related to inflammation, dryness or other damage to the skin, mucous membranes or conjunctiva of the eye.
In general, pruritus involves activation of the pruriceptors of specialized nerve cells called C-fibers. These C-fibers are identical to those associated with the sensation of pain, but they are functionally distinct and only convey the itch sensation—they comprise about 5 percent of the total C-fibers in human skin. When stimulated superficially on the skin, these C-fibers carry signals along the nerve to the spinal cord and on to the brain, where they are processed, generating a scratching or rubbing reflex response. Scratching and rubbing then interfere with the sensations arising from pruriceptors by stimulating various pain and touch receptors in the same areas. Though it is helpful in relieving the itch, scratching offers only temporary relief and may cause the skin to become further irritated and possibly tear, which could result in an infection.