| MadSci Network: Medicine |
Doctors and scientists have been studying itch for a long time, but there still is no completely satisfactory answer to the question of what is itch. Itch can be caused by a wide variety of physical and chemical stimuli, such as light pressure, warming, exposure to acids or bases, or histamine release (allergic reactions). As a practical matter, a major factor in itching is damaged skin. Dryness, either from outdoor exposure or dry heated air, and contact with detergents or hot water can break down the protective barrier that skin provides and allow nerve endings in the skin surface to become irritated. Because itch can be elicited in so many ways, scientists have constructed a fairly detailed classification system. In general though, itch can be divided into 2 categories, sensory itch and allergic itch. Sensory itch is distinguished from allergic itch by the lack of external skin rash or irritation at the itch site.
Very early experiments on cutaneous sensation ignored the phenomenon of itch. Later experiments showed that the body was covered by "itch spots", small spots of the skin about 1mm apart which convey a sensation of itch when lightly touched. However, any part of the skin can become itchy if exposed to the correct stimulus. One extremely important bit of information is that a spot that is itchy usually doesn't have pain sensitivity. Itch is not merely a "subliminal" sort of pain. This is well demonstrated by the observation that opioids, such as morphine, are quite good at blocking even severe pain, but also produce itchy skin as a side effect.
Researchers have put together a fairly detailed description of the physical aspects of itch, but the physiological mechanism behind itch remains elusive. It is known that itch can only be evoked from superficial layers of skin, mucous membranes or conjunctiva. Therefore, only certain populations of nerves carry itch signals. We know that humans can feel itch when large diameter myelinated (insulated) nerve fibers are blocked , which means that itch is transmitted to the spinal cord and brain through smaller, thinly insulated or uninsulated fibers. This in turn means that sensory endings which are excited by weak mechanical stimulation ( Merkel's discs, Meissner corpuscles, Pacinian corpuscles, etc...) do not transmit the itch sensation. More likely itch is transmitted by lightly insulated or uninsulated nocioceptors (pain receptors) in the superficial skin. Recordings of the activity of these types of neurons have shown that they respond well to many of the stimuli that cause itch.
It turns out that mild scratching is one of the best ways to excite the pain neurons with uninsulated axons. There are several competing theories regarding why this relieves itch. Essentially, scientists believe that the massive signal that mild scratching sends to certain populations of pain neurons temporarily overwhelms or masks the itch sensation.
The information in this response was primarily derived from "Itching for an Explanation" by S.B. McMahon and M. Koltzenburg, Trends in Neuroscience, Vol 15, No. 12, 497-501, 1992. If you have any questions or comments, please send me an email.