ASK A SCIENTIST
Question: What is an itch and why do you get them?
An itch is certainly a common occurrence; and, without thinking, people scratch them. But why? As you probably know, sensory nerves send signals (messages) to the brain about things we are feeling in our body. These nerves have receptors that are able to detect changes like pressure, temperature, vibration, or pain; and there are different types of nerves for different kinds of sensations. So, itch signals actually travel to the brain using different nerves than pain. An itch leads to scratching or rubbing whereas pain causes the person to pull away. Itch receptors are located almost exclusively in the outermost layers of the skin while pain receptors are located throughout the body. So unlike pain, the sensation of itch is never felt in muscles, joints, or inner organs of the body. Many types of stimuli are known to cause itching: mechanical (tight clothing), chemical (drugs or detergents), thermal (sunburn) and electrical (electrical shock). Itching is also associated with many types of skin conditions (poison ivy) or diseases that affect entire body systems (end-stage kidney disease).
Scientists have identified four types of itch based on the anatomic location of the cause. The first type, called puritoceptive itch, is caused by inflammation, dryness or other damage to the skin and generally associated with a mosquito bite. A second type, called neuropathic itch, involves injury to the peripheral nerves that carry itch signals from the skin to the spinal cord and brain; and occurs in diseases like multiple sclerosis. A third type is called neurogenic itch meaning that the itch signal originates in the central nervous system (brain and spinal cord) but does not involve injury or damage to the peripheral nerves. This type is often associated with disorders of body systems as in liver disease. The last, psychogenic itch, is associated with psychologic conditions, for example, if someone was delusional or having a hallucination.
You are probably most familiar with the first type of itch which has its origin in the skin. When your skin is exposed to certain stimuli, chemicals such as histamine are released by inflammatory cells that are located near "itch" nerve endings. Histamine binds with specialized receptors on the nerve ending and initiates a nerve signal. The signal travels along the "itch" nerve to the spinal cord and travels up to the brain. In the brain, the signal is processed by the thalamus and sent to the cerebral cortex, the part of the brain responsible for cognition or conscious thought. At this point, a person becomes aware of the "itchy" feeling. Scratching or rubbing is a reflex response that is thought to relieve the itch by simultaneously stimulating pain nerves and interfering with the transmission of the "itch" signal to the brain. Of course, scratching can offer temporary relief, but can also cause your skin to become reddened, inflamed, and tear which will only worsen the problem and could result in an infection. So, it is better to apply blunt pressure or a cool towel than to scratch!
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail email@example.com. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).