Nic McGregor of Palmerston North asks :-

What causes a random itch that isn't caused by external events such as an insect bite or a rash?

Rodger Pack, a physiologist at Massey University, responded.

That irritating skin sensation urging you to scratch (an itch or pruritis) is related to several phenomena of the central nervous system, including the sensation of prickle. This occurs when skin receptors are stimulated and is most noticeable when you dress in the morning and your brain is aware of material on your skin's surface. This sensation disappears quickly and is different from an itch because it does not lead you to scratch. An itch is stimulation of skin receptors (either mechanical or chemical) that is projected to the central nervous system, provoking scratching.

You have a wide range of different receptors in your skin. One group consists of free nerve endings which have small diameter fibres known as C fibres. When stimulated strongly by tissue damage or inflammatory mediators such as histamine, these receptors are involved in pain. One school of thought suggests sub-maximal stimulation of these receptors gives rise to the sensation of itching. Another suggestion is that there could be a subpopulation of these nerve endings that function as specific `itch' receptors. We do not know for sure which situation is the case.

Another factor in the cause of random itches is the role of sensory filtering, which occurs both in the spinal cord and the brain and is so comprehensive that very little sensory information actually gets projected to consciousness.

The major neuronal pathway for itch in the central nervous system is not fully understood. Repeated itching and scratching may form as a habit. Interestingly, some chronic systemic diseases that do not appear to directly affect the skin increase the tendency to itch.

Have you ever noticed that if you see somebody scratching, possibly for a valid reason like an insect bite, you feel itchy yourself and get a huge urge to scratch? Is it all in the mind?