On Saturday night I got my first bug-bites of the summer – this is quite a miracle. No, not that I got bitten – more the fact that I hadn’t been bitten yet this summer. I am one of those people who seems to attract every mosquito within a mile radius, and is usually being eaten alive as soon they step out the door. I think my record is getting about 70 bites in 10 minutes. This resulted in me to breaking out in hives and going on a mad dash to the nearest pharmacy to buy antihistamines. It earned me the name bug-girl…
There’s nothing worse that then that persistent tingling itch which calls on you to scratch it; enticing you to run your fingernails over the affected area to bring momentary relief. But all it is is a temporary fix to an itchy problem. However, I’m, intrigued to find out why scratching helps. Its time to dive into the physiology of the itch!
It was once thought that there was no specialized mechanism behind the itch, but technological advances allowed scientists to discover a specific (annoying) pathway which is responsible for this aggravating sensation. A collection of molecules in a layer of the skin called the epidermis constitutes an ‘itch receptor’. Stimulation of this receptor, or the injection of irritating molecules when an insect bites you, leads to the release of histamine from specialized cells in the skin called dermal mast cells. In turn, this activates a small subset of neurons (nerve cells) called C neurons.
The advent of [a specialised] technique, in which action potentials in individual C neurons can be recorded in human volunteers using ultra-fine glass electrodes, has established the existence of dedicated slow-conducting unmyelinated C neurons which only transmit itch (and temperature changes) in response to histamine – thus disproving the notion that itch is merely a mild version of pain.
So why does scratching stop these neurons sending their itchy message? Well scratching activates larger neurons in the skin. Their activation inhibits the C neurons, preventing their message reaching the brain – ta da, itch stopped! Just got to keep scratching though…
While you scratch away, here’s an extra little nugget of information – have you ever experienced a phenomena where you scratch one itch and it causes another irritation to pop up? This sensation, referred itch, is called Mitempfindungen (how awesome is that word?!). There is a wonderful account of one doctor’s personal investigation of it here. Any science article which has a sentence is “I was also unaware of Kovalevsky’s work when I started to record my own Mitempfindungen in 1940,” is worth a read right?
Greaves, MW (2007) Recent Advances in Pathophysiology and Current Management of Itch Ann Acad Med Singapore 36:788-92
Evans, PR (1976) Referred itch (Mitempfindungen) British Medical Journal, 2:839-841