Why do healing wounds itch?
We are told that if a cut is itching it is healing, and that scratching itches is bad for healing, why have we evolved this tendency to cause the healing cut to itch?
Chris - There are special nerves in the skin, and this has only been discovered in fairly recent times as in the last few years, but there are itch specific nerve fibres in the skin, and their job solely is to signal to the spinal cord that an area of the skin is being irritated in a way that we would perceive as itchy.
Those nerve cells are activated in a number of ways. One of them is mechanically. So, if you have an insect crawling on you for example, the insect crawling over your skin elicits the right kind of stimulus that those nerve cells are interested in, and they are triggered. So you are therefore paying attention to that bit of skin because there might be a bug which is biting you, which might be about to give you malaria or something. So that's the first point, it's mechanical.
The nerve cells are also sensitive to chemicals and there are certain chemicals which when you put them in your skin, they're irritants. As a result, they make you think, "Oh! I've got an itchy patch of skin again." You pay attention and you brush away the irritant chemical.
Now when you have a wound, the wound closes by cells around the margins of the wound, proliferating, in other words, growing and they then migrate from the margins of the wound, down into the base of the wound. They actually follow the electrical gradient, a guy in Aberdeen discovered in the last 5 years or so that the inside of the wound is at a different voltage than the margin, and the cells flow down this electrical gradient, so they know where the base of the wound is. They then unite with their cellular counterparts, and stitch themselves into place. Then they start to contract, contractile filaments which pull the wound closed.
So as they do that, they're eliciting a mechanical stress which the itch sensitive nerves will respond to and at the same time, there are various other factors which get released in a healing wound, chemicals which provoke healing in the wound, but also, upregulate the activity of these itch sensitive nerves.
So therefore, a wound that's closing up will feel itchy for mechanical and chemical reasons which are precisely the reasons why those nerve cells get stimulated in the first place. So that's the reason.