Why does scratching an itch feel good?

Cat scratching
Cat scratching
Source: David Wagner

We have all experienced itching at some point in our lives. You may even feel this rather unpleasant sensation at the very moment.

One of the ways to relieve it, alas not in every case and only temporarily, is to scratch it. Scratching an itching spot (or even an area some distance from it) usually feels good, or at least better than the itch itself. But how does it work, exactly?

An itch, known also by its Latin name pruritus, is a sensation to which your body wants you to respond which scratching. You may either feel an intense desire to scratch, or simply have an involuntary reaction – a scratching reflex.

Itch signals are received by the same part of the brain that receives pain signals. It is believed that about 5% of our pain nerves are preset to send itch signals. Itching is one of our defense mechanisms, in many ways like tickling and pain itself. Itching, however, is unique among these three. What happens when you experience pain? Do you think to yourself: “Oh, look! I just cut my leg really bad, after falling down on this shattered glass. I’m bleeding heavily and I think I can actually see something white down there, peeking at me from beneath all the ruptured flesh. It may be my own bone! Wow, isn’t it cool? I really need to touch it, and rub it a little”. That’s unlikely. Touching would make it hurt more. And what about tickling? When something (or someone) tickles you, you rarely scream “More!” with an ecstatic look on your face. Well, unless you “have” knismolagnia, which is the experience of arousal from tickling (there is also acarophilia – or arousal from scratching). Pain, in most people, causes withdrawal, while itch causes people to scratch.

So, why do we scratch?

For one, we do it to remove irritants from the skin. It may be a bug, for example. It’s one of the ways to remove potential external parasites from the surface of our bodies. Animals scratch too, and it’s easy to see why. There is a big hint – a coat, or fur. Scratching is an excellent way for animals to reach down to their skin and get rid of those annoying little insects. We, humans, no longer have such lush full body hair, but we still have the scratching reflex.

Two – delicate scratching helps skin’s regeneration process by removing old, dead cells.

And why does it feel good when you scratch an itch?

In short – scratching blocks the itching sensation, and it works on several levels. Itch may be caused by various irritants. When you scratch, you remove those irritants, but also cause pain. The receptors responsible for pruritus are called pruriceptors. The signals from pruriceptors are carried along the spinal cord to the brain through nerve cells called C-fibers. Scratching interferes with those signals by activating pain receptors in the same area where the itch occurred. However, the relief is usually temporary. It is especially true when the itch results from the allergic reaction. In such case, our bodies attempt to fight the invasion with histamine. It causes blood vessels around the affected area to swell. We often scratch such spots too intensely, effectively breaking the skin, because allergy-related itch usually lasts longer. This could lead to infection.

But, there is also another mechanism at work. More recent research has showed that scratching reduces activity in certain areas of the brain, namely the anterior cingulate cortex and posterior cingulate cortex. The posterior area is associated with memory. The anterior area is active when people think of unpleasant experiences or past pain. So, it appears, the scratching “suppresses the emotional components and misery of pain” – in the words of one researcher. In another finding, scientists established that the itching takes place deep within the central nervous system – in the spinothalamic tract (STT) neurons in the spinal cord. Scratching “calms” the nerves and relieves the itch. You can see that the itch-scratch relationship is a complicated one and we still have a lot to learn about it.

As with everything, too much scratching is not a healthy thing, so don’t scratch yourself beyond what’s necessary.

Further reading:
Scratch reflex on Wikipedia
Itching at Patient.co.uk