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Have you ever noticed that your skin feels itchier when it’s dry? Or when the weather is warm? Even simply watching someone else scratch, or reading about itchy skin can make you want to scratch (sorry about that). But what’s actually going on in your body when you feel an itch?
The best explanation for why we itch is that it evolved as a way to dislodge potentially harmful things from our skin, like biting insects or parasites. For a long time, the sensation of itch (known as pruritus) was thought to be a mild version of pain; get a slight signal and you feel an itch, but turn up the dial and it becomes pain. Today, however, that theory has largely been discarded in favour of the idea that the sensations are closely related, but distinct processes—like sisters that occasionally share clothing. Below is a detailed, neurological explanation of how itchy skin happens.
Beneath our skin, millions of nerve cells extend throughout our bodies like an intricately branching subway system. Just as a subway has stations where passengers board the train, each sensory nerve has multiple receptors, designed to pick up different stimuli. An itch typically begins when one of these receptors is triggered by a pruritic (or itch-causing) factor. Examples of pruritic factors include histamine, neuropeptides, serotonin, and various cytokines, but researchers are constantly uncovering more. These itch factors are usually released by our own cells in response to something external. For example, histamine and other mediators, released by our mast cells, are responsible for the itch after a mozzie bite .
To avoid congestion on your body’s neuron railway, there are separate lines for the different types of signals: Touch, Pain, Temperature, and Itch. The sensation of touch rides the express train, which travels at around 35-75 metres per second through high speed Aβ-fibres. Pain and temperature signals take second place, reaching speeds of between 5 and 35 metres per second through Aδ-fibres, while itch is relegated to the slow train, which travels at 0.5 – 2 metres per second on C-fibres, about the speed of a casual stroll. However, pain and temperature also have their own dedicated C-fibres, and can even hijack the itch pathway. More on this later.
The itch signal rides these C-fibres through the body until it reaches the dorsal root ganglion (DRG), a gateway to the central nervous system, located in the spinal cord. After a quick change of trains, where the signal jumps across several receptor sites, it’s picked up by a projection neuron, which relays the signal to its final destination, the thalamus in the brain.
Once a signal reaches the brain it causes a reaction impulse. For pain, that impulse is to withdraw (if you’ve ever touched a hot pan straight out of the oven, you’ll know what I mean), while for itch the impulse is to scratch. These impulses can be extremely compelling and difficult to ignore.
Recall that pain signals can travel on both Aδ-fibres and C-fibres. In fact, it’s believed that our brains give pain signals a priority ticket that allows them to travel on the same track as an itch, effectively commandeering the line and derailing the itch. This is why scratching, which causes a very mild pain signal as you scrape off the top cell layers of your skin, can temporarily knock out an itch. The brain even fires a little serotonin, a neurotransmitter thought to contribute to feelings of happiness, back down the tracks after you scratch—a feel-good tip for a job well done. Or so it would seem…
Thwarting an itch with a quick scratch is all well and good, but unfortunately scratching does more than just dissipate the itch. Your body’s response to damage is to release inflammatory mediators like neuropeptides, which unfortunately are themselves itch factors. As is serotonin. This can begin a new itch signal in the area, which, of course, leads to more scratching, and so begins the vicious itch-scratch cycle . For chronic itch, a symptom in eczema and other dermatological conditions, this poses a real problem as the itch signal becomes jammed and can’t be switched off, so the urge to scratch is persistent and unabating. Once the itch-scratch cycle takes hold, the offending area can quickly be scratched raw, opening up the possibility of secondary bacterial infection.
Since itch is often caused by some irritating factor on the skin, it follows that a weakened skin barrier will make the skin more susceptible to these factors, since they’ll more easily be allowed access into the skin. You can read a more detailed discussion about the skin barrier here , but dry skin is basically your body’s equivalent of a cracked and leaky roof—too much water gets out, and allergens and bacteria can get in. With eczema, or atopic dermatitis, the situation is even worse. In addition to dryness, atopic skin has a greater density of nerve fibres in affected areas, resulting in an intense itch signal that won’t let up.[11,12] Think of the train station after the footy Grand Final—more trains running, and every one of them full.
Because there are so many different itch-causing factors, different receptors and possible pathways for a signal to take, there is currently no sure-fire way to stop itchy skin in all cases. Closing down one station, say with an antihistamine, might halt one type of itch, but a different mediator can simply hop on at the next station. Scratching can temporarily dislodge the itch train, but as we’ve seen, that’s not an ideal solution, particularly for chronic itch. Placing something cold on the site is another way to override an itch signal, but this, too, is a short term solution. One way to help prevent itchy skin is to keep it moisturised. Also, if you’re prone to itchy skin try to avoid triggers, like overheating, taking long hot showers or baths, or wearing wool or rough natural fibres. Watch this video for more tips on how to manage red, itchy skin. Finally, if you’re suffering from chronic itch due to a dermatological condition, talk to your healthcare professional about the best way to manage your symptoms.
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