Eli5: What on the skin makes the itchy feeling, and how does scratching stop it?
In: Biology
[deleted]
Smash rash will make your skin itchy but scratching won’t help that. You would need to see a Dr.
Your skin has lots of sensors in it. There are different types of sensors for pressure, stretching, temperature, etc. These sensors are connected to the brain by ‘wires’ which are part of the nerve cells, with lots of signals travelling up to the brain, and lots of signals also travelling to the skin from the brain. All these sensors are working together to tell the brain when things are going fine or if something needs attention, like broken skin from injury, or something crawling or stuck on you.
The signals can change not just from outside simulation, but also from internal changes like inflammation chemicals that can make nerves send a bigger signal for the same amount of stimulation (this is why areas that are sore or itchy are often also hot, red or swollen: the same inflammation chemicals also change bloodflow in that area).
We don’t actually feel or notice these sensor signals until the brain interprets what they mean. The brain generates a sensation based on the combination of all these signals AND what the brain guesses the signals mean. Some types of signals usually happen when something is crawling on us, stuck on or in the skin, or some other situation where the brain decides it would be best if we scratched it. So the feeling of ‘itch’ is what the brain generates as a perception.
This ‘itch’ feeling is not just the physical sensation, but a whole set of responses in the brain attached to the sensation, including a sense of discomfort, focused attention so that the feeling is hard to ignore, and an urge to make the feeling stop. It’s like an alarm bell indicating ‘this area of skin is in trouble, and your job right now is to fix it’. Since we usually experience the sensation/emotion/attention/urge at the same time, we label that whole combined experience ‘itch’ (although we might not realise that all of that is happening as well as just the pure physical sensation; in fact all of that is PART of the physical sensation).
Then when we scratch it, at the body level our skin is receiving new signals that it sends up to the brain (in fact rubbing pain or scratching an itch can send so many competing signals that it partly or fully blocks the itch/pain related signals from reaching the brain). The brain receives this change in signals, while also knowing that it’s ‘fixed the problem’ by sending instructions to scratch. The brain’s best-guess understanding of what’s happening based on these signals AND other clues (like looking to see if something is there) combine to form the physical sensation of relief from the itch.
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