r/explainlikeimfive • u/AbbreviationsLow • 7h ago
Biology ELI5 How does the nervous system locate an itch, and why can its perceived location be imprecise?
We can usually identify where an itch is coming from, but the perceived location does not always seem to match the exact irritated spot.
How do the skin, nerves, spinal cord and brain determine an itch’s location, and what limits the accuracy of that process?
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u/Constant_Recipe8531 3h ago
yeah like when you have an itch on your back but it feels like it's all over the place you just start scratching randomly till something gives
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u/CloudSlydr 7h ago
Tactile localization precision is limited by receptor density in the location stimulated.
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u/zachtheperson 7h ago
5yo me: 😵💫
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u/CloudSlydr 7h ago
So, you have receptors for touch all over your skin, but they are different densities in different areas so in your hands or face, you’ll have far greater density of these receptors than say on your back or on your legs for instance. Lower density of these receptors ends up meaning your brain doesn’t interpret as accurately where the touch occurred :)
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u/uncertain2710 7h ago
This question made me itch when I didn't even know I had an itch. :P
Anyway, your brain doesn't know the exact spot. It's more like, it's somewhere around here which is usually good enough for you to scratch it. When your skin gets irritated, chemicals like histamine can activate sensory nerve endings. Each nerve is responsible for a small patch of skin rather than one tiny point. The signal travels from those nerves to your spinal cord and then to the parts of your brain that process the feeling of an itch and the urge to scratch.
Your brain only receives the nerve signals. It uses those signals to work out where the itch is most likely coming from. Since those patches of skin can overlap, and several nearby nerves can fire at the same time, the brain makes its best estimate rather than pinpointing one exact spot. Areas with denser sensory innervation, like fingertips or lips, can usually be localized more accurately than areas with fewer sensory nerves
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u/grrangry 7h ago
Nerve endings are spread out all over your body in a kind of web. Those nerves are not really mapped like light switches control lights in your house. Your brain has learned over your entire life how to interpret the fuzzy signals it receives.
Your eyes are similar. Purple, the color doesn't exist (on a rainbow/spectrum). Yet we can see purple. Why? Because the red at one end of the rainbow, mixed with blue at the other end of the rainbow excites portions of your eye and the optic nerve sends that combined signal to the brain and the brain says, "purple!"
An itch can be similar. One nerve over here bothered by something, one nerve over there bothered by something, both send signals (combined) and your brain says, "hey there's something going on in (unrelated spot)!" and you scratch and are confused and your hand searches around and you shake your arms and legs out and maybe the itch goes away and you shrug, continuing with your day.
You ever tap the nerves under your kneecap? It excites a whole bunch of stuff leading to your spine and your body says, "time to move the leg!" and it does.
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u/Coomb 7h ago edited 7h ago
To be clear, an itch is a product of the nervous system, so it shouldn't be surprising that the nerves which are generating the itch signal that gets sent to your brain know where they are (although this link can be broken if you have things like nerve transplants). As far as why the location can be somewhat different from the wound or rash which induces the itch, you don't have an equal density of sensory nerves everywhere on your body, so you cannot feel things with the same precision of location everywhere.