If you have shampoo in your eyes, why does it only hurt when they’re open?
In: Biology
Pain receptors and “squeeze” receptors come from the same place, so more squeeze receptors firing equal less pain receptors firing. Same reason why you get a paper cut and it feels better to put pressure on it
When you open your eyes, the oxygen in the air combines with the sodium laureth sulphate in the shampoo to create sulphuric acid and table salt, stinging the delicate surface of the eye. Source: my friend is a scientist.
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Add a bit of context, your eyes do not have pain sensors on them, this is not the same as in the back of your eye or other areas, just the front of your eyeball. The pain you feel is 100% from your eyelids, which have quite a few pain and sensation receptors. Most of these are at the central margin.
When you squeeze your eyes shut you are forcing the liquid out from the margin of your eyes, and hopefully out of your eyes entirely. Still, it is no longer in contact with the margin because of the force squeezing it out, so it stops hurting. Start blinking and you have fresh tears which move the soapy solution around, and it gets all over your lid margins and the pain comes back. Repeat until resolved.
Soap does a number of other things like dries your eye out, changes the lipid balance and can make your eyelids hurt for a bunch of other reasons as well. Its fun stuff.
You can touch your eyeball and not feel it. Contact lenses aren’t felt by the eye itself – only the lids ( which are VERY sensitive – to protect the eye.) Closing the eyelids sweeps most irritants away – to be felt thereafter only by the lids.
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Because your eyelids closing pushes the shampoo toward a central location. Now instead of having the entire surface area of your eye covered in shampoo, it’s just a smaller portion between your eyelids. This minimizes the pain.
The shampoo cleans your “eye thingies” so you see the world how it really is and the pain is too much
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