Today while surfing I accidentally rubbed the top layer of skin in my fingertips off from friction against the surfboard.

I didn't think much of it until later in the day when I was using my laptop. I brushed against a USB peripheral (the external case of which is made of metal) which was plugged into my laptop with one of my fingertips and felt an immediate searing pain, not like a burning, but very much like electrical shock.

To verify, I quickly touched it with another of my raw fingertips and experienced pain so great that it caused me to cry out in shock. I adjusted pressure with the tip of my finger, and the more surface contact there was with my finger, the less the pain was. That is, as I pulled my finger away slowly leaving only a tiny bit of skin in contact with the metal it got EXTREMELY painful, as if the fewer the affected nerves the worse the pain.

I then touched it with one of my fingers which had somehow escaped being rubbed raw, and I felt nothing. It simply felt like normal room temperature metal (there was no noticeable heat).

This led me to believe that what I was feeling through my raw fingertips was not simply heat, but electroconductivity against which is usually protected by the outermost epidermis.

Can anyone shed some light on this?

  • $\begingroup$ In case this gets closed, here is some basic info, and in more detail here. The basic reason you feel more (and different) in your raw fingertip is that the protective layer of squamous epithelial cells has been removed, exposing raw nerve endings which don't get triggered when the epithelium is intact. If this is not closed, I'll give a full explanation when I have more time. $\endgroup$ Commented Apr 19, 2023 at 16:55

1 Answer 1


Sensation comes from receptors, and there are many different types, including free nerve endings, which are responsible for sensations of heat and pain 1. These free nerve endings can be directly stimulated by voltage. (So can other receptors, but these receptors are more readily stimulated by electricity 2.) Everything you "feel" is initially detected by your receptors and transmitted to your brain. What this means is that your skin itself does not feel anything - all sensation (through touch on skin) is conducted by these receptors.

[Most] Receptors lie in the 2nd layer of skin called the dermis, which lies under the epidermis 3. The epidermis has a few sublayers itself, including a [relatively and variably] thick sub-layer composed of cellular remnants and primarily the protein keratin 4. (All receptors lie below this sublayer 5). This layer protects the rest of your body from hazards in the environment. So to gain information from the environment through touch, your receptors have to be extra sensitive.

Now, when you blister or lose a layer of skin, part or all of the epidermis is what is usually lost 6. The net effect of this is you lose distance (and a layer of heat and electrical resistance) between your receptors and the environment - it is no longer protected by the epidermis. So your receptors will be more easily activated by stimuli - such as hot surfaces and your USB port. Additionally, fluids from your extracellular space can more easily "leak"7 and form a thin, sweat-like layer of fluid over the raw skin, which can actually enhance electrical conductivity.

As for why the smaller the contact, the greater the pain? Perhaps someone else can supplement, but my speculation is that when you apply pressure, other receptors (pressure, etc.) are also firing signals. This can cause a net dampening of heat/pain signal from the free nerve endings. Although not exactly the same process, you can think of like lessening the pain in one body part by inflicting pain in another (e.g. squeezing hands really tightly and clenching jaw after stubbing toe). When you are only slightly making contact with the port, only your free nerve endings may be firing, causing uninhibited signal. The pain may not be able to be replicated with intact skin simply because the free nerve endings under the epidermis are not firing - whatever miniscule current passing through the port cannot activate the receptors through an epidermal layer.

Again, this last part is just my reasoning. I don't think this has been studied, and there is so much variability in the human body that you would probably need a personal nerve conduction study for a definite answer to this phenomenon you've experienced.

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    $\begingroup$ "...you would probably need a personal nerve conduction study..." No. There's not that much variation in the nervous systems of individuals. $\endgroup$ Commented Apr 20, 2023 at 9:30
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    $\begingroup$ Btw, I'm not a neurologist, but I think your explanation is probably correct. I would liken it to, say, the same amount of pressure applied to a 1 cm^2 section of the fingertip vs. the point of a needle. The first would be noticeable, the second painful. $\endgroup$ Commented Apr 20, 2023 at 21:59
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    $\begingroup$ I don't think it's necessary to speculate about any of this, and I'm not sure what all has been done in humans but certainly has been extensively studied in animal models, I don't think speculation is necessary here. $\endgroup$
    – Bryan Krause
    Commented Apr 20, 2023 at 22:06

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