Connected nerves: scratch one spot, feel it somewhere else?

Forgive me if this has been asked before, or if none of you know what I’m talking about…

What’s the reason why sometimes, if you scratch a particular area of your body, you can feel a sympathetic scratch somewhere seemingly completely unrelated? I can’t find a suitable spot right no (and besides, I’m in the office…) but say you’ll be scratching behind your ear and feel a related sensation somewhere in your lower back, or scratch your shin and feel it in your shoulder.

Is it related to the theory of acupuncture/acupressure? And other people do get this occasionally, right, it’s not just me? It only rarely happens, but the effect seems prettty real to me.

It happens to me on my arm where I cut myself quite badly a few years ago. I can scratch the skin below the scar and feel it above the scar.

It’s not just you, but I can’t help with why.
Got a spot on my back that hurts my face in front of my right ear when I scratch it, for instance

If I vigorously scratch to top of my ass-crack, I get a sharp pain in the tip of my penis.

I opened a thread about it years ago, I don’t recall if any conclusion was drawn. IIRC it ended up being a pretty funny thread, though.

I can help with part of it although the rest is hard to answer. Remember that the sensation your are feeling isn’t created in your skin, the brain is the primary sense organ for that. You may say duh! but think about it. The nerves all over your body just pick up your environmental signal. It is the brain that makes them into something that your conscience recognizes. Nerves run and wander all over your body and some of them are quite long. Your brain does an amazing job of taking the input from a never that exists across parts of your body and making a given signal into, say, a pinch one one exact spot on your skin. No one knows how all of this works and that is where it gets hard. The nerves are the transmission lines so if the signal gets altered or crossed, the brain can an incorrect interpretation of what they are transmitting. That is what is happening here. I get fantom hot spots on my skin from gentle rubbing and that is much the same thing. However, the details for all of this are still being explored.


This doesn’t work for the scar that you feel in two places - for that, you need Shagnasty’s explanation. Which was lucid and helpful.

Dermatomes are pieces of skin that got a nerve to grow to them while you were a fetus; the same nerve also grew to other tissues. For instance, a classic sign of blood accumulating beneath the diaphragm is pain over the top of the shoulder. This is because the same nerve runs to the diaphragm as to that spot on the skin of the shoulder. I missed this once when I was an intern. Guy who was in hospital after getting beaten up told me it was weird, but his shoulder was starting to hurt. I told him maybe he got hit there during the fight and didn’t notice it. He gave me a look and told me he didn’t think he could get hit without noticing it.

He was quite right. When I told my second-year resident about it, he immediately recognized phrenic pain and got the guy a CT scan. And I’ve never forgotten it.

Pain from the fifth sacral vertebra is often alleviated by scritching in a circle just around the anus. Pain from the fifth lumbar vertebra can often by temporarily alleviated by scritching a broad band over the top of the hip and down the outer side of the top of the leg. Pain from the first sacral vertebra is a broad band that is slightly lower across the hip and buttock and then runs along the inside of the back of the leg. I can often tell which nerves my husband’s compressed vertebral fractures are affecting most today by scritching his ass and leg, and seeing what relieves the pain.

Not all odd skin/other place combinations are explained by dermatomes, but quite a number of them are.

Many years ago, I worked for a neurobiologist who was doing pain research. I used to tell people my job was to design torture equipment, which always got me a few strange looks. The main purpose of most of this equipment was to figure out some basic things about how different sensations were processed by the human body. The experts in the field still don’t understand it all, and I only understand what little bit rubbed off on me while I was working there.

There’s a lot of interesting things that go on in your nervous system. For one thing, there’s an integration effect of sorts in your nerve cells. When you start to scratch, for example, at first the nerves are reluctant to fire off signals. But then, the more they fire off signals, the more they want to fire off signals, so if you scratch one area long enough you’ll feel a faint “phantom” scratching after you’ve stopped.

The integration effect also causes some other weird things. For example, if you precisely control the temperature of a small probe (which is what one of my “torture” machines could do) and you varied the temperature so that it went from 25 deg C (about room temp) to 40 deg C (not quite to the heat pain threshold, but rather warm) and alternated them back and forth at about a 1 second interval, people think that the temperature is getting hotter and hotter when it isn’t. In other words, while the temperature is going 25-40-25-40-25-40, what your nerves tell your brain is that the temperature is going 25-40-26-41-27-42-28-43 (approximately).

Another thing that happens is what engineers call “crosstalk”, which is when one signal accidentally gets coupled onto another signal. This happens all the time in electronics, and happens in your nerves too. Ever talk on the telephone and heard someone else’s conversation in the background? Same thing. If two nerves run side by side for a long time, it’s very easy for the signal for one to get coupled onto the other one. This is what causes SnakesCatLady to get confused about whether the sensation is above or below the scar. Both of those nerve signals end up in the same nerve bundle and you end up with crosstalk between them. People with sore throats often feel that their ears are itchy or sore. There’s nothing wrong with their ears, it’s just that it’s the same nerve bundle. People often feel heart attack pains in areas that are not their heart, like their shoulder or arm. Same thing. The neurobiologist did an interesting test on me once when I asked him exactly what he did with some of these machines I made for him. He put two probes on my forearm, very carefully choosing two locations that ended up in the same nerve bundle (which he didn’t tell me until later). The two spots he chose weren’t righ next to each other, either. They were a good 5 inches or so apart. Then it was a very simple test. One probe or the other would get hot. All I had to do was press a button for which probe it was. Simple enough, right? I missed 8 out of 20. I was like what? :dubious: which is when he explained the whole crosstalk in nerve bundles thing to me.

If you think the nerves themselves are complicated, once you reach the brain it gets even worse. Some parts of your nerves are processed way before your conscious brain is even aware of what is going on. This is how your reflexes work. There’s a bunch of different layers of how things are processed, and your consciousness is pretty much the last thing in a long line of stuff.

I’d go with the crosstalk model. Sometimes I feel an itch and have a very difficult time figuring out where to scratch. I can think it’s on the bottom of my foot, then when I scratch my foot it’s not there, it’s at the back of my head, then my side. It doesn’t feel like it moves around, I just can’t localize it.