When I pinch my knee, why do I feel a tingle in my shoulder?

James - Thanks for your question, Jim. You've touched on something that's actually quite common there. In the nervous system, nerves receive activation at the level of our skin. They send their signal all the way to the spinal cord where they're processed, and the signals then go on an ascending pathway to the brain to be interpreted. That critical point in the middle there, at the spinal cord, is probably the source of this confusing phenomenon. Here's Mark Hoon, senior investigator of the neuroscience of sensation at the National Institutes of Health in the US.

Mark - Along the spinal cord, there's something called a dermatome, which is just a fancy word to say that there's different units all the way along the spinal cord. And each of those correspond to a grouping of nerves that go to a specific part of the body. It's a very defined area of the body that goes to a single part of the spinal cord. And that unit of information then goes up to the brain where there's this discrimination of location.

James - So what's happening in Jim's case where the stimulus is occurring in one part of the body, but he's feeling the sensation in another? Is it literally a case of the wires or the nerve pathways getting crossed?

Mark - In some ways, yes, you could refer to it as that. It's basically that there isn't a separation of some nerves coming into very precise locations, and that's called referred pain. And a very good example of referred pain is people who have pain that's actually the result of something going wrong in their heart. So a heart attack patient will often not feel the pain close to where their heart is, but will actually feel the pain in the shoulder, the neck and the back. And very often, if you have a shoulder pain for instance, the doctors will be quite alarmed and also have to test you to see that you haven't got a heart problem.

James - And this is why, as you say, there are parts of the body that are more commonly mixed up with each other. The dermatomes for the parts of the body they relate to happen to sit next to each other in the spinal column?

Mark - Yes, that's exactly how it is.

James - Jim's feeling, he referred to as more of a tingling than a pain. Some people might have an itch. Do we have a handle on the distinction between them?

Mark - A tingling sensation is better described as paraesthesia. And that is something that most people will recognise as something that happens, for instance, when they sleep on their arm and you're actually cutting the blood flow and the nerves are actually affected by that blockage. When you wake up with your tingling arm it's when the blood flow starts to go back to the nerves and they all start to have fire. And so you get activation of nerves that produce the sensation of pain, itch and also touch neurons. So it's a funny little activation that causes a tingling sensation.