Melissa Newton, of Green Island School, asks :-

What causes pins and needles?

Brian Hyland, a physiologist at the University of Otago Medical School, responded.

These feelings are produced by the nerves carrying information from our skin to our brain ("sensory nerves"). Nerves are made up of many axons (stretched-out cells). Each axon is responsible for sending messages about a small patch of skin.

Touching the skin activates the axons responsible for that bit of skin. These axons carry the message about the touch up the arm and eventually it reaches the brain. Each part of the body sends messages through different axons, so the brain knows which bit has been touched.

Axon messages are electrical signals, a bit like those in telephone wires. However axons are living cells, not wires; they need oxygen and food from the blood to keep working.

In healthy people, the most common cause of pins and needles is temporary reduction in blood supply. For instance, sitting in a certain way can reduce the blood supply to the nerves. The nerve axons stop working, because they are no longer getting the supplies they need. Messages from the skin and other parts of the leg no longer reach the brain and so all sensation is lost. We say that "my leg has gone to sleep". Sometimes the same thing happens in the arms, if students study their books for a long time with their elbows on the table.

When you change position so blood can flow to the nerve again, the axons begin to recover. But while they are recovering they send off `messages' by themselves ("spontaneously"), even though nothing is touching the skin. These messages are random. Because the brain believes what the axons send it, it feels as if random spots on the skin are being touched. Depending on how strongly the axons are working, the sensation can be mildly painful, as if someone was randomly pricking the skin with a pin (very fine point) - or if more intense, as if they were using a needle (a broader, stronger point).

Another way that pins and needles can be triggered is by hitting a nerve hard. This blow briefly stops the nerve axons working normally, and as they recover they again produce the random, spontaneous activity that gives rise to pins and needles sensation. We call this the "funny bone" sensation, even though it is a nerve, not a bone that causes it. This is because the most common place this happens is at a nerve that runs close to bone you can feel at the back of the elbow.

In either case, it is completely harmless: after a while, the nerve axons recover back to normal activity, and the sensation stops.

Diseases which affect the nerve can also cause pins and needles feelings. These are different than the common sort as they are present a lot of the time, and have no obvious cause. People with pins and needles feelings that happen a lot in the same places with no obvious cause should consult their Doctor.