Sarah Smith, of Ardgowan 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. Nerves are made up of many axons (stretched-out cells).

Touching the skin activates the axons responsible for that bit of skin. These 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.

Axons 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 (which is why legs 'go to sleep' after being sat on in certain ways) and they do not like being hit (your 'funny bone' is actually a nerve containing axons coming from your hand).

In a leg that has 'gone to sleep', the axons have stopped working and the skin that those axons come from feels numb, since no messages are carried. When you move so blood can get in, the axons begin to recover. But while they are recovering they send off 'messages' by themselves, even though nothing is touching the skin. Since the brain 'believes' what the axons send it, it feels as if the little spot of skin associated with each axon were being prickled.