How do we react so fast when we put our hand on a hot surface? © Getty Images

How do we react so fast when we put our hand on a hot surface?

Asked by: Tom Hampton, Townsville, Australia

Nerve impulses travel at about 50 to 60 metres per second in your arm, which means they can go from your fingertip to your brain, and back again, in about 27 milliseconds. But in most situations, that nerve impulse also has to be processed by the brain so that you can decide on the appropriate muscle movements to react to the sensory input. This can add another 130 to 160 milliseconds to your reaction time.

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If your hand is touching a hot stove, that extra delay is enough to burn you badly. So, for certain extreme stimuli like heat or a sharp pain, the nerve impulses bypass your brain altogether. The sensory nerves and the motor nerves become short-circuited together in the spine so that you can yank your hand out of the way without thinking.

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