Actually, can I chip in on this one? The reason the message goes so fast down the nerve is because the nerve exploits a trick. Nothing moves down the length of the nerve – that’d be too slow. Instead it does the *molecular equivalent of a Mexican wave*. Ever seen that? In a stadium, some people start the wave off by lifting up their arms, and people next to them life their arms right after that, and if enough people do it it looks like a wave is passing through the crowd? The speed the wave travels is way faster than any one person can run, right?
A nerve is a long thin thing. It contains metal ions like sodium and potassium. These ions are also on the outside of the nerve, but at a slightly different concentration. When a nerve is stimulated, and needs to send a signal, a little molecular pump at the start of the nerve fires into action and pumps ions across the membrane, changing the concentrations of ions from normal. This triggers the next pump along to kick into action, and so on. The ion flux is what the signal is. Sounds crazy, but you have to remember that molecular machines are really fast and responsive.
So it’s a wave, caused by stuff moving in/out of the nerve in a very tightly controlled sequence. That’s how the signal can move so fast.