Action potentials of an earthworm going negative first?

Ok so for a bit of a background, I am doing a science project looking at the action potentials of the earthworm. I anaesthatized the worms then hooked them up to a spiker box (forums aren't letting me link the website. Just type in spikerbox and it is the first result) and stimulated them. This is an average result:



What I am confused about is why the action potential goes negative first before then going back up. From my readings online, I thought that the action potentials were meant to go up first then travel down and then continue on its course. If it helps I used a program called audacity to record the action potentials.

A bit more information on the earthworm if it at all influences this is that they have 3 main nerve cords where all the nerves bundles are held, 2 of them carrying the signals down the worm and the other transmitting them to the front.

Thanks very much guys. Sorry if I'm not being clear. Just ask anything if you need more information!