People don't really understand what the terms 'up' or 'down' mean, in general. Down is the direction of the sum of the gravitation forces. That's why things fall down. The brain doesn't care about up or down location of nerves, because it is not affected much by gravity, it is held in place. If a nerve is above or below another, the brain doesn't care. On a side note, you should also realize that the direction 'down' is dependent on your location, unless you are on the discworld.