I don't know about mania but I know for depression the problem is that the nerves are firing incorrectly and the seritonin and norepinephrin cannot cross the gap from one nerve to the next in the proper manner. That's what I remember from taking psychology like 12 years ago... so it's a bit fuzzy.

I have also heard, but not sure if it is true, but I heard somewhere along the lines that ever manic episode causes damage to our brain or nervous system. But, I can't remember where I heard it so could be totally wrong.