When I’m on a road trip and don’t get the chance to satisfy the urge immediately, I notice a pattern:
If you don’t go when you first get the urge, it eventually goes away. When it comes back the second time, it builds up more strongly and then you really have to go.
I was told when young that the reason for this is you have a nerve at one point along your bladder; when it’s filled to a certain level, this nerve is triggered to send you the signal to go. But if you ignore it, the continual filling of the bladder bypasses it. Until it fills up to the point of a second nerve, which again signals you to void.
This sounds like a strange explanation to me, and I can’t see how it could work as described. Does anyone know the Straight Dope on urine urgency phases?
I apologize for the autobump, but this thread sank like a stone off the first page. This question may not be a matter of life and death, but it is something I’ve been wondering about for most of my life. Anybody?
Well, hey, I’ll take a shot at it. Google, “bladder nerve”.
http://www.urolog.nl/patienten-eng/bladder.asp
Dang, it won’t let me copy and paste. 
Well, anyway, that “sweet spot” is called the “trigone”. Go and read more than you ever wanted to know about how your bladder works (scroll down some). 
Yes, if you ignore the first signal, it eventually bypasses the trigone and starts dealing directly with the spinal cord (it’s not a second nerve, it’s the Big Kahuna.)
if you sprinkle when you tinkle
be a sweety
and wipe the seatie! 
sorry…off the track…but figured it needed a bump!!
Thanks for the explanation, Duck Duck Goose. I too had tried to Google the question, but found sites with extremely technical language on “urodynamnics” & stuff that didn’t really answer the question. What I did find was that the neurological wiring of the bladder is far more complex than ever suspected.
So it’s the trigone that triggers the first gotta-go signal, and the spinal cord that sets off the second, more powerful one. But what about in between? What turns off the trigone signal if you ignore it for a while?
I guess it’s just that a nerve that keeps firing continually will eventually lose its effect. Like when you try smelling too many different perfumes in a row, and then your smeller quits and you can’t tell the difference between them. Or when there’s a constant noise in the environment, you eventually quit perceiving it (until it suddenly quits and then the silence sounds paradoxically loud).
I don’t mean to hijack, but there’s something I’ve been wondering about as well. It may well be closely related to the original question, and may therefore illuminate the whole topic; besides, I didn’t think it was worth starting a whole new thread.
Actually, there are two questions. First: When you have to pee, why do you dance around? Well, adults don’t really, but I certainly fidget and twitch when I really have to go. The phenomenon is more obvious in kids.
And speaking of kids, I discovered at an early age that pushing on my belly just below my navel helps suppress the need to pee for a minute or two. Again, I don’t really do that any more; I’m too worried about bladder and kidney damage. But why does it work?
(Again, I apologize for the hijack. But like I said, expanding the discussion may make the functioning of the system clearer overall.)