[extropy-chat] why do we stop laughing?

[Spike]

I was cutting up with some guys at lunch today.  A
question occurred to me: why is it that when someone
does or says something funny, people start to laugh,
then something must happen in our brains to adjust
something.  Otherwise we would keep on laughing at
that joke indefinitely.  Sometimes some event is so
funny we cannot stop laughing for some time, but after
a period of time we always do stop.  Good thing, eh?  
But what actually occurs in our brains to allow us
to stop laughing?

spike