Re: Why do we cry when we laugh? (i.e laughing to the point of tears)

Karen,
	Good question.  Such a good question, in fact, that I couldn't find a 
straight answer.  Laughing to the point of tears involves emotions, and 
although scientists are discovering a lot of interesting things about 
emotions, they're hard to study scientifically.
	What I can tell you is that we can shed tears for many reasons.  Most of 
them are reflexes of your body to things like a speck of dust in your eye, 
biting cold wind, cutting onions, etc. Like I said, these are reflexes that 
you can't control.  Interestingly, there are certain nerves in the head 
that you can sever that will cause you to lose this reflex (Specifically, 
the afferent path runs in the trigeminal nerve and the efferent path runs 
in the facial nerve).  
	Now, crying is a whole different way of shedding tears.  Even if you cut 
those nerves that make your eyes water in response to physical stimuli, you 
can still cry over something that makes you sad, and you can still cry 
because you're laughing.  This is called psychical or psychogenic tearing, 
and it's not related to reflex tears.
	Now, this is where my sources got kind of hazy.  They basically said 
that you cry in response to emotion, either good or bad, because of signals 
from higher centers in your brain that tell your tear glands (the lacrimal 
gland) that you're sad or happy.  These signals travel through the 
parasympathetic pathways, the part of your nervous system that controls, 
among many other things, bodily secretions such as tears. These signals are 
part of complicated patterns of emotion that your body instinctively 
recognizes.  If we look at other things your body is doing while you weep, 
it's not too surprising that you can cry while laughing also. The facial 
expressions of laughing and crying are similar, and even your breathing 
pattern is similiar (First you take a long deep breath, and then you let 
out short, sharp breaths, in both laughing and weeping).  
	I guess after all that scientific explanation, my answer is that it's 
hard to say.  Why do people in all cultures smile when they're happy?  Why 
do we cry when we laugh?  Is it in our genes somewhere?  I'm not sure we'll 
ever be able to really understand emotions, and until we do, I'm not sure 
anyone will be able to really answer your question.  But it's interesting 
to keep looking for answers!

Tom