|MadSci Network: Anatomy|
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
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