|MadSci Network: General Biology|
There are some old experiments that confirm what you have noticed; it seems that the muscle tone in a person's arms drops by about 1/2 when they are laughing. Very little is known about how a person's brain organizes and controls laughter, so I can't give you a definitive answer. However, I do have some speculations that I will share with you.
It is pretty clear that the brain circuits that are responsible for emotionally based movements, such as laughter, are largely different from the ones that control voluntary or goal directed movements, such as picking up a glass of water. This difference is clearly seen in some people who have had strokes. They might be able to laugh or smile if someone says something funny, but not be able to laugh or smile on command. Or they might have great difficulty speaking a sentence, but be able to say swear words or other emotional exclamations without difficulty.
It also seems that people often have little control over the activity of the "emotionally based" motor circuits. Every laugh you make is pretty much like all of the other laughs you make. You don't really have any voluntary control over how they come out. You have also probably noticed that it can be very hard to supress a laugh if someone does something you think is funny. And we have all probably experienced a situation in which we just can't stop laughing or crying, even though we genuinely want to stop.
These observations tells us that the brain circuits that generate emotionally based behaviors like laughter seem to have preferential access to the parts of the motor system that actually make our muscles move. So, it seems plausible that the reason you feel weak is that the part of your brain that controls laughter is interfering or suppressing other commands you might send to your muscles.
That only answers how it occurs. Why your body muscles should become weak is a tougher question, and one I don't have an answer to. I suspect that if we understood the purpose of laughter, we might be able to find a reason. One hypothesis about laughter states that laughter is a way of signaling submission to others; if that were true, it might make sense to relax your muscles during the laugh. The relaxed muscles might send a visual signal that you are not preparing to fight. I don't find that expalnation very satisfactory, but its the best one I can think of considering the available evidence.
For more reading, you can go to this article by Robert Provine. You may also be interested in the extensive bibliography about laughter found here.
If you have any questions or comments, please send me an email.
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