|MadSci Network: Anatomy|
Hiccups are caused by a spasmotic contraction of the muscles of the diaphragm. The diaphragm is usually used to increase the volume of the rib cage to draw air into the lungs.
Nobody knows what causes hiccups, but they usually go away spontaneously. Those that last for more than three days are considered to be chronic and need medical intervention. There have been over a hundred medical reports of different medical treatments published in the past several years.
There have also been innumerable "home remedies" advocated for stopping hiccups. How a spoon full of sugar might stop hiccups is unknown. I suppose it depends on what you do with the spoon full of sugar. If you deposit a spoon full of sugar into your mouth you will probably be a little uncomfortable as to how to try to swallow it. There could be various nerves in the throat and esophagus that would be stimulated and as a consequence of this, affect the nerves of the diaphragm. Most "home remedies" involve putting something into the mouth which probably activates a nerve reflex that affects the diaphragm.
If you put the spoon full of sugar on the counter top then the only way that could stop the hiccups would just be a coincidence in timing since most hiccups stop spontaneously anyway. Actually, this is probably the explanation for the effectiveness of all the "home remedies".
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