I have poked around on the interwebs and I find the answer to that question is generally related to the stomach or the diaphragm. Distending the stomach too quickly or pressing on the diaphragm or something related to that. But what is not explained is how those events get translated into hiccups, rather than anything else. I suspect the vagus nerve is involved, but I haven’t found anything that directly relates eating to hiccups. Do any dopers have insight here? Thanks.
Distension or irritation of the lower esophagus to distal stomach sends signals up into the brain centers involved in hiccuping which sends messages back out to the diaphragm and the other muscles involved in the reflex.
The input side (afferent) can travel by way of the phrenic nerve, the vagus, and the sympathetic chain.
The brain center is in the medulla.
The main nerve out from the brain (efferent) to the muscles involved is the phrenic.
Other input can trigger hiccups too.
This article suggests the reflex exists to get rid of gas in the stomach. I am not convinced. They also present other previous best guesses for why it exists.
Most interesting article. Thank you. Answer: we don’t know. But illuminating was the mention that hiccups can be suppressed by an increase in serum CO2. I think that explains why holding your breath can help stop the hiccups. I always figured that it had to do with forced tension of the diaphragm for a few extra seconds. Maybe a little of both?
The CO2 angle also explains why the remedy of breathing into paper bag might help.