Why do we hiccup?
A twitchy muscle under your lungs and a door in your throat that slams shut.
A sudden twitch
Under your lungs sits a big flat muscle called the diaphragm. Normally it moves slowly, down and up, to pull air in and let it out. A hiccup is that muscle twitching all by itself. It jerks hard and fast, and air gets sucked in much quicker than usual. Watch it jump.
A hiccup (singultus) is an involuntary spasm of the diaphragm, often with the external intercostal muscles joining in. The contraction is sudden and strong, so air is drawn in fast. Common triggers stretch or irritate the stomach and the nerves near the diaphragm: eating quickly, a big meal, fizzy drinks, alcohol, a sudden change of temperature, or excitement. The twitch shown here is exaggerated so you can see it.
The "hic!"
A blink after the twitch, a little door at the top of your windpipe slams shut. That door is your vocal cords, the same flaps you use to talk. The rushing air smacks into the closed door and stops dead. That sudden stop is the "hic!" sound.
About 35 milliseconds after the diaphragm contracts, the glottis (the gap between the vocal folds) snaps shut. The column of air rushing in is stopped against the closed folds, which produces the characteristic sound. The vocal folds sit inside the larynx, stretched between the thyroid cartilage at the front and the two small arytenoid cartilages at the back; the muscles shown pull the arytenoids together to close the gap. The tiny air train here stops at the cricoid ring, just below the folds.
Why, and how to stop them
Nobody knows for sure why we hiccup. Babies do it even before they are born, so it may be practice for breathing. The order comes from the bottom of your brain and travels down nerves to the diaphragm. Most hiccups stop on their own in a few minutes. Holding your breath or sipping water slowly can help calm the nerves.
The hiccup is a reflex arc: sensory fibres in the vagus and phrenic nerves report to a centre in the medulla and upper spinal cord (C3 to C5), and the phrenic nerve carries the order back down to the diaphragm. (The phrenic nerve is not in this model; the vagus nerve is shown.) The purpose is unknown: hypotheses include a rehearsal of breathing in the fetus and a leftover of gill breathing in our distant ancestors. Most bouts end within an hour; hiccups lasting more than 48 hours are called persistent and are worth a visit to a doctor. Holding your breath raises carbon dioxide in the blood, which damps the reflex.