HOW COME? Lots of things make you hiccup

Why do people get hiccups? asks Salma Khan, of Woodside.HOW COME?One slice too many of that birthday cake and pepperoni pizza? Startled by your little brother leaping from behind a door? Laughing so hard you can't stop? Next up: Hiccups crash the party.

Most people blame the diaphragm for those annoying "hics." But according to William Whitelaw, professor of medicine at Canada's University of Calgary, hiccupping involves the whole upper body, in a kind of coordinated spasm.

Take the diaphragm, the domed muscle under the lungs. The diaphragm helps us breathe, expanding upward to shove used air out of the lungs, then collapsing downward to allow fresh air in. When we hiccup, the diaphragm suddenly contracts strongly.

But the diaphragm isn't alone. Other muscles that help your body import air from the atmosphere squeeze at the same instant. As the muscles spasm in concert, the vocal cords in your throat also snap shut - making that annoying "hic" sound.

Hiccups are repetitive, and the brain has circuits that generate patterns for the body's other repetitive actions (think breathing, sneezing, coughing). The brainstem, where the brain connects to the spinal cord, is home to these pattern generators. So along with a Sneeze Center, there may a Hiccup Central lurking in the brainstem, too.

The brain, like a lighthouse, sends out a repeating signal telling the vocal cords (aka glottis) to snap shut. But something must set off the generator to begin with. Common triggers seem to be eating and drinking. When the stomach expands too far, irritated nerves probably set off the hiccups.

While the hiccupping pattern is set by the brain, it seems to be tied to the beating of the heart. In a study, Whitelaw found that in most hiccupping volunteers, each hiccup struck in mid-heartbeat. So as the heart muscle contracts, the glottis clamps shut. Hic-cup! And when hiccups continue, they follow a regular pattern, with a certain numbers of heartbeats - say, four - between each "hic" and the next.

Whitelaw and other researchers think the brain's blueprint for hiccups came from our ancestors who used gills for breathing. Just look at modern tadpoles. A half-grown tadpole has both gills and lungs. When a tadpole breathes underwater, its glottis snaps shut as its mouth fills with water. The water is then pushed out through the gills, its dissolved oxygen absorbed into the tadpole's bloodstream.

But above the surface of the water, a tadpole can open its mouth and fill it with air. Then, since tadpoles don't have diaphragms, it shuts its mouth (and nose); the gill passageway closes, too. Squeezing its mouth down, the tadpole actually forces the air into his lungs. These choreographed, hiccup-like moves seem to originate in a central pattern generator in the tadpole brain, much like Hiccup Central in the human brain.