Nation & World

The mechanics of life-saving sighs

It’s a great day for sigh-ence. In a study published Monday in Nature, researchers claim to have found the brain circuit that controls sighing in mice. You probably haven’t given much thought to the involuntary exhalations that signal everything from wistfulness to disapproval, but this type of breathing is more than just an emotional cue: It’s actually a life-saving reflex.

Sighs — the quick addition of a second inhalation before an exhalation, creating an unconscious deep double breath — can have emotional triggers, but they also happen involuntarily every few minutes regardless of your mood. Some scientists now think these sighs serve a monumentally important purpose.

The lungs contain some 500 million tiny, balloon-like structures called alveoli, which all together have the surface area of a tennis court. Alveoli hardly ever collapse, but then again there are a half-billion of them. At that scale, “hardly ever” becomes pretty often. And if enough alveoli collapse, breathing becomes difficult.

It’s thought that sighs serve to reinflate errant alveoli and keep us breathing easy.

According to the new study, this mechanism could rely on a surprisingly small group of brain cells. Just 200 or so neurons in the brain stem trigger the process that leads to a sigh — at least in rodents, which have brains and respiratory systems quite similar to our own.

“Unlike a pacemaker that regulates only how fast we breathe, the brain’s breathing center also controls the type of breath we take,” study author Mark Krasnow of Stanford University said in a statement. “It’s made up of small numbers of different kinds of neurons. Each functions like a button that turns on a different type of breath. One button programs regular breaths, another sighs, and the others could be for yawns, sniffs, coughs and maybe even laughs and cries.”

Researchers learned that peptides they’d identified serve to trigger a second set of 200 neurons, which then produces a sigh by controlling breathing muscles. Blocking one of the two peptides identified by Stanford cut sighs in half, and blocking both cut sighing off entirely.

The researchers hope that others will use their work to help those who sigh too much — or not enough. It’s possible that patients unable to breathe deeply on their own could benefit from artificial sighs to keep their alveoli inflated, and while there isn’t any indication that excess sighing is bad for you, it can cause a lot of stress. UCLA researcher Jack Feldman, who collaborated on the study, thinks the basic science behind the new paper is just as noteworthy as potential medical applications.

“This greatly interests us because it’s happening on top of the normal breathing rhythm,” Feldman said. “If we can understand how this double breath comes, it could give us a back door into understanding how the basic rhythm is generated.”