(Sigh...) How should I know? Don't I already have enough on my mind?
As we all well know, sighing is a long, deep involuntary inhalation accompanying sensations of yearning, sadness, relief, boredom, exhaustion, or (see above) exasperation. Fewer of us know (at least I didn't, but now I do!) that the typical person also sighs spontaneously about every five minutes or so.
If you're a mouse, you do it much more often - as much as 40 times per hour. (Nobody said it would be easy, little mousie.)
Those spontaneously sighs (and all the other ones), it's thought, may be helping to keep our half-billion or so alveoli - the tiny sacs through which our lungs exchange oxygen and carbon dioxide with the atmosphere that surrounds us - pumped up and operating efficiently.
In a series of experiments described in a Nature study, Krasnow's team, along with colleagues at Stanford and UCLA, painstakingly employed genetic, pharmacological and surgical techniques to map out a precise set of nerve circuits in the brain that are essential to the act of sighing. They showed that a sigh results when inhalation-initiating nerve impulses generated rhythmically within these circuits double up: One impulse effectively laps another and rides piggyback on top of it, producing a deeper, drawn-out inhalation.
The experiments were performed in mice. But the brain circuits involved are sufficiently ancient that our common ancestors no doubt had them, too - and therefore we (probably) do, too, or at least very similar ones.
The sigh-generation apparatus consists of just several hundred individual nerve cells in the brain stem (a minuscule fraction of the estimated 100 billion of them in our entire brains), making it possibly the most compact microcircuit for what is, when you think about it, a fairly complex behavior that's ever been identified.
It may be possible to use this new understanding to medical advantage: For instance, stimulating sighing in patients who can't breathe deeply on their own could, in principle, be life-saving.
The work fundamentally changes our understanding of the brain's breathing-rhythm generator, says Krasnow, who knows a thing or two about alveoli: "Unlike a pacemaker that only regulates how fast we breathe, the brain’s breathing center also controls the type of breath we take. It's made up of small numbers of many different kinds of nerve cells, each kind functioning like a button that when punched turns on a different type of breath. One button is for regular breaths, another for sighs, and the others could be for yawns, sniffs, coughs, and maybe even laughs and cries."
All very cool. Now, a hundred bucks to whoever finds the button I can push to turn off the hiccups.