In the vocabulary of body language, the sigh surely ranks among the most versatile: a single, heavy breath calibrated to convey everything from relief to—eye roll, please—disdain.
But most sighs aren’t so nuanced, or noticed. They’re involuntary impulses that draw in a double-dose of oxygen that is vital to keeping the alveoli—the half-billion balloon-like sacs within the lungs—working properly. The typical person heaves a life-preserving sigh every five minutes or so.
Now researchers at Stanford and UCLA have pinpointed two tiny clusters in the brain—totaling just a few hundred neurons—that are responsible for these involuntary sighs.
The circuit appears to be the fewest number of neurons ever linked to a basic human behavior, a needle-in-the-haystack discovery that offers fundamental insight into the brain’s mechanics as well as the potential for treatments to control the reflex.
Turning on sighing could be vital for people who can’t do so on their own. “If you don’t sigh every five minutes or so, the alveoli will slowly collapse, causing lung failure,” says Jack Feldman, a UCLA professor of neurobiology who co-authored the study along with Stanford biochemistry professor Mark Krasnow. “That’s why patients in early iron lungs had such problems, because they never sighed.”
The mechanism behind the roots of conscious sighing remains a mystery. Alas.