The wistful sigh is really a survival mechanism

A fluorescent-green marker on each side of the brain stem illuminates the two networks of 200 neurons that control the sighing reflex(Credit: Krasnow lab/Stanford)

A sigh may do more for your health than provide emotional relief. Researchers in California claim to have identified the source of the sigh in the brain, which they say is a life-sustaining reflex for healthy lung functioning. Humans sigh around 12 times per hour to reinflate the half-billion or so tiny, balloon-like sacs in the lungs called alveoli, which are vital in regulating the flow of oxygen and carbon dioxide. A sigh is mostly an involuntary deep breath, or a regular breath with another added on top before an exhale.

According to the study conducted by researchers from UCLA and Stanford University, sighs are a response to the lungs' signal to inflate the alveoli, and originate from two tiny clusters of 200 neurons in the brain stem – the area also responsible for autonomic functions such as heart rate, breathing and sleeping. The researchers believe the sigh has the fewest number of neurons linked to any basic human behavior. The findings also provide insights into mechanisms that may be behind other complex behaviors.

Mark Krasnow, one of the study's authors and a biochemist at Stanford University School of Medicine, said the brain's breathing center controls the rate of breath, as well as the type of breath taken. "It's made up of small numbers of different kinds of neurons," he said. "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."

Using laboratory mice, which sigh up to 40 times per hour, the researchers looked at 19,000 patterns found in mice brain cells with possible links to genetic activity. From this they found 200 neurons in the brain stem that produce and release one of two neuropeptides, which facilitate communication between brain cells.

While the researchers were unclear on which brain cells the neurons were talking to or why, they knew the strain of peptides was also found in humans and very active in the area of the brain involved with sighing. By blocking one of the peptides in the mice, their rate of sighing was reduced by half, and shut down completely when blocking both peptides.

"These molecular pathways are critical regulators of sighing, and define the core of a sigh-control circuit," said Krasnow. "It may now be possible to find drugs that target these pathways to control sighing."

Without sighing, our lungs would eventually fail. Early breathing devices did not provide regular deep breaths to patients, who sometimes died as a result. Today's ventilators compensate for sighs by delivering a large volume of air.

The study was published in the online edition of the journal Nature.

Source: UCLA Health

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