Though we often sigh out of relief, tiredness or sadness, sighing is actually crucial to lung function.
Jack Feldman, a professor of neurobiology at the David Geffen School of Medicine at the University of California-Los Angeles (UCLA), and colleagues publish the details of their discovery in the journal Nature.
According to Feldman, a sigh is a deep breath, but it is not a voluntary one. "It starts out as a normal breath, but before you exhale, you take a second breath on top of it."
The average person sighs every 5 minutes - equating to around 12 times an hour. This may sound excessive, but we need to sigh this frequently in order for our lungs to function properly.
Sighing is required in order to inflate the alveoli in the lungs - the tiny sacs that enable oxygen and carbon dioxide to move between the lungs and the bloodstream. However, some of these alveoli can collapse.
"When alveoli collapse, they compromise the ability of the lung to exchange oxygen and carbon dioxide," explains Feldman. "The only way to pop them open again is to sigh, which brings in twice the volume of a normal breath. If you don't sigh, your lungs will fail over time."
Two groups of neurons found to control sighing
While sighing is crucial to health, there are situations when it can become a problem.
There are two groups of around 200 neurons (highlighted) in the brain stem that control sighing.
Image credit: Krasnow lab/Stanford
On the other hand, there are some conditions that make it difficult for a person to sigh, such as respiratory problems, meaning their lung function may be compromised.
For their study, Feldman and colleagues set out to gain a better understanding of what role the brain plays in sighing and breathing rhythm - information that could one day help people who experience debilitating sighs or who have difficulty sighing.
Previous research has identified peptides in the brain - including frog bombesin - that can influence sighing in rodents, though the mechanisms underlying the release of such peptides have been unclear.
With the aim of unraveling this mystery, the researchers analyzed more than 19,000 gene expression patterns in the brain cells of mice, identifying around 200 neurons, or brain cells, in the brain stem that are responsible for the production and release of bombesin-like peptides.
Further investigation revealed that these peptides stimulated another group of 200 neurons that led the breathing muscles of mice to significantly increase the number of sighs produced, from around 40 an hour to more than 400.
"These molecular pathways are critical regulators of sighing and define the core of a sigh-control circuit," says study coauthor Mark Krasnow, a professor of biochemistry at Stanford University School of Medicine in California.
Findings may open door to drug target
The researchers say their findings offer insight into how networks of cells in the brain stem regulate breathing rhythm.
"Unlike a pacemaker that regulates only how fast we breathe, the brain's breathing center also controls the type of breath we take," explains Krasnow.
"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."
"Sighing appears to be regulated by the fewest number of neurons we have seen linked to a fundamental human behavior. One of the holy grails in neuroscience is figuring out how the brain controls behavior. Our finding gives us insights into mechanisms that may underlie much more complex behaviors."
Additionally, because the peptides identified in the brain cells of mice are also found in the human brain, the researchers say it may be possible to target the peptide-producing pathways with drugs in order to control sighing in humans.
Still, the researchers note that the underlying brain mechanisms of conscious sighing remain unclear, and this is something that warrants further investigation.
"There is certainly a component of sighing that relates to an emotional state. When you are stressed, for example, you sigh more," says Feldman. "It may be that neurons in the brain areas that process emotion are triggering the release of the sigh neuropeptides - but we don't know that."
Medical News Today recently reported on a study that suggests breathing through the mouth during sleep may raise the risk of tooth decay.