Sighing is a fundamental biological reflex that's a lot more important than most people care to think. We don't just sigh when we're in a position of weariness or relief, but quite regularly for no particular reason -- about 12 times an hour. Sighing opens up the lungs, and is thus vital to life. Now, researchers say they've found the neural pathways thatgovern the reflex. Those who suffer from breathing problems as well as compulsive sighers will benefit the most from the findings.
When we sigh, the million of tiny sacks inside the lungs called the alveoli inflate causing oxygen to enter and carbon dioxide to leave. Sometimes these sacks collapse and it takes a sigh to open them which is typically two breaths in for one breath out. If we didn't sigh, we'd be dead in under an hour.
Joining forces, researchers from labs at UCLA and Stanford sought to unravel the neural mechanism that leads to sighing. The researchers screen some 19,000 mouse genes that are involved in brain cells. They singled out 200 neurons in the brain stem that produce one of two neuropeptides -- small protein-like molecules (peptides) used by neurons to communicate with each other -- but couldn't tell at this point which were involved in sighing.
Later they found some peptides triggered a second set of 200 neurons, some of whom were already involved in controlling breathing. A handful of neurons were found to activate the mouse's breathing muscles to produce a sigh -- roughly 40 times an hour. When one of the peptides was blocked, the sighing rate was cut in half. Silencing both peptides halted sighing completely, the researchers reported in Nature.
"Sighing appears to be regulated by the fewest number of neurons we have seen linked to a fundamental human behavior," explained Jack Feldman, a professor of neurobiology at the David Geffen School of Medicine at UCLA and a member of the UCLA Brain Research Institute. "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."
"Unlike a pacemaker that regulates only how fast we breathe, the brain's breathing center also controls the type of breath we take," Mark Krasnow, a professor of biochemistry and Howard Hughes Medical Institute Investigator at the Stanford University School of Medicine said. "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."
Drugs could be designed that target these peptides to either suppress or enhance their generation on a case by case basis. For instance, there are anxiety disorders and other psychiatric conditions where sighing grows debilitating. Conversely, in some cases poor breathing is caused by a poor sighing reflex. As for conscious sighing triggered by emotional states, this is still a subject for debate among scientists.
"There is certainly a component of sighing that relates to an emotional state. When you are stressed, for example, you sigh more," Feldman said. "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."