For centuries, people have slowed their breathing to calm their minds. For some of us, this takes the form of meditation or yoga; for others, it's 10 deep breaths before a panic attack sets in.
Regardless of what you call it, scientific evidence has backed up the fact that our breath can induce a feeling of tranquillity - although no one has ever been able to figure out exactly how that happens. Now, researchers think they might have finally found the answer, pinpointing a small group of neurons in the brain stems of mice that connect the breath with feelings of calm.
To be clear, the research so far is limited to mice - scientists are yet to replicate the results in humans.
But the mouse brain has many similarities to the human brain, so it's a good starting point that could begin to explain on a physical level how practices such as meditation and pranayama yoga can bring on feelings of calm and euphoria.
"This study is intriguing because it provides a cellular and molecular understanding of how that might work," said lead researcher Mark Krasnow from Stanford University School of Medicine.
The group of cells in question belongs to the pre-Bötzinger complex, an area of neurons deep within the brain stem that are known to fire each time we breathe in or out - like a breathing pacemaker.
This structure was first discovered in mice back in 1991, but a similar structure has also been found in humans.
"The respiratory pacemaker has, in some respects, a tougher job than its counterpart in the heart," said Krasnow.
"Unlike the heart's one-dimensional, slow-to-fast continuum, there are many distinct types of breaths: regular, excited, sighing, yawning, gasping, sleeping, laughing, sobbing."
"We wondered if different subtypes of neurons within the respiratory control centre might be in charge of generating these different types of breath," he added.
Last year, Krasnow and his team found evidence that a small group of neurons within this pre-Bötzinger complex were solely responsible for generating sighs - without them, mice never sighed, and when they were simulated, the animals couldn't stop sighing.
In this latest paper, they found a separate group of neurons in the complex that have a more zen function - they appear to regulate states of calm and arousal in response to our breath.
To figure this out, the team identified two genetic markers called Cdh9 and Dbx1 that they noted were active in the pre-Bötzinger complex and appeared to be linked to breathing.
They then genetically engineered mice without any of the neurons that expressed these two genes - taking out a subpopulation of about 175 neurons in the brain stem.
Interestingly, the mice without these neurons still breathed normally, but with key one difference - they breathed more slowly than normal mice.
"I was initially disappointed," said Kevin Yackle, one of the research team, now at the University of California, San Francisco.
But after a few days, the team noticed something else strange going on - the mice without the Cdh9 and Dbx1 neurons were extraordinarily calm compared to their control group peers. They still showed varieties of breathing, but they were all at a much slower pace.
"If you put them in a novel environment, which normally stimulates lots of sniffing and exploration," said Yackle, "they would just sit around grooming themselves." For mice, that's taken as evidence of a zen state of mind.
"We were totally surprised," Yackle told Diana Kwon over at Scientific American. "It certainly wasn't something we expected to find."
Upon further investigation, the team found evidence that the neurons were forming connections with the locus coeruleus - a region of the brain stem that's involved in modulating arousal and emotion, and is responsible for waking us up at night and triggering anxiety and distress.
The team concluded that rather than regulating breathing, this little group of neurons was responding to it and reporting their findings to the locus coeruleus so that it could regulate our mood in response.
"If something's impairing or accelerating your breathing, you need to know right away," said Krasnow. "These 175 neurons, which tell the rest of the brain what's going on, are absolutely critical."
You can see below the pathway (green) that directly connects the brain's breathing centre to the arousal centre and the rest of the brain.
Krasnow Lab
The work is definitely a promising step forward, but we need to keep in mind that there's still a lot we have to learn about how the pre-Bötzinger complex works, particular in humans.
Still, the new paper raises the possibility that "any form of practice - from yoga, pranayama to meditation - that is actively manipulating respiration might be using this pathway to regulate some aspects of arousal," neurobiologist Antoine Lutz from the French National Institute of Health and Medical Research, who wasn't involved in the research, told Scientific American.
While other teams will now need to pursue this research further in mice and eventually humans, Krasnow and his team are now continuing to get a better understanding of what other secrets could be hiding in the pre-Bötzinger complex.
"The pre-Bötzinger complex now appears to play a key role in the effects of breathing on arousal and emotion, such as seen during meditation," said Feldman.
"We're hopeful that understanding this centre's function will lead to therapies for stress, depression and other negative emotions."
The research has been published in Science.