Researchers at the Stanford University School of Medicine and their colleagues have identified a handful of nerve cells in the brainstem that connect breathing to states of mind.
The finding, published in the journal Science, explains how slow breathing induces tranquility.
Medical practitioners sometimes prescribe breathing-control exercises for people with stress disorders. Similarly, the practice of pranayama, controlling breath in order to shift one's consciousness from an aroused or even frantic state to a more meditative one, is a core component of virtually all varieties of yoga.
The tiny cluster of neurons linking respiration to relaxation, attention, excitement and anxiety is located deep in the brainstem. This cluster, located in an area Mark Krasnow, professor of biochemistry at Stanford, calls the pacemaker for breathing, was discovered in mice by study co-author Jack Feldman, a professor of neurobiology at University of California, Los Angeles, who published his findings in 1991. An equivalent structure has since been identified in humans.
The lead author of the new study is former Stanford graduate student Kevin Yackle, now a faculty fellow at the University of California, San Francisco.
"The respiratory pacemaker has, in some respects, a tougher job than its counterpart in the heart," Krasnow was quoted as saying in a news release. "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 center might be in charge of generating these different types of breath."
On that hunch, Yackle searched through public databases to assemble a list of genes that are preferentially activated in the part of the mouse brainstem where the breathing-control center resides. The center's technical term is the pre-Bötzinger complex, or preBötC. He pinpointed a number of such genes, allowing the investigators to identify more than 60 separate neuronal subtypes, physically differentiated from one another by their gene-activation signatures but comingling in the preBötC like well-stirred spaghetti strands.
The researchers used these genes, and the protein products for which they are recipes, as markers allowing them to zero in on the different neuronal subtypes.
They were able to systematically assess the role of each neuronal subpopulation in laboratory mice, selectively destroy any one of these neuronal subtypes and only that subtype based on its unique signature of active genes, and observe how this particular subtype's loss affected the animals' breathing. In 2016, they reported in the journal Nature that they succeeded in isolating a subpopulation of neurons in the preBötC that explicitly controls one type of breathing: sighing. Knocking out these neurons eliminated sighing but left other modes of breathing unaffected.
Krasnow and Yackle then set out to discover the respiratory role of another subpopulation of about 175 preBötC neurons distinguished by their shared expression of two genetic markers called Cdh9 and Dbx1, and bioengineered mice in which they could wipe out, at will, the neurons bearing both of these markers. But once these rodents had their Cdh9/Dbx1 neurons eliminated, they seemed to take the loss in stride. Unlike their sigh-deprived brethren, there was no lacuna in these mice's portfolio of breathing variations.
"I was initially disappointed," said Yackle. However, a few days afterward, he noticed something: for mice, the animals were extraordinarily calm. "If you put them in a novel environment, which normally stimulates lots of sniffing and exploration," he said, "they would just sit around grooming themselves." Further analysis showed that while these mice still displayed the full palette of breathing varieties from sighs to sniffs, the relative proportions of those varieties had changed. There were fewer fast "active" and faster "sniffing" breaths, and more slow breaths associated with chilling out.
Surmising that rather than regulating breathing, these neurons were spying on it instead and reporting their finding to another structure, the locus coeruleus, in the brainstem, which in turn sends projections to practically every part of the brain and drives arousal, the researchers proved that the preBötC neurons that express Cadh9 and Dbx1 not only project to the locus coeruleus but activate its long-distance-projections, promoting brainwide arousal.
Neurons in the locus coeruleus are known to exhibit rhythmic behavior whose timing is correlated with that of breathing.
"The preBötC 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 center's function will lead to therapies for stress, depression and other negative emotions."
