Pre-Bötzinger Complex: How Deep Breathing Promotes Tranquility
Taking a deep breath can calm you down. Now, researchers have found a group of nerve cells that regulates this effect.
Physicians and stress relief experts commonly prescribe breathing-control exercises for people diagnosed with stress disorders. In the same manner, in yoga and meditation, the practice of controlling the breath in order to shift consciousness from an aroused or even distressed state to a more calm one is a core component.
Conversely, breathing rapidly and excitedly causes tension to mount. But why?
The handful of nerve cells in the brainstem that researchers at Stanford University School of Medicine have identified seem to finally answer the question.
“The respiratory pacemaker has, in some respects, a tougher job than its counterpart in the heart. 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.”
The miniscule cluster of neurons linking respiration to relaxation, attention, excitement and anxiety is located deep in the brainstem. This cluster, located in an area Krasnow calls the pacemaker for breathing, was discovered in mice by study co-author Jack Feldman, PhD, a professor of neurobiology at UCLA, who published his findings in 1991. An equivalent structure has since been identified in humans.
Lead author Kevin Yackle, MD, PhD, now a faculty fellow at the University of California-San Francisco, 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. This center’s technical term is the pre-Bötzinger complex, or preBötC.
The pathway (in green) that directly connects the brain’s breathing center to the arousal center and the rest of the brain. Credit: Krasnow lab
Yackle 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 scientists were able to use these genes, and the protein products for which they are recipes, as markers allowing them to zero in on the different neuronal subtypes.
Knocking Out Neurons
Systematic assessment of the role of each neuronal subpopulation in laboratory mice was the next step. Using advanced technologies, they could selectively knockout any one of these neuronal subtypes – and only that subtype — based on its unique signature of active genes.
Then they could observe how this particular subtype’s loss affected the animals’ breathing. In 2016, in collaboration with Feldman, they succeeded in isolating a subpopulation of neurons in the pre-Bötzinger complex 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. They 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.
Yackle was initially disappointed, he said.
But then, after a few days, he noticed something: For mice, the animals seemed extraordinarily mellow.
“If you put them in a novel environment, which normally stimulates lots of sniffing and exploration,” Yackle said, “they would just sit around grooming themselves”
That is good evidence of what passes for calmness for a mouse.
Investigating still further, researchers found 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.
The investigators conjectured that instead of regulating breathing, these neurons were spying on it instead and reporting their finding to another structure in the brainstem. This structure, the locus coeruleus, sends projections to practically every part of the brain and drives arousal: waking us from sleep, maintaining our alertness and, if excessive, triggering anxiety and distress.
It’s known that neurons in the locus coeruleus exhibit rhythmic behavior whose timing is correlated with that of breathing. In a series of experiments, the Stanford researchers proved that the preBötC neurons that express Cadh9 and Dbx1 not only project to the locus coeruleus — a new finding — but activate its long-distance-projections, promoting brainwide arousal.
“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.”
“The preBötC now appears to play a key role in the effects of breathing on arousal and emotion, such as seen during meditation. We’re hopeful that understanding this center’s function will lead to therapies for stress, depression and other negative emotions.”