Breathing for Mental & Physical Health & Performance | Dr. Jack Feldman

The episode opens with Andrew Huberman, who introduces himself as a professor of neurobiology and ophthalmology at Stanford School of Medicine, presenting his guest, Dr. Jack Feldman, a distinguished professor of neurobiology at the University of California, Los Angeles, known for pioneering work on the neuroscience of breathing. The introduction treats breathing as more than oxygen delivery. It claims that how we breathe, including how often, how deeply and the ratio of inhales to exhales, "actually predicts how focused we are, how easily we get into sleep, how easily we can exit from sleep."

The introduction credits Dr. Feldman with discovering the two major brain centers that control different patterns of breathing, and describes him as a practitioner as well as a researcher. It also claims that before he came into neuroscience from physics, there "really wasn't much information about how the brain controls breathing," while acknowledging there was "a little bit of information." Listeners are promised protocols for focus, disengaging from high-stress work and calming down quickly, along with the underlying science needed to customize them.

The opening also includes a paid resource mention. The host says he is not associated with Our Breath Collective, but that Dr. Feldman is an advisor to it. It is described as offering daily live guided sessions, an on-demand library and free workshops, with a code giving $10 off the first month.

The discussion begins on a note of scientific humility: "there's a lot that we don't understand, which still keeps me employed and engaged, but we do know a lot."

The first explanation concerns purpose. Metabolism needs oxygen, and aerobic metabolism produces carbon dioxide. Getting rid of CO2 matters because it affects the acid-base balance of the blood, the pH, and "all living cells are very sensitive to what the pH value is." Breathing therefore has two jobs: supplying enough oxygen for normal metabolism and removing the CO2 that metabolism produces.

Airflow is explained with a balloon analogy. Expanding the lungs lowers the pressure in the air sacs, the alveoli, and air flows in because pressure outside the body is higher. The main muscle doing this is the diaphragm, which sits just below the lung and contracts downward during inhalation, while the ribcage rotates up and out to enlarge the thoracic cavity.

At rest, exhalation is described as passive, like letting go of a stretched spring: "Inhale, relax, and exhale." Asked directly whether the exhale is passive, the answer is qualified: "At rest, it's passive," with a different picture promised for situations such as exercise, when ventilation and metabolism rise.

Skeletal muscles do nothing unless the nervous system tells them to. Specialized neurons in the spinal cord and the brain stem, called motor neurons, drive the diaphragm and the external intercostal muscles of the ribcage, which are active for a period and then fall silent, letting the muscles relax back to rest. The question of where that motor neuron activity originates is described as one that had been "bandied around for thousands of years," and one that was a high priority early in an assistant professorship, because the mechanism could not be understood until researchers knew where to look.

The answer described is a small region in the brain stem, just above the spinal cord, called the pre-Bötzinger complex. In humans it contains a few thousand neurons, sits on either side and works in tandem. When its neurons finish their burst of activity, inspiration stops and passive recoil of the lung and ribcage produces exhalation. The story of how the region was named is mentioned but deferred.

And every breath begins with neurons in this region beginning to be active.

Two side questions sharpen the picture. On nose versus mouth breathing, the answer begins "I don't think we fully have the answer to that." Nasal breathing tends to dominate at rest because the airflow needed is easily managed through the nasal cavities, while larger ventilation needs, as in exercise, shift breathing to the mouth because its airway is larger. But the contraction of the diaphragm and intercostals is described as "almost agnostic" to whether the nose or mouth is open, and there is no apparent modulation in the pre-Bötzinger complex based on where the air comes from.

On muscle type, the breathing muscles are described as skeletal muscles that need neural input to move, contrasted with muscles that can contract and relax on their own; the transcript groups the gut and heart together here and notes that the heart beats without neural input, which only modulates its strength and frequency. Smooth muscle does affect breathing by lining the airways, and inappropriate constriction of that smooth muscle is what happens in asthma. Whether brain breathing centers play a role in asthma is answered cautiously: "the preponderance of evidence is that it's not involved, but we've not really fully investigated that."

The account then turns to a correction of the original model. After the pre-Bötzinger complex was discovered, it was thought to be the single source of all rhythmic breathing, both inhalation and exhalation, likened to day following night. Experiments in the early part of 2000 suggested another region dominated expiratory movements. The fundamental mistake, as described, was not accounting for the fact that exhalation is passive at rest, so neurons that generate active expiration, driving muscles like the abdominals and internal intercostals, are simply silent and appear absent.

This second oscillator generates what is called active expiration: the forceful push of air out during exercise, or a sound like "Shh." The group of cells is "silent at rest" and becomes active when needed, and "it appears that it's an independent oscillator."

An oscillator is defined as anything that goes in a cycle, such as a pendulum or the earth's rotation producing day and night. Oscillators can be highly regular, like a watch, or sporadic and episodic. Breathing is one that must run 24/7: it starts late in the third trimester so it is working at birth, and if it stops without intervention for more than a few minutes, "it will likely be fatal."

The second oscillator was found in a region around the facial nucleus. That area was first thought to be a central chemoreceptor, sensing carbon dioxide, because the brain is "extraordinarily sensitive to changes in pH," and a big shift in CO2 causes a big shift in brain pH. Others had identified the ventral surface of the brain stem as critical for this sensing. A nearby structure, unnamed in anatomical atlases and close to the trapezoid nucleus, was given a name "out of the hat": the retrotrapezoid nucleus. The discussion clarifies that "trapezoid" refers to the shape of a cluster of neurons, not the trapezius muscles.

Why would an exhalation center sit near facial structures? The explanation is evolutionary. Control systems for facial muscles developed near the motor nuclei in primitive creatures that took food into their mouths, and at one point facial muscles were probably important for moving fluid and air in and out of the mouth. Some of the many subnuclei in this area now seem, in mammals, to be involved in controlling expiratory muscles.

Mammals are described as the only class of vertebrates with a diaphragm. Amphibians and reptiles breathe the opposite way to mammals at rest: they actively expire and passively inspire, lacking a powerful inspiratory muscle. How the diaphragm developed is left open: there are "lots of theories," and "I don't think it's particularly clear," though a structure in alligators and lizards could have turned into it.

The core argument is about surface area. Oxygen passes passively across the alveolar capillary membrane into the blood, at a rate that depends on the membrane's characteristics and the distance to the capillary, but "the key element is the surface area." Lungs pack a large area into a small chest by branching from the trachea into two, then four tubes and onward, ending in alveoli. A larger elastic membrane takes more force to pull apart, so amphibians, unable to generate much force, have relatively few branches and an unimpressive lung surface area relative to body size.