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

Andrew Huberman opens by calling neuroscientist Jack Feldman his go-to source on how the brain and breathing interact, and asks him to begin with the basics of what generates a breath. Feldman starts with the purpose. Air has to flow in and out because the body's metabolism needs oxygen, and aerobic metabolism produces carbon dioxide. That carbon dioxide has to be removed in particular because it shifts the acid-base balance, or pH, of the blood, and Feldman notes that all living cells are very sensitive to pH. So the body works hard to regulate it.

The mechanics follow from pressure. Feldman compares the lungs to a balloon being pulled apart. As the lungs expand, pressure inside the air sacs, the alveoli, drops below the pressure outside the body, and air flows in. The main muscle doing this is the diaphragm, which sits just below the lungs. When it contracts it pulls down and exerts force on the lungs, while the rib cage rotates up and out to enlarge the thoracic cavity. At rest, exhaling is passive. At the end of inspiration you simply relax, and Feldman likens it to letting go of a spring you have pulled down.

The rhythm itself starts in the brainstem, in a region just above the spinal cord called the pre-Bötzinger complex. Feldman describes it as a small site that, in humans, contains a few thousand neurons, present on both sides and working in tandem. Every breath begins when neurons there become active. Those neurons ultimately connect to the motor neurons that drive the diaphragm and the external intercostal muscles, producing the inspiratory effort. When the pre-Bötzinger neurons finish their burst, inspiration stops and exhalation follows through the passive recoil of the lungs and rib cage.

Asked whether the diaphragm and intercostals are activated differently in nasal versus mouth breathing, Feldman starts with a caveat: "I don't think we fully have the answer to that." He grants that the two clearly differ. At rest people tend to breathe through the nose because normal airflow passes easily through the nasal cavities. When ventilation has to rise, as in exercise, people switch to the mouth because its airway is much larger and can move more air. At the level of the breathing muscles, though, he says contraction is almost agnostic to whether the nose or mouth is open.

Feldman then describes how his group's view of the breathing circuitry changed. When they discovered the pre-Bötzinger complex, they thought it was the main source of all rhythmic breathing movements, both inspiration and expiration. Later experiments turned up a second oscillator that generates what he calls active expiration, the forceful pushing out of air you do during a sharp exhale or when you start to exercise. This group of cells is silent at rest, becomes active to drive those muscles, and appears to be an independent oscillator in a region around the facial nucleus.

When this region was first identified, Feldman says, it was thought to sense carbon dioxide, acting as a central chemoreceptor. The reasoning is that the brain is "extraordinarily sensitive to changes in pH," so a large shift in carbon dioxide would shift brain pH and, as he jokingly puts it with "the technical term," throw the brain "out of whack." Regulating something in the brain calls for a sensor in the brain. Other researchers found that the ventral surface of the brainstem was critical for this, and Feldman's group identified a structure near the trapezoid nucleus. Since it had no name in the neuroanatomical atlases, they called it the retrotrapezoid nucleus.