Hering-Breuer Reflex

What Is the Hering-Breuer Reflex?

The Hering-Breuer reflex is an automatic reflex that shortens a breath in once the lungs have stretched enough, so the lungs do not overinflate. It is one of the lung reflexes that feed information into the wider system described in control of breathing, and it is part of our respiratory system glossary.

The reflex is named after two physiologists working in Vienna, Ewald Hering and his assistant Josef Breuer, who described it in 1868. They showed that inflating an animal's lungs stopped its breathing movements, and that the effect disappeared when the vagus nerves were cut. This was one of the first examples of a feedback reflex found in physiology.

Explained simply

Think of filling a balloon with a sensor in its skin. When the balloon stretches enough, the sensor sends a message to stop blowing. In the lungs, the sensors are stretch receptors in the airway walls, the message travels up the vagus nerve, and the brain answers by ending the breath in.

Where Are the Receptors of the Hering-Breuer Reflex Located?

The receptors of the Hering-Breuer reflex are located in the smooth muscle of the airway walls, from the trachea through the bronchi to the bronchioles. They are called slowly adapting pulmonary stretch receptors.

Slowly adapting means they keep firing for as long as the airway stays stretched instead of falling silent after a moment. Their firing rate rises as the lungs fill, so the brainstem receives a running measure of lung volume during each breath in. They are different from the rapidly adapting, or irritant, receptors in the airway lining, which respond to dust, smoke and sudden changes and help drive the cough reflex.

Hering-Breuer Reflex Arc: Afferent and Efferent Pathways

The Hering-Breuer reflex arc runs from stretch receptors in the airways, along vagal afferent fibers to the medulla, and back out along the phrenic and intercostal efferent nerves to the breathing muscles. Each link of the arc has a fixed job.

1. Stimulus: the lungs and airways stretch during a breath in.
2. Receptor: slowly adapting stretch receptors in airway smooth muscle start firing faster.
3. Afferent pathway: large myelinated sensory fibers of the vagus nerve, cranial nerve X, carry the signal to the brainstem.
4. Integration center: the signal reaches the nucleus of the solitary tract in the medulla, home of the dorsal respiratory group, which works with the pontine respiratory group in the pons.
5. Efferent pathway: the inspiratory neurons are inhibited, so firing falls in the phrenic nerve and the intercostal nerves.
6. Effector and response: the diaphragm and the external intercostal muscles stop contracting, the breath in ends and passive exhalation begins.

Because a bigger stretch produces a stronger stop signal, the reflex is a classic negative feedback loop.

What Is the Function of the Hering-Breuer Reflex?

The function of the Hering-Breuer reflex is to protect the lungs from overinflation and to help set the depth and timing of each breath. It acts as an off-switch for inspiration that is triggered by lung volume rather than by blood gases.

• Prevents overinflation: it stops a breath in before the lungs are stretched too far.
• Limits tidal volume: by cutting inhalation short, it keeps each breath, the tidal volume, within a safe range.
• Sets the rhythm: shorter breaths in allow breaths to follow each other faster, so the reflex tends to raise the respiratory rate.
• Prolongs exhalation: continued stretch at the end of a breath in delays the start of the next one.

How does it affect ventilation?

The Hering-Breuer reflex mainly changes the pattern of ventilation rather than its total amount. It trades depth for rate, giving shallower and more frequent breaths, while the overall level of breathing is set by the respiratory chemoreceptors that sense carbon dioxide, oxygen and acidity in the blood. In experiments, animals whose vagus nerves were cut breathed more slowly and much more deeply, showing what breathing looks like without the reflex.

Hering-Breuer Inflation vs Deflation Reflex

The Hering-Breuer inflation reflex ends a breath in when the lungs are well inflated, while the Hering-Breuer deflation reflex shortens a breath out when the lungs become unusually small. Most textbooks mean the inflation reflex when they say Hering-Breuer reflex.

The deflation reflex works in the opposite direction: strong deflation of the lungs speeds up the start of the next breath in. It is thought to involve rapidly adapting receptors as well as the drop in firing of stretch receptors, and it may help keep the lungs from sinking to too low a volume at the end of each exhalation.

Head's paradoxical reflex

A third, related reflex was described by the British physiologist Henry Head. In Head's paradoxical reflex, a quick inflation of the lungs makes the subject take an even deeper breath instead of stopping. It is seen mainly in newborns, and it is thought to help with the first deep breaths that open the lungs after birth and with the occasional deep sighs that reopen small airspaces.

How Does the Hering-Breuer Reflex Differ in Infants and Adults?

The Hering-Breuer reflex is strong in newborns and young infants, where it helps regulate every breath, but in adults it is weak during quiet breathing and becomes active mainly when breaths are much larger than usual. This change with age is one of its best-known features.

In newborns, the reflex helps control the size and timing of breaths and is thought to help hold the lungs at a stable volume between breaths, which matters because a baby's chest wall is soft. In adults, a normal resting breath does not stretch the lungs enough to trigger much of a response. During heavy exercise, when breaths become deep, the reflex is thought to help limit tidal volume, as covered in lung volumes and capacities.

Evidence from people who have received a lung transplant supports this view. Their new lungs have no vagal connection to the brainstem, yet their resting breathing pattern is close to normal, which suggests that adults depend little on the reflex at rest.

Common Conditions That Affect the Hering-Breuer Reflex

Conditions that damage the vagus nerve, change how stiff the lungs are or remove their nerve supply can alter the Hering-Breuer reflex and the other vagal lung reflexes. These changes affect the pattern of breathing more than its overall control.

• Lung transplantation: the transplanted lung is cut off from its vagal nerve supply, so stretch signals from it no longer reach the brainstem.
• Stiff lungs: in conditions such as pulmonary fibrosis, rapid, shallow breathing is common, and stretch receptors together with other vagal receptors are thought to contribute.
• Overinflated lungs: in emphysema and COPD, air trapping leaves the lungs larger than normal, which changes the signals that stretch receptors send.
• Newborn breathing: because the reflex is strong early in life, it is studied in the breathing patterns of premature and newborn babies.

Different breathing patterns and wider respiratory diseases are covered on their own pages.