The Hering–Breuer inflation reflex, named for Josef Breuer

Josef Breuer

Josef Breuer was an Austrian physician whose works laid the foundation of psychoanalysis.Born in Vienna, his father, Leopold Breuer, taught religion in Vienna's Jewish community. Breuer's mother died when he was quite young, and he was raised by his maternal grandmother and educated by his father...

 and Ewald Hering

Ewald Hering

Karl Ewald Konstantin Hering was a German physiologist who did much research into color vision and spatial perception...

, is a reflex

Reflex

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 triggered to prevent over-inflation of the lungs. Pulmonary stretch receptors

Pulmonary stretch receptors

Pulmonary stretch receptors are mechanoreceptors found in the lungs.When the lung expands, the receptors imitate the Hering-Breuer reflex, which reduces the respiratory rate...

 present in the smooth muscle of the airways respond to excessive stretching of the lung during large inspiration

Inhalation

Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli....

s.

Once activated, they send action potential

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

s through large myelin

Myelin

Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

ated fibers of the paired vagus nerve

Vagus nerve

The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

s to the inspiratory area in the medulla and apneustic area  of the pons

Pons

The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...

. In response, the inspiratory area is inhibited directly and the apneustic area is inhibited from activating the inspiratory area. This inhibits inspiration, allowing expiration to occur.

The Hering–Breuer inflation reflex ought not be confused with the deflation reflex discovered by the same individuals, Hering and Breuer. The majority of this page discusses the inflation reflex; the deflation reflex is considered separately at the end.

History

Josef Breuer and Ewald Hering reported in 1868 that a maintained distention of the lungs of anesthetized animals decreased the frequency of the inspiratory effort or caused a transient apnea. The stimulus was therefore pulmonary inflation.

Anatomy and physiology

The neural circuit that controls the Hering–Breuer inflation reflex involves several regions of the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

, and both sensory and motor components of the vagus nerve.

Increased sensory activity of the pulmonary-stretch lung afferents (via the vagus nerve) results in inhibition of the central inspiratory drive and thus inhibition of inspiration and initiation of expiration. The lung afferents also send inhibitory projections to the cardiac vagal motor neurones (CVM) in the nucleus ambiguus (NA) and dorsal motor vagal nucleus (DMVN). The CVMs, which send motor fibers to the heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 via the vagus nerve, are responsible for tonic inhibitory control of heart rate

Heart rate

Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

. Thus, an increase in pulmonary stretch receptor activity leads to inhibition of the CVMs and an elevation of heart rate (tachycardia). This is a normal occurrence in healthy individuals and is known as sinus arrhythmia.

Rate and depth of breathing

Early physiologists believed the reflex played a major role in establishing the rate and depth of breathing in humans. While this may be true for most animals, it is not the case for most adult humans at rest. However, the reflex may determine breathing rate and depth in newborns and in adult humans when tidal volume is more than 1 L, as when exercising.

Hering–Breuer deflation reflex

The Hering–Breuer deflation reflex serves to shorten exhalation when the lung is deflated. It is initiated either by stimulation of stretch receptors or stimulation of proprioceptors activated by lung deflation. Like the inflation reflex, impulses from these receptors travel afferently via the vagus. Unlike the inflation reflex, the afferents terminate on inspiratory centers rather than the pontine apneustic center. These reflexes appear to play a more minor role in humans than in non-human mammals.