The Hering–Breuer inflation reflex, named for Josef Breuer and

Ewald Hering Karl Ewald Konstantin Hering (5 August 1834 – 26 January 1918) was a German physiologist who did much research into color vision, binocular perception and eye movements. He proposed opponent color theory in 1892. Born in Alt-Gersdorf, Ki ...

, is a reflex triggered to prevent the over-inflation of the lung.

Pulmonary stretch receptors Pulmonary stretch receptors are mechanoreceptors found in the lungs. When the lung expands, the receptors initiate the Hering-Breuer reflex, which reduces the respiratory rate. This signal is transmitted by vagus nerve. Increased firing from the ...

present on the wall of bronchi and bronchioles of the airways respond to excessive stretching of the lung during large inspirations. Once activated, they send

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

s through large

myelin Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be ...

ated fibers of the

vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...

to the inspiratory area in the medulla and apneustic center of the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...

. In response, the inspiratory area is inhibited directly and the apneustic center is inhibited from activating the inspiratory area. This inhibits inspiration, allowing expiration to occur. The Hering–Breuer inflation reflex should 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 Hering-Breuer reflex, put simply, is what keeps the lungs from over-inflating with inspired air. The neural circuit that controls the Hering–Breuer inflation reflex involves several regions of the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

, and both sensory and motor components of the

. 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 The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...

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

heart rate Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...

. Thus, an increase in pulmonary stretch receptor activity leads to inhibition of the CVMs and an elevation of heart rate (

tachycardia Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal ( ...

). This is a normal occurrence in healthy individuals and is known as sinus arrhythmia.

Rate and depth of breathing

Early physiologists believed the reflex plays 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 Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass. Mechanical vent ...

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.

Clinical use

The absence of this reflex contributes to the diagnosis of brain death.

References

External links

* {{DEFAULTSORT:Hering-Breuer reflex Respiratory physiology Reflexes Vagus nerve