Hering–Breuer reflex

The Hering–Breuer inflation reflex, named for Josef Breuer and Ewald Hering, is a reflex triggered to prevent over-inflation of the lung. Pulmonary stretch receptors present in the smooth muscle of the airways respond to excessive stretching of the lung during large inspirations.

Once activated, they send action potentials through large myelinated fibers of the vagus nerve to the inspiratory area in the medulla and apneustic center of the pons. 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.

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.

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

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