Hering-Breuer reflex

The Hering-Breuer reflex is a reflex triggered to prevent overinflation of the lungs. 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 paired vagus nerves to the apneustic center of the pons, inhibiting the inspiratory neurons present there. This inhibits inspiration, allowing expiration to occur.

Anatomy and physiology
The neural circuit that controls the Hering-Breuer reflex involves several regions of the central nervous system, and both sensory and motor components of the vagus nerve. Increased sensory activity of the vagus nerve increases activity of the nucleus of the solitary tract (NTS), which itself sends inhibitory input to the nucleus ambiguus. The nucleus ambiguus, which sends motor fibers to the heart via the vagus nerve, is responsible for tonic inhibitory control of heart rate. Increased pulmonary stretch receptor activity, which leads to inhibition of the nucleus ambiguus, ultimately elevates the heart rate. Thus during inspiration, it is normal for the heart rate to increase.

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 human when tidal volume is more than 1 L, as when exercising.