Hering-Breuer reflex
Summary
The Hering-Breuer (inflation) reflex is a protective vagally mediated reflex in which lung stretch receptors (slowly adapting pulmonary stretch receptors in airway smooth muscle) sense large tidal volumes and signal via the vagus nerve to the medullary respiratory centers to terminate inspiration and prolong expiration. It prevents overinflation of the lungs and slows respiratory rate. In adult humans it is mainly active at high tidal volumes (>~800 mL, e.g., exercise), but it is more prominent in neonates.
Detail
**Mechanism (afferent → center → efferent):** - Stimulus: Lung inflation/distension, which stretches the airways (bronchi and bronchioles). - Receptors: Slowly adapting pulmonary stretch receptors (SARs) located in smooth muscle of the airways. They are mechanoreceptors and continue firing as long as the lung is inflated. - Afferent: Large myelinated fibers of the vagus nerve (CN X). - Integration: Medulla. Signals go to the dorsal respiratory group (nucleus tractus solitarius) and the pontine respiratory group (pneumotaxic center/pre-Bötzinger region interplay). This inhibits the inspiratory neurons (the apneustic center drive is inhibited). - Efferent: Phrenic nerve and intercostal motor neurons are inhibited, so inspiration ends and expiration begins. - Net effect: Shorter inspiratory time, longer expiratory time, and decreased respiratory rate. This is a negative feedback loop.
**Deflation reflex (Hering-Breuer deflation reflex):** Lung deflation or collapse decreases stretch receptor firing, which promotes inspiration. This is less well characterized than the inflation reflex.
**Clinical significance and associations:** - Vagotomy (bilateral) removes the reflex, causing slow, deep breathing (increased tidal volume, decreased rate). If the pontine pneumotaxic center is also lesioned, the result is apneusis (prolonged inspiratory gasps with brief expiration). - Neonates and infants have a more active reflex. It contributes to rhythmic breathing and is implicated in apnea of prematurity and in the response to ventilation. It is also relevant to ventilator weaning and to mechanical ventilation strategies (preventing overdistension). - In adult humans at rest, the reflex is weak. It is thought to be recruited at tidal volumes above ~800 mL (about 3x resting tidal volume), such as in exercise. - Do not confuse it with the **Head's paradoxical reflex** (inflation causes further inspiration, seen in neonates), the **cough/irritant reflex** (rapidly adapting irritant receptors), or **J-receptor (juxtacapillary) reflex** (pulmonary edema/capillary engorgement causes rapid shallow breathing, dyspnea, and apnea). - Do not confuse it with the **Bainbridge reflex**, which is cardiac (atrial stretch increases heart rate), or the **Bezold-Jarisch reflex** (cardioinhibitory chemoreflex).
**Respiratory control summary (high-yield):** - Dorsal respiratory group (DRG; NTS): inspiration; receives input from CN IX and X. - Ventral respiratory group (VRG): expiration (active) and forced breathing. - Pneumotaxic center (upper pons; nucleus parabrachialis/Kölliker-Fuse): limits inspiration and increases respiratory rate. - Apneustic center (lower pons): prolongs inspiration. - Pre-Bötzinger complex: rhythm generator.
**Board pearl:** Stretch of the lung → vagal afferents → inhibits inspiration (negative feedback). Bilateral vagotomy → slow, deep breaths.
Sources
- Guyton and Hall Textbook of Medical Physiology (Regulation of Respiration chapter)
- West's Respiratory Physiology: The Essentials
- Costanzo Physiology (Respiratory Physiology: Control of Breathing)
- First Aid for the USMLE Step 1 (Respiratory – Physiology)
- Boron & Boulpaep Medical Physiology (Control of Ventilation)
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