Pulmonary Stretch Reflexes
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Direct answer
Inflate a lung beyond about 1-1.5 litres tidal volume in an adult and stretch receptors in the airway smooth muscle — slowly adapting receptors (SARs) — fire through the vagus to the medulla, switching off inspiration before the lung overinflates: the Hering-Breuer inflation reflex. In quietly breathing adults this reflex is dormant (it guards against large volumes, not normal 500 mL breaths), but it is tonically active in neonates and infants, where it helps maintain functional residual capacity. Its mirror, the deflation reflex, fires when the lung collapses and reinforces inspiration. Beside them sit three more vagal populations worth full marks: rapidly adapting irritant receptors (cough, bronchoconstriction), C-fibre bronchial receptors, and the juxtacapillary (J) receptors in the alveolar wall, which are stimulated by pulmonary congestion and oedema and drive the rapid shallow breathing and dyspnoea of left heart failure.
What you must remember
- Hering-Breuer inflation reflex: SARs in airway smooth muscle, vagal afferents, medullary integration, apneustic inhibition — a volume-protective brake that in adults engages only when tidal volume exceeds roughly 1-1.5 litres.
- Developmental point: the reflex is active at normal tidal volumes in neonates and infants, contributing to the control of breathing and to functional residual capacity maintenance — a favourite one-liner in paediatric vivas.
- Deflation reflex: lung collapse excites receptors that augment inspiratory effort — the reflex defending against atelectasis, also vagally mediated.
- Slowly adapting receptor portfolio: besides the inflation brake, SARs reflexly dilate airways, speed the inspiratory-expiratory switch and tachycardise the heart — airway-to-vascular crosstalk examiners occasionally probe.
- J receptors (juxtacapillary): unmyelinated C fibres in alveolar walls interstitial to capillaries; stimulated by congestion, oedema, emboli and exercise-induced interstitial distension — producing rapid shallow breathing, dyspnoea and the bradycardia-hypotension components described by Paintal.
- Irritant receptors (rapidly adapting): between epithelial cells of larger airways; triggered by dust, smoke, cold air and histamine — cough, bronchoconstriction, augmented breathing; hyperpnoea of asthma attacks is partly this reflex.
- Apneusis experiment: sectioning the vagus plus an upper pontine lesion in animals produces apneustic (prolonged inspiratory) breathing — the classical demonstration that vagal stretch input and the pontine pneumotaxic centre jointly terminate inspiration.
A worked case from heart failure to the reflex arc
A patient with acute left ventricular failure becomes breathless with rapid, shallow breaths. The physiology runs through the J receptors: rising left atrial pressure backs into the pulmonary capillaries, interstitial fluid engorges the alveolar walls, J-receptor C fibres fire, and the medulla receives a false alarm of lung congestion — which is precisely true. Breathing turns rapid and shallow (a pattern that minimises elastic work on a stiff, congested lung), and the sensation is the air hunger of orthopnoea. Treat the preload (diuretics, nitrates) and the reflex falls silent because its stimulus — interstitial distension — has gone.
Contrast the asthma emergency: inhaled allergens and histamine excite rapidly adapting irritant receptors in the bronchi, provoking cough, reflex bronchoconstriction and hyperpnoea; the wheeze is pharmacological (muscle) plus reflex (neural) at once, which is why anticholinergic ipratropium — blocking the reflex vagal bronchoconstriction — adds to beta-agonist therapy. Finally, the preterm neonate with apnoea of prematurity: an immature brainstem plus active laryngeal chemoreflexes makes the breathing pattern unstable — the bridge examiners build to neonatology.
Where students slip
The standard error is claiming the Hering-Breuer reflex governs normal adult breathing — it does not; in adults it is a reserve reflex for high lung volumes, demonstrated experimentally by inflating the lung with volumes above about 1 litre, and its normal-tidal activity belongs to neonates. The second slip is naming J receptors "juxta-alveolar": they are juxtacapillary, sitting in the alveolar interstitium near capillaries, and their stimulus is interstitial distension, not alveolar gas pressure. Third, students forget all these afferents run in the vagus — a single-nerve answer that examiners accept for the whole topic, and the reason bilateral vagal section in animal experiments prolongs inspiration into apneusis.
Frequently asked questions
What is the Hering-Breuer inflation reflex?
Stretch receptor (slowly adapting) discharge from airway smooth muscle travels through the vagus to inhibit medullary inspiration, preventing overinflation — active at normal tidal volumes in neonates, but only above roughly 1-1.5 litres in adults.
Where are J receptors located and what stimulates them?
In the alveolar interstitium juxtacapillary position; pulmonary congestion, oedema, microemboli and exercise distension stimulate them, producing rapid shallow breathing, dyspnoea and hypotension.
Which receptors mediate the cough and bronchoconstriction response to inhaled irritants?
Rapidly adapting irritant receptors between airway epithelial cells, vagally mediated, triggered by dust, smoke, cold air and histamine — responsible for cough, reflex bronchoconstriction and hyperpnoea in asthma.
Why is the Hering-Breuer reflex important in neonates?
It operates tonically at normal tidal volumes in infants, contributing to ventilatory rhythm and maintenance of functional residual capacity, whereas in adults it only guards against very large inflations.
What breathing pattern results from vagal section with an upper pontine lesion?
Apneusis — prolonged inspiratory gasps — because both the vagal stretch input and the pneumotaxic centre's inspiratory off-switch are removed, demonstrating the dual termination of inspiration.