Control of Respiration

🫁 Control of Respiration

Respiratory centres, central vs peripheral chemoreceptors, the hypoxic drive in COPD, and the danger of high-flow oxygen — core respiratory physiology for MRCS.

Respiratory Control Centres

Breathing is controlled by an integrated network of neurones in the brainstem. Unlike cardiac automaticity which is entirely intrinsic, respiratory rhythm requires intact brainstem function. The centres generate the rhythmic pattern of inspiration and expiration, modulated by chemical signals, mechanical reflexes, and higher cortical inputs.

Brainstem Respiratory Centres

Centre	Location	Function	Clinical Relevance
Pre-Bötzinger Complex	Medulla oblongata (ventrolateral)	Pacemaker for respiratory rhythm — generates the basic inspiratory drive. Contains autonomous pacemaker neurones with intrinsic bursting activity.	Essential for normal breathing. Damage → apnoea. Opioids suppress this centre → respiratory depression. Critical target of sedative/anaesthetic agents.
Dorsal Respiratory Group (DRG)	Medulla (nucleus tractus solitarius)	Predominantly inspiratory. Integrates afferent signals from peripheral chemoreceptors (CN IX/X), lung stretch receptors, and baroreceptors. Drives phrenic nerve → diaphragm.	Integration centre for chemical and mechanical control of breathing. Receives input from carotid/aortic bodies via CN IX/X.
Ventral Respiratory Group (VRG)	Medulla (nucleus ambiguus + retroambiguous)	Active in both inspiration and expiration. Inactive during normal quiet breathing — recruited during exercise and increased ventilatory demand. Controls intercostal and abdominal muscles.	Provides additional ventilatory drive during exercise, hypercapnia, hypoxia. Botzinger complex (expiratory) and pre-Bötzinger (inspiratory) are sub-groups.
Pneumotaxic centre	Upper pons (nucleus parabrachialis)	Inhibits inspiration — switches off the inspiratory centre, limiting the depth of each breath and increasing respiratory frequency. Fine-tunes respiratory pattern.	Damage → apneustic breathing (prolonged inspiratory gasps). Over-rides the apneustic centre.
Apneustic centre	Lower pons	Promotes inspiration — prolongs inspiratory phase. Normally overridden by the pneumotaxic centre and Hering-Breuer stretch reflex.	If pneumotaxic centre damaged + vagus nerves cut → apneustic breathing. Rarely clinically significant alone.

Hering-Breuer Inflation Reflex

Hering-Breuer Reflex — Mechanical Protection

Pulmonary stretch receptors (slow-adapting receptors in airway smooth muscle) respond to lung inflation → send signals via the vagus nerve (CN X) to the DRG → inhibit further inspiration → prevent over-inflation. This is the Hering-Breuer inflation reflex. Deflation reflex (paradoxical reflex) → promotes inspiration when lungs are deflated. In adults: operates mainly at large tidal volumes (>800 mL) and during anaesthesia/mechanical ventilation. In neonates: active at normal tidal volumes (important in controlling breathing pattern). Clinical: over-inflation of anaesthetic breathing circuits can trigger reflex apnoea.

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