Medullary dorsal and ventral groups, with pontine pneumotaxic and apneustic centres
Mnemonic
Three centres, two in the pons and one in the medulla:
- Medullary respiratory centre - the dorsal group drives inspiration and sets the basic rhythm; the ventral group is largely silent at rest and drives forced expiration
- Apneustic centre, lower pons - prolongs inspiration
- Pneumotaxic centre, upper pons - limits inspiration, so it sets the rate
“Apneustic prolongs, pneumotaxic stops.”
Chemoreceptors supply the input:
- Central, in the medulla, respond to CSF pH, driven by carbon dioxide, and provide about 80 per cent of the normal drive
- Peripheral, in the carotid and aortic bodies, respond to hypoxia below about 8 kPa, and also to carbon dioxide and acid. The carotid body signals via IX, the aortic via X
“Carbon dioxide drives you centrally, hypoxia drives you peripherally.” That is why chronic carbon dioxide retention blunts the central drive and leaves the patient dependent on hypoxic drive, and it is the physiological basis of target saturations of 88 to 92 per cent in that group.
Expansion
Medulla - generates the basic rhythm
- Dorsal respiratory group - mainly inspiratory; receives vagal and glossopharyngeal afferents
- Ventral respiratory group - both inspiratory and expiratory; active mainly in forced breathing
- Pre-Botzinger complex - the likely site of rhythm generation
Pons - modulates the rhythm
- Pneumotaxic centre (upper pons) - switches off inspiration, so it limits tidal volume and increases rate
- Apneustic centre (lower pons) - prolongs inspiration
Because the rhythm is medullary, a lesion above the pons leaves breathing intact, whereas medullary damage is rapidly fatal.
Voluntary control comes from the cerebral cortex via corticospinal tracts, bypassing the brainstem entirely. This is why breath-holding is possible, and why a patient with Ondine’s curse loses automatic breathing during sleep while retaining voluntary breathing when awake.