Control of breathing

Vivian Imbriotis | Sept. 23, 2025

Physiological factors that affect breathing include PaCO2 (most potently), PaO2, pH, temperature and fever, fear, pain, anxiety, and stimulation of pulmonary nerve fibres (chemical irritation, pulmonary oedema).

  1. Aortic Arch chemorecetors. Respond to low oxygen content, high PaCO2, low arterial pH, high temperature. Via vagus nerve.
  2. Carotid body chemoreceptors. Respond to low PaO2, high PaCO2, high temperature. Via glossopharyngeal nerve.
  3. Central chemoreceptors in medulla. Respond to local (CSF) pH. As the BBB is impermeable to most acids, but permeable to CO2, and has minimal protein buffering, this is a very sensitive barometer of PaCO2. Via medullary interneurons.
  4. J fibers and other pulmonary sensory nerves. Respond to chemical and physical irritants, especially pulmonary oedema. Via vagus nerve
  5. Cerebral cortex and thalamus, responds to fever, anxiety, pain, fear, and voluntary control. Via white matter tracts.

Respiratory control center in medulla, composed of a collection of neuron groups:


  1. Dorsal respiratory group: upper motor neuron to contralateral inspiratory muscles
  2. Ventral respiratory group: upper motor neurons to contralateral expiratory muscles
  3. Nucleus Ambiguus: Airway dilator muscles via viagus nerve \(\to\) larynx, pharynx
  4. Pre-Botzinger Complex: central pattern generator
  5. Pontine respiratory group: integrates cortical and thalamic inputs


$$\require{AMScd}$$

$$\begin{CD} @. @. {Cortex} @. @. \\ @. @. @VVV @. @.\\ {Thalamus} @>>> {Pontine Resp group} @. @. \\ @. @VVV @. \\ {CNX} @>>> {Pre-Botzinger Complex (CPG)} @. @. \\ @. @VVV @. \\ {DRG} @<<< {Interneurons} @>>> {VRG} \\ @VVV @VVV @VVV \\ {Inspiratory mm} @. {N Ambiguus} @. {Expiratory mm} \\ @. @VVV @. \\ @. @. {Airway dilators} @. @.$$

Muscles of inspiration

Phrenic nerves \(\to\) diaphragm \(\to\) increased vertical + lateral + AP dimensions of chest \(\to\) responsible for 75% of ventilation during TV breathing

Spinal nerves \(\to\) external intercostals \(\to\) increased lateral and AP dimensions of chest

Cervical plexus \(\to\) scalenes, SCM, pectoralis major \(\to\) elevention of shoulder girdle, first 2 ribs, sternum


Muscles of expiration

Thoracoabdominal nerves \(\to\) rectus, obliques, transversus \(\to\) increased IAP \(\to\) increased intrathoracic pressure

Spinal nerves \(\to\) internal intercostals \(\to\) decreased AP and lateral dimension of chest

CO2 response curve in linear, \(MV \propto P_aCO_2\)



O2 response curve is flat for PaO2 > 80, then sharply rises at a PO2 of 60