7th February 2024

Receptors

Receptors responsible for the control of breathing can be categorised into:

1. Central chemoreceptors
2. Peripheral chemoreceptors
3. Other receptors

Central chemoreceptors

Central chemoreceptors are in the anterolateral surface of the medulla within the blood-brain barrier. It directly detects changes in hydrogen concentration (and, in effect, the changes in pH) in the cerebrospinal fluid (CSF). Indirectly, central chemoreceptors detect changes to blood CO2 concentration.

CO2 in the blood can pass the blood-brain barrier. After passing the blood-brain barrier into CSF, it reacts to eventually produce hydrogen ions (H+) and bicarbonate ions (HCO3-). H+ diffuses into chemoreceptor cells, directly stimulating chemoreceptor cells. Stimulation of chemoreceptor cells will then activate the respiratory centre.

H+ ions in the blood cannot pass the blood-brain barrier. Hence, respiratory acidosis will stimulate central chemoreceptors, and metabolic acidosis will not.

Compensation in hypercapnia – long-standing hypercapnia will result in compensation in the CSF and intravascular space. Long-standing hypercapnia will result in HCO3- being absorbed into CSF space to normalise pH and reabsorption of HCO3- by the kidneys.

Peripheral chemoreceptors

Peripheral chemoreceptors are in the carotid bodies and aortic bodies. Both carotid and aortic bodies respond to changes in partial pressure of carbon dioxide (PaCO2)and partial pressure of oxygen (PaO2) [not oxygen content in the blood]. However, peripheral chemoreceptors are only responsible for 20% of the body’s response to high PaCO2. Central chemoreceptors are responsible for the rest.

Carotid chemoreceptors are also stimulated by increased blood hydrogen centration (decrease in blood pH). This is the reason for hyperventilation in metabolic acidosis.

Carotid bodies send afferent signals to the respiratory centre via the glossopharyngeal nerve (CN IX)

Aortic bodies send afferent signals to the respiratory centre via the vagus nerve (CN X)

Other receptors

• Lung stretch receptors – located in bronchial smooth muscles, part of the Hering-Bruer reflex. The Hering-Bruer reflex is a reflex where overinflation of the lung (i.e. during positive pressure ventilation) causes stimulation of lung stretch receptors. Lung stretch receptors send afferent signals via the vagus nerve (CN X) to the apneustic centre, causing a reduction in the depth of breathing.
• Juxtacapillary receptors (J receptors) – These are unmyelinated C-fibres in the alveolar wall. Stimulation of these receptors leads to increased ventilation, dyspnoea, bradycardia and hypotension. These receptors are thought to be stimulated in pulmonary oedema and pulmonary embolism.
• Laryngeal epithelial irritant receptors – irritants (i.e. noxious gas) stimulate receptors, causing bronchoconstriction and increasing ventilation.