6.1 Central chemoreceptors

Changes in PCO₂, and therefore in pH, are detected largely by chemoreceptors within the respiratory centres of the brain (Figure 16). During increased metabolic activity, such as exercise, the PCO₂ in the arterial blood increases.

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Figure 16 Neurons in the pontine and medullary respiratory centres of the brain synapse onto the diaphragm and intercostal muscles to regulate breathing rate.

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On the left a drawing of the ribcage with the diaphragm within the base of the cage, and intercostal muscles between the ribs.

On the right a diagram of part of the hind-brain (pink) with the pons at the top and the medulla oblongata below, leading down to the spinal cord. Within the pons is a yellow dot, the pontine respiratory group; at the top of the medulla is a red dot, the dorsal respiratory group, and a blue dot, the ventral respiratory group; the dorsal and ventral respiratory groups form the medullary respiratory centre.

A black line (representing neuronal links) from the pontine respiratory group to the dorsal respiratory group and thence to a synapse at the base of the medulla oblongata links to a dark blue line to the diaphragm from the lower end of the medulla oblongata; this represents the phrenic nerve to the diaphragm.

A black line from the ventral respiratory group to synapse at the base of the medulla oblongata links to a light blue line (intercostal nerves) to internal intercostal muscles (involved in expiration), and to a green line (intercostal nerves) to external intercostal muscles (involved in inspiration).

Figure 16 Neurons in the pontine and medullary respiratory centres of the brain synapse onto the diaphragm and intercostal muscles to ...

As CO₂-rich blood reaches the brain, CO₂ diffuses across the blood–brain barrier into the interstitial fluid and cerebrospinal fluid that surrounds the medulla.

Neurons within the medullary and pontine respiratory centres will fire action potentials in response to the change in pH, via activation of receptors that are sensitive to protons (Guyenet and Bayliss, 2015). These neurons synapse onto the phrenic and intercostal nerves which innervate the diaphragm and intercostal muscles (see Section 1.2) and stimulate increased breathing (Figure 16).

As the pH returns to homeostatic levels, the chemoreceptors stop being activated and the breathing rate returns to normal. Therefore, the respiratory centres act as the ‘pacemakers’ of respiration during both resting and stimulated conditions, via communication with the muscles that control the expansion and contraction of the lungs (McKay et al., 2003). Fine-tuning of the breathing pattern is controlled by inputs from the pontine respiratory group (Figure 16). Information from stretch receptors in the lungs is also used by the respiratory centres to determine when the lungs have expanded to full capacity.

Some neurodegenerative diseases, such as motor neurone disease, are characterised by respiratory problems that are caused by the gradual loss of innervation to the diaphragm and intercostal muscles, despite the fact that the respiratory centres are intact. In other cases, when the respiratory centres of the medulla are damaged, individuals may require artificial ventilation of the lungs to regulate their breathing rate.