Central Chemoreceptors

Overview
  • The Central Chemoreceptors are an anatomical collection of neuronal chemoreceptors located just beneath the ventral surface of the brainstem's medulla, a few hundred microns away from the brainstem respiratory centers. The central chemoreceptors are critical sensors of arterial carbon dioxide and are the key sensory component of a negative feedback loop which controls respiratory activity in an attempt to maintain relatively constant levels of arterial carbon dioxide as described in integrated respiratory control.
Sensory Mechanism
  • Experimental studies have demonstrated that the central chemoreceptors are most directly sensitive to changes in their surrounding extracellular fluid pH, which given their anatomical location would mean the CSF. However, the blood brain barrier is relatively impermeable to hydrogen and bicarbonate ions; consequently, the central chemoreceptors cannot respond quickly to changes in the blood pH. Although blood pH is not closely linked to the CSF pH, the partial pressure of arterial carbon dioxide displays a tight relationship with the CSF pH through a notable mechanism. Dissolved CO2 within the arterial blood can readily diffuse through the blood brain barrier into the CSF where it is converted by carbonic anhydrase into carbonic acid (H2CO3).
  • Carbonic Acid spontaneously releases a free hydrogen ion which reduces the pH of CSF. Consequently, changes in the arterial partial pressures of CO2 indirectly modulate the pH of the CSF through CO2 diffusion and conversion to carbonic acid. When arterial carbon dioxide increases above normal, CSF pH decreases and the central chemoreceptors send stimuli above their basal rate. Conversely, when arterial carbon dioxide decreases below normal, CSF pH increases and the central chemoreceptors reduce their activity below their basal rate.


Mechanism of Central Chemoreceptor Sensitivity to Arterial CO2 Tension
The central chemoreceptors do not directly detect the arterial CO2 tension. Instead, they detect decreases in the CSF pH. These two values are connected because arterial CO2 diffuses past the blood brain barrier, into the CSF, and is converted by carbonic anhydrase to carbonic acid that in turn decreases the CSF pH.

Effects
  • The central chemoreceptors send signals to the brainstem respiratory centers, especially the Inspiratory Center, and thus aid in control of respiration by modifying the respiratory drive. When CSF pH lowers, due to increased arterial carbon dioxide partial pressures, stimuli sent by the central chemoreceptors activates the Inspiratory Center to increase the respiratory drive, thus enhancing alveolar ventilation and aiding in elimination of accumulated CO2. When CSF pH rises due to decreased arterial carbon dioxide partial pressures, decreased stimuli sent by the central chemoreceptors lowers basal stimulation of the Inspiratory Center, thus reducing respiratory drive which in turn reduces alveolar ventilation and pulmonary elimination of CO2.