Question

Describe how the central chemoreceptors act to regulate ventilation. Be sure to describe the integrating centers and sensors involved, their locations, and explain their roles in determining ventilation rate. Be sure to include any chemical equations (and enzymes!) that are necessary to explain ventilation regulation and explain how the mechanisms work. Then explain how the system responds when arterial blood pH is elevated, and explain how the response corrects arterial CO₂. Explain how the system responds when arterial blood pH is decreased, and explain how the response corrects arterial CO₂.

Describe how the peripheral chemoreceptors act to regulate ventilation. Be sure to name the sensors involved, their locations, and explain their roles. Be sure to include the molecules sensed, and any chemical equations (and enzymes!) that are necessary to explain their roles. Then explain how and when the system acts to increase ventilation. Explain how and when the system acts to decrease ventilation.

Answer 1

Central chemoreceptors are located on the ventrolateral medullary surface of the central nervous system. Chemoreceptors are stimulated by a change in the chemical composition of their immediate environment. There are different chemoreceptor throughout the body which help to control different processes like taste, smell and breathing. The process of linking chemosensory mechanisms to the central nervous mechanisms controlling breathing is called chemosensory transduction.

Our respiratory system is regulated by central and peripheral chemoreceptors. Both these chemoreceptors have different mechanisms but work together to help our bodies control the pH, partial pressure of oxygen (pO2) and partial pressure of carbon dioxide (pCO2) within our blood.

Peripheral and central respiratory chemoreceptors are ultimately responsible for maintenance of constant levels of arterial PO2, PCO2 and [H+]. They thus protect the brain from hypoxia and ensures that our breathing is always appropriate for metabolism. A common mediator of peripheral and central chemosensory transduction is purine nucleotide ATP . Glomus cells of the carotid body release ATP to activate chemoafferent fibres of the carotid sinus nerve ,in response to a decrease in PO2 (hypoxia). They then transmit this information to the brainstem respiratory centres. When there is an increase in PCO2/[H+] (hypercapnia) chemosensitive structures present on the ventral surface of the medulla oblongata rapidly release ATP, these ATP thus acts locally within the medullary respiratory network

Peripheral Chemoreceptors :

They are located in both the carotid body and the aortic body, as the arterial blood supply leaves the heart, these receptors detect large changes in pO2 . Effects of these chemoreceptors are relatively insensitive but almost instantaneous.If an abnormally low pO2 is detected, afferent impulses travel to the respiratory centres of the brainstem. There a number of responses coordinate which leads increase the pO2 again.

Central Chemoreceptors :

These are located in medulla oblongata of the brainstem, they are more sensitive and detect smaller changes in arterial pCO2. They constantly initiate negative feedback loops which act to control our respiratory system. An increase in pCO2 is the reason behind increase in ventilation. Thus more CO2 being blown off and hence pCO2 returns to normal.On the other hand a decrease in pCO2 leads to a decrease in ventilation. This causes m\ore CO2 to be retained in lungs and to return pCO2 to normal.

Central chemoreceptors detect the arterial pCO2 by detectiong changes in the pH of the Cerebral Spinal Fluid (CSF).

pH Control :

Central chemoreceptors are sensitive to increases in arterial carbon dioxide and decreases in arterial pH.

The pH of the CSF is given by the ratio of pCO2 : [HCO3–].

The pH of the CSF is inversely proportional to the arterial pCO2. Thus small drop in pCO2­ leads to an increase in pH of the CSF and thus stimulates the respiratory centres to decrease ventilation and vice versa.

If pCO2 levels stay abnormal over a substantial period of time, e.g. three days or more, some specialised cells within the blood brain barrier ,called choroid plexus cells, makes HCO3– ions enter CSF. In this way the system can be ‘reset’ to a different pCO2 by manipulation of the pH. _

In the chemoreceptor with the help of enzyme called carbonate dehydratase CO2 undergoes chemical reaction, turning CO2 into water and bicarbonate.

CO2 + H2O → H2CO3 → H+ + HCO3–

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