Describe the role of peripheral chemoreceptors in the regulation of respiration.

Outline:

·        Anatomical structure of the peripheral chemoreceptors

·        Mechanism of stimulation

·        Effects of oxygen and carbon dioxide on activity of chemoreceptors

Essay:

            The ultimate goal of respiration is to maintain proper concentrations of oxygen, carbon dioxide, and hydrogen ions in the tissue. The center of control for respiration lies in the respiratory center of the brain and the peripheral chemoreceptors located in areas outside the brain.

            The peripheral chemoreceptors adjust ventilation in response to changes in blood levels of oxygen concentration and they are located in the carotid and aortic bodies which also contain the baroreceptors involved in the reflex control of blood pressure. The chemoreceptors transmit nervous signals to the respiratory center in the brain to help regulate respiratory activity.

            There is a carotid body near the carotid bifurcation on each side, and there are usually 2 or more aortic bodies near the arch of the aorta. Each carotid and aortic body (glomus) contains islands of 2 types of cells, type I and type II cells, surrounded by fenestrated sinusoidal capillaries. The type I or glomus cells are closely associated with cuplike endings of the afferent nerves. They have dense core granules containing catecholamines that are released upon exposure to hypoxia and cyanide.

            The glomus cells contain gated potassium channels that are oxygen sensitive. An oxygen sensor on the extracellular fluid side of the cell membrane is associated with the channel. When the sensor is combined with oxygen, the channel stays open and potassium ions leave the cell, hyperpolarizing it. If oxygen levels in the blood decrease, fewer of the sensors are combined with oxygen and more the potassium channels close. The resultant decrease in potassium permeability depolarizes the cells, causing them to fire repetitive action potentials, which open voltage-gated calcium channels. The entry of calcium triggers the release of the neurotransmitter dopamine in the axon terminal which initiates action potentials in sensory neurons leading to the central nervous system, signaling the respiratory control centers to increase ventilation.

            Under most circumstances, oxygen is not an important factor in modulating ventilation. Arterial PO₂ must drop below 60 mmHg before ventilation is stimulated. The chemoreceptors do not play a role in the everyday regulation of ventilation as arterial PO₂ seldom drops below 60 mmHg except under high altitude and pathologic conditions. An increase in either carbon dioxide concentration or hydrogen ion concentration also excites the chemoreceptors and increases respiratory activity.

            A low oxygen level (below 60 mmHg) is detected by the chemoreceptors in the carotid and aortic bodies and they increase their impulse firing rate to the dorsal respiratory area through the glossopharyngeal nerves and the vagi.