Chemical regulation and chemoreceptors
The leading factor stimulating the activity of the respiratory center is carbon dioxide ($CO_2$). The classic cross-circulation experiment by Fredericq in dogs proved that if one animal experiences asphyxia, the second animal develops dyspnea due to changes in the gas composition of the shared blood. The mechanism of $CO_2$ action on the brain is specific: hydrogen ions ($H^+$) themselves cross the blood-brain barrier with difficulty, but carbon dioxide crosses it easily. In the extracellular fluid of the brainstem, it locally generates hydrogen ions, which are the true active principle.
Chemoreceptors are specific and divided into two groups:
- Central chemoreceptors: located on the ventral surface of the medulla oblongata (medulla oblongata) at a depth of about 0.2 mm, as well as in the reticular formation. They respond predominantly to shifts in pH and $pCO_2$ in the cerebrospinal fluid (CSF).
- Peripheral chemoreceptors: located in the aortic arch and the sinocarotid zone (the bifurcation area of the common carotid artery). The carotid bodies are richly vascularized and innervated by the sinus nerve (a branch of the glossopharyngeal nerve — n. glossopharyngeus). These receptors respond primarily to a decrease in oxygen levels. When $pO_2$ drops below 70–60 mm Hg, their firing frequency increases sharply.
Airway receptors
Neurons of the respiratory center receive constant afferent information from mechanoreceptors, which are divided into three main types:
- Slowly adapting stretch receptors: located in the smooth muscles of the trachea and bronchi, activated during inspiration.
- Irritant (rapidly adapting) receptors: located in the mucous membrane of the airways; their stimulus is sharp changes in lung volume.
- Juxtacapillary (J-receptors): located in the pulmonary interstitium near the capillaries, activated when pressure in the pulmonary circulation increases.
Neuronal organization and phase switching of respiration
The mechanism of respiratory phase switching is based on the complex interaction of four pools of brainstem neurons:
- Pool I ("early" inspiratory neurons): receive signals from chemoreceptors, trigger inspiration, activate respiratory muscles and Pool II.
- Pool II ("late" inspiratory neurons): activated by Pool I, collect afferentation from pulmonary mechanoreceptors and transmit excitation to the third pool.
- Pool III: exerts an inhibitory effect on Pool I, providing the mechanism for switching off inspiration.
- Pool IV (pneumotaxic center): localized in the pons (pons), receives signals from the cerebral cortex and modulates the activity of all respiratory neurons.
Mechanism of inspiration termination and reflex arc
As the lungs fill with air, alveolar stretch receptors are excited. Impulses from them travel along the afferent fibers of the vagus nerve (n. vagus). This forms the basis of the Hering–Breuer reflex: lung inflation causes reflex inhibition of inspiration.
The process depends on the firing frequency. When the frequency in a single fiber reaches 80–100 imp./s, it becomes pessimal for inspiratory neurons. Pessimal inhibition develops: inspiration stops, and passive expiration ensues. If forced breathing is necessary, active expiration is engaged via expiratory neurons of the ventral region of the medulla oblongata.
In general, the effector link of this system includes the phrenic nerve (n. phrenicus), which controls the diaphragm (diaphragma), and intercostal nerves (nn. intercostales) innervating the intercostal muscles (mm. intercostales).
Briefly: answers to topical questions
What functions do the pneumotaxic and apneustic centers of the pons perform?
The centers of the pons (pons) perform functions of regulating respiratory rhythm, automatism, and inspiration stimulation.
- Pneumotaxic center regulates the switching of respiratory phases (transition from inspiration to expiration), modulates the activity of all respiratory neurons of the medulla oblongata, and helps maintain automatism. This center receives impulses from the cerebral cortex.
- Apneustic center is located in the lower part of the pons and stimulates inspiration.
How does the cerebral cortex perform voluntary regulation of respiration?
The cerebral cortex (cortex) exerts voluntary control over breathing by sending impulses from the motor areas of the cortex to spinal motoneurons. The impulses pass through the pyramidal tract. An important feature of this mechanism is that the signal bypasses the bulbar centers of respiratory automatism. This provides voluntary control of breathing, such as breath-holding, as well as the participation of breathing in speech and conditioned reflex regulation.
To memorize the work of respiratory pools: "Pool one begins, two continues, three cuts off (inhibits) inspiration, and four from the pons manages everyone."
Class and exam questions
What is the essence and significance of Fredericq's experiment?
In establishing cross-circulation in two dogs. The experiment proved the humoral regulation of respiration: asphyxia of one animal caused dyspnea in the second due to changes in the gas composition of the cross-circulating blood.
How does carbon dioxide affect central chemoreceptors?
Carbon dioxide itself easily crosses the blood-brain barrier into the brainstem. There, in the extracellular fluid, it generates hydrogen ions ($H^+$), which locally excite chemoreceptors.
What role does the Hering–Breuer reflex play?
During lung inflation, signals from mechanoreceptors travel along the vagus nerve and inhibit inspiratory neurons. During forced breathing, this reflex protects the lung tissue from excessive overexpansion.
What else to study on this topic
- Dependence of peripheral chemoreceptor activity on oxygen tension
- Histological structure of type I and type II cells in carotid bodies
- Effect of the sympathetic and parasympathetic nervous systems on receptor sensitivity
- Mechanisms of cerebral cortex participation in voluntary breathing control
- Functional differences between the dorsal and ventral regions of the medulla oblongata
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