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The respiratory center is a complex group of nuclei located within the pons and medulla oblongata.Upon receiving this information, the dorsal respiratory group stimulates the phrenic nerve to contract the diaphragm, as well as the thoracic spinal nerves to contract the intercostal muscles.The majority of its neurons are found within the nucleus of the solitary tract, which receives information from the peripheral chemoreceptors about the blood oxygen saturation.However, when in need of increased pulmonary ventilation, the dorsal respiratory group stimulates the ventral group, which in turn stimulates the accessory respiratory muscles.The pontine pneumotaxic center lies within the parabrachial nucleus of the rostral pons and it is connected with the dorsal respiratory group of neurons.The ventral respiratory group consists of the rostral part of the nucleus ambiguus, and a small satellite nucleus called the nucleus retroambiguus which lies caudally to the former.The former two are found within the dorsal and ventral medulla, respectively, while the latter lies within the rostral pons.It consists of three parts: the dorsal respiratory group, ventral respiratory group and pneumotaxic center.The dorsal respiratory group is in charge of inspiration or inhaling the air, and it plays the most fundamental role in the breathing process.These neurons are inactive during normal, non-forced breathing.The end result is inspiration.

The respiratory center is a complex group of nuclei located within the pons and medulla oblongata.
It consists of three parts: the dorsal respiratory group, ventral respiratory group and pneumotaxic center. The former two are found within the dorsal and ventral medulla, respectively, while the latter lies within the rostral pons.
The dorsal respiratory group is in charge of inspiration or inhaling the air, and it plays the most fundamental role in the breathing process. The majority of its neurons are found within the nucleus of the solitary tract, which receives information from the peripheral chemoreceptors about the blood oxygen saturation.
Upon receiving this information, the dorsal respiratory group stimulates the phrenic nerve to contract the diaphragm, as well as the thoracic spinal nerves to contract the intercostal muscles. The end result is inspiration.
The ventral respiratory group consists of the rostral part of the nucleus ambiguus, and a small satellite nucleus called the nucleus retroambiguus which lies caudally to the former.
These neurons are inactive during normal, non-forced breathing. However, when in need of increased pulmonary ventilation, the dorsal respiratory group stimulates the ventral group, which in turn stimulates the accessory respiratory muscles.

The pontine pneumotaxic center lies within the parabrachial nucleus of the rostral pons and it is connected with the dorsal respiratory group of neurons. The main function of the pneumotaxic center is to “turn off” the inspiratory signal from the dorsal respiratory group, thus dictating the respiratory cycle and the length of inspiration. Depending on the physiological condition, the pneumotaxic center allows 0.5 to 5 seconds long inspiration [2].
Chemoreceptors are specialized sensory cells, sensitive to molecules, that detect chemical changes in the body, responding to chemical stimuli and interpreting them into electrical impulses. They monitor the oxygen and carbon dioxide levels, as well as the pH in the blood or the environment, there are two types, central and peripheral and are found in various organs throughout the human body. Chemoreceptors are particularly important for regulating processes like respiration and maintaining the osmolarity of blood and cerebrospinal fluid in balance, as well as functions related to smell (olfaction) and taste (gustation).
Central Chemoreceptors: Central chemoreception involves the detection of alterations in carbon dioxide (CO2) and hydrogen ion (H+) levels within the brain, which subsequently influences respiratory rate and depth. The central chemoreceptors are primarily located on the ventral medulla and especially in the retrotrapezoid nucleus. They monitor the acidity (pH) of the cerebrospinal fluid, which is a good indicator of blood carbon dioxide levels. Although H+ can’t cross the blood-brain barrier, CO2 can easily diffuse across the barrier and enter the brain, where it reacts with water to form carbonic acid. This acid then breaks down into hydrogen ions (H+) and bicarbonate ions. The increase in H+ ions in the cerebrospinal fluid triggers the central chemoreceptors.
When CO2 levels rise, the pH decreases, and the central chemoreceptors send signals to the respiratory centers to increase breathing rate and depth, expelling more carbon dioxide and restoring normal pH. They provide real-time feedback to the brainstem's respiratory control center. The elevated CO2 levels, particularly in arterial blood and alveolar gas, stimulate these receptors and this stimulation, in turn, leads to increased alveolar ventilation. The hyperventilation helps to reduce CO2 levels and restore balance. By continuously monitoring and adjusting ventilation, these chemoreceptors ensure adequate oxygenation and carbon dioxide elimination, maintaining a delicate equilibrium between metabolism and respiration.
On the contrary, when the level of CO2 in the blood decreases, the amount of H+ ions in the CSF decreases, so the CSF becomes less acidic. The chemoreceptors are less stimulated and the brain sends fewer signals to the breathing muscles, which causes the rate and depth of breathing to slow down. In short, lower blood CO2 levels lead to slower breathing and this helps maintain a balance in blood gasses, ensuring proper oxygen levels and CO2 removal.

Peripheral Chemoreceptors: located in the carotid and aortic bodies they detect changes mainly in blood concentration of PO2 and to a lesser extent of CO2 and H+.
Peripheral chemoreceptors are sensors located outside the CNS that act faster than the central chemoreceptors and help regulate blood osmolarity. They are found in carotid bodies, bilaterally in the division of the common carotid arteries, and in the aortic bodies in the aortic arch.
The carotid body (CB) is composed of clusters of type I cells, the primary chemoreceptors for oxygen and carbon dioxide, surrounded by supporting type II cells. Information from the carotid and aortic bodies reaches the brain via the glossopharyngeal and vagus nerves, respectively.
Peripheral chemoreceptors primarily detect changes in blood oxygen levels (O2), which is the most powerful stimulus, with lesser sensitivity to changes in CO2 and pH.
A decrease in the partial pressure of oxygen (PO2) in the arterial blood or, respectively, an increase in CO2 levels triggers these peripheral chemoreceptors to transduce the signal. The electrical impulse is transmitted to the respiratory center in the brainstem and the breathing rate and depth are adjusted to restore blood osmolarity and pH in normal.

When PO2 falls below a certain threshold, around 60 mmHg a condition called hypoxemia, the peripheral chemoreceptors increase their firing rate. This signal is transmitted to the respiratory control center in the brainstem, where the respiratory control center then increases the rate and depth of ventilation in an attempt to increase O2 intake and eliminate CO2. Peripheral chemoreceptors are primarily sensitive to low oxygen levels.
However, when oxygen levels become excessively high, a condition called hyperoxia, their activity diminishes, as they are not designed to regulate breathing in such conditions.
In hypoxia, additionally, the peripheral chemoreceptors send signals to the cardiovascular control center, which increases heart rate and contractility to improve oxygen delivery to tissues [3].