respiratory center of the medulla
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The respiratory center is located in the medulla oblongata and
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
, in the brainstem. The respiratory center is made up of three major respiratory groups of
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
s, two in the medulla and one in the pons. In the medulla they are the dorsal respiratory group, and the ventral respiratory group. In the pons, the pontine respiratory group includes two areas known as the pneumotaxic centre and the apneustic centre. The respiratory centre is responsible for generating and maintaining the rhythm of respiration, and also of adjusting this in
homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
response to physiological changes. The respiratory center receives input from
chemoreceptors A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorec ...
, mechanoreceptors, the
cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consistin ...
, and the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
in order to regulate the rate and depth of breathing. Input is stimulated by altered levels of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
,
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
, and
blood pH Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the ...
, by hormonal changes relating to stress and anxiety from the hypothalamus, and also by signals from the cerebral cortex to give a conscious control of respiration. Injury to respiratory groups can cause various breathing disorders that may require
mechanical ventilation Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV), is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move a ...
, and is usually associated with a poor prognosis.


Respiratory groups

The respiratory centre is divided into three major groups, two in the medulla and one in the pons. The two groups in the medulla are the dorsal respiratory group and the ventral respiratory group. In the pons, the pontine respiratory group is made up of two areas – the pneumotaxic centre and the apneustic centre. The dorsal and ventral medullary groups control the basic rhythm of respiration. The groups are paired with one on each side of the brainstem.


Dorsal respiratory group

The dorsal respiratory group (DRG) has the most fundamental role in the control of respiration, initiating inspiration (inhalation). The DRG is a collection of neurons forming an elongated mass that extends most of the length of the dorsal medulla. They are near to the
central canal The central canal (also known as spinal foramen or ependymal canal) is the cerebrospinal fluid-filled space that runs through the spinal cord. The central canal lies below and is connected to the ventricular system of the brain, from which it r ...
of the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, and just behind the ventral group. They set and maintain the rate of respiration. Most of the neurons are located in the
nucleus of the solitary tract In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, (SN or NTS) is a series of purely sensory nuclei (clusters of nerve cell bodies) forming a vertical column ...
. Other important neurons are found in the adjacent areas including the reticular substance of the medulla. The solitary nucleus is the end-point for sensory information arriving from the pontine respiratory group, and from two
cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...
– the
vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
, and the
glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. ...
. The solitary nucleus sends signals to the respiratory centre from
peripheral chemoreceptors Peripheral chemoreceptors (of the carotid and aortic bodies) are so named because they are sensory extensions of the peripheral nervous system into blood vessels where they detect changes in chemical concentrations. As transducers of patterns of va ...
, baroreceptors, and other types of receptors in the lungs in particular the
stretch receptor Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles, and are neurologically linked to the Medulla oblongata, medulla in the brain stem via Afferent nerve fiber, afferent nerve fibers. Examples include stre ...
s. Thus, the dorsal respiratory group is seen as an integrating centre that gives the ventral respiratory group output to modify the breathing rhythm.


Ventral respiratory group

In the medulla, the ventral respiratory group (VRG) consists of four groups of neurons that make up the
exhalation Exhalation (or expiration) is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of the lungs, ...
(expiratory) area of respiratory control. This area is in the ventrolateral part of the medulla, about 5  mm anterior and lateral to the dorsal respiratory group. The neurons involved include those in the
nucleus ambiguus The nucleus ambiguus ("ambiguous nucleus" in English) is a group of large motor neurons, situated deep in the medullary reticular formation named by Jacob Clarke. The nucleus ambiguus contains the cell bodies of neurons that innervate the muscle ...
, the nucleus retroambiguus, and the interneurons in the pre-Bötzinger complex. The VRG contains both inspiratory and expiratory neurons. The ventral respiratory group of neurons are active in forceful breathing and inactive during quiet, restful respirations. The VRG sends inhibitory impulses to the apneustic center.


Pontine respiratory group

In the
pontine tegmentum Pontine may refer to: * Having to do with the pons, a structure located in the brain stem (from ''pons'', "bridge") * Pontine Marshes, a region of Italy near Rome * Pontine Islands The Pontine Islands (, also ; it, Isole Ponziane ) are an ar ...
in the pons, the pontine respiratory group (PRG) includes the pneumotaxic and apneustic centers. These have connections between them, and from both to the solitary nucleus.


Pneumotaxic center

The pneumotaxic center is located in the upper part of the pons. Its nuclei are the subparabrachial nucleus and the medial parabrachial nucleus. The pneumotaxic center controls both the rate and the pattern of breathing. The pneumotaxic center is considered an antagonist to the apneustic center, (which produces abnormal breathing during inhalation) cyclically inhibiting inhalation. The pneumotaxic center is responsible for limiting inspiration, providing an ''inspiratory off-switch'' (IOS). It limits the burst of
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
s in the phrenic nerve, effectively decreasing the
tidal volume Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass. Mechanical vent ...
and regulating the
respiratory rate The respiratory rate is the rate at which breathing occurs; it is set and controlled by the respiratory center of the brain. A person's respiratory rate is usually measured in breaths per minute. Measurement The respiratory rate in humans is me ...
. Absence of the center results in an increase in depth of respiration and a decrease in respiratory rate. The pneumotaxic centre regulates the amount of air that can be taken into the body in each breath. The dorsal respiratory group has rhythmic bursts of activity that are constant in duration and interval. When a faster rate of breathing is needed the pneumotaxic centre signals the dorsal respiratory group to speed up. When longer breaths are needed the bursts of activity are elongated. All the information that the body uses to help respiration happens in the pneumotaxic centre. If this was damaged or in any way harmed it would make breathing almost impossible. One study on this subject was on anaesthetized paralyzed cats before and after bilateral
vagotomy A vagotomy is a surgery, surgical procedure that involves segmental resection, removing part of the vagus nerve. Types A plain vagotomy eliminates the parasympathetic supply from the stomach to the left side of the transverse colon. Other techni ...
. Ventilation was monitored in awake and anaesthetized cats breathing air or CO2. Ventilation was monitored both before and after lesions to the pneumatic centre region and after subsequent bilateral vagotomy. Cats with pontine lesions had a prolonged inhalation duration. In cats, after anaesthesia and vagotomy, pontine transaction has been described as evoking a long sustained inspiratory discharges interrupted by short expiratory pauses. In rats on the other hand, after anaesthesia, vagotomy and pontine transaction, this breathing pattern was not observed, either in vivo or in vitro. These results suggest interspecies differences between rat and cat in the pontine influences on the medullary respiratory centre.


Apneustic center

The apneustic centre of the lower pons appears to promote inhalation by constant stimulation of the neurons in the medulla oblongata. The apneustic centre sends signals to the dorsal group in the medulla to delay the 'switch off, the ''inspiratory off switch'' (IOS) signal of the inspiratory ramp provided by the pneumotaxic centre. It controls the intensity of breathing, giving positive impulses to the neurons involved with inhalation. The apneustic centre is inhibited by pulmonary stretch receptors and also by the pneumotaxic centre. It also discharges an inhibitory impulse to the pneumotaxic centre.


Respiratory rhythm

Breathing is the repetitive process of bringing air into the lungs and taking waste products out. The oxygen brought in from the air is a constant, on-going need of an organism to maintain life. This need is still there during sleep so that the functioning of this process has to be automatic and be part of the autonomic nervous system. The in-breath is followed by the out-breath, giving the respiratory cycle of inhalation and exhalation. There are three phases of the respiratory cycle: inspiration, post-inspiration or passive expiration, and late or active expiration. The number of cycles per minute is the
respiratory rate The respiratory rate is the rate at which breathing occurs; it is set and controlled by the respiratory center of the brain. A person's respiratory rate is usually measured in breaths per minute. Measurement The respiratory rate in humans is me ...
. The respiratory rate is set in the respiratory center by the dorsal respiratory group, in the medulla, and these neurons are mostly concentrated in the solitary nucleus that extends the length of the medulla. The basic rhythm of respiration is that of quiet, restful breathing known as eupnea. Quiet breathing only requires the activity of the dorsal group which activates the diaphragm, and the
external intercostal muscle The external intercostal muscles, or external intercostals (Intercostales externi) are eleven in number on both sides. Structure The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end ...
s. Exhalation is passive and relies on the elastic recoil of the lungs. When the metabolic need for oxygen increases, inspiration becomes more forceful and the neurons in the ventral group are activated to bring about forceful exhalation.
Shortness of breath Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing di ...
is termed dyspnea – the opposite of eupnea.


Clinical significance

Depression of the respiratory centre can be caused by:
brain trauma A traumatic brain injury (TBI), also known as an intracranial injury, is an injury to the brain caused by an external force. TBI can be classified based on severity (ranging from mild traumatic brain injury TBI/concussionto severe traumatic br ...
, brain damage, a brain tumour, or ischemia. A depression can also be caused by drugs including
opioid Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid use ...
s, and sedatives. The respiratory centre can be stimulated by amphetamine, to produce faster and deeper breaths. Normally at therapeutic doses, this effect is not noticeable, but may be evident when respiration is already compromised.


See also

*
Control of respiration The control of ventilation refers to the physiological mechanisms involved in the control of breathing, which is the movement of air into and out of the lungs. Ventilation facilitates respiration. Respiration refers to the utilization of oxygen and ...
* Cough center *
Gag reflex The pharyngeal reflex or gag reflex is a reflex muscular contraction of the back of the throat, evoked by touching the roof of the mouth, the back of the tongue, the area around the tonsils, the uvula, and the back of the throat. It, along with ot ...


References


Further reading

* * * {{Pons Brainstem Medulla oblongata Pons