Involuntary respiration is controlled by the respiratory centers of the upper brainstem. The cerebral cortex of the brain controls voluntary respiration.
Two main parts of the brain control involuntary ventilation; the medulla oblongata and the pons. The brainstem contains central chemoreceptors that detect pH levels in the blood and send signals to the respiratory centers of the brain to adjust the ventilation rate to change acidity by increasing or decreasing the removal of carbon dioxide by exhalation. This removal of carbon dioxide causes the removal of hydrogen ions from the blood by the bicarbonate buffer system.
The medulla oblongata is the primary respiratory control center. Its principal function is to send signals to the muscles that control respiration that causes breathing to occur. There are two regions in the medulla that control breathing: the ventral respiratory group stimulates expiratory movements, and the dorsal respiratory group stimulates inspiratory movements. The medulla also controls the reflexes for nonrespiratory air movements, such as coughing and sneezing reflexes, as well as other reflexes, like swallowing and vomiting.
The pons is the other respiratory center and is located underneath the medulla. Its primary function is to control the rate or speed of involuntary ventilation. It has two main functional regions that perform this role: The apneustic center sends signals for inspiration for long and deep breaths. It controls the intensity of breathing and is inhibited by the stretch receptors of the pulmonary muscles at a maximum depth of inspiration, or by signals from the pnuemotaxic center. It increases tidal volume. The pnuemotaxic center sends signals to inhibit inspiration that allows it to control the respiratory rate finely. Its signals limit the activity of the phrenic nerve and inhibit the signals of the apneustic center. It decreases tidal volume. These two centres work antagonistically against each other to control the respiratory rate.
The primary motor cortex is the neural center for voluntary respiratory control. More broadly, the motor cortex is responsible for initiating any voluntary muscular movement. Different parts of the cerebral cortex control various forms of voluntary respiration. Initiation of the voluntary contraction and relaxation of the internal and external intercostal muscles takes place in the superior portion of the primary motor cortex.
There are several nerves responsible for the muscular functions involved in respiration. There are three types of important respiratory nerves: The phrenic nerves stimulate the activity of the diaphragm, the vagus nerve innervates the diaphragm as well as movements in the larynx and pharynx and the posterior thoracic nerves which stimulate the intercostal muscles located around the pleura.
Practice Questions
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MCAT Official Prep (AAMC)
Biology Question Pack, Vol 2. Question 61
Key Points
• The ventral respiratory group controls voluntarily forced exhalation and acts to increase the force of inspiration.
• The dorsal respiratory group controls mostly inspiratory movements and their timing.
• Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate.
• Chemoreceptors can detect changes in blood pH that require changes in involuntary respiration to correct. The apneustic (stimulating) and pnuemotaxic (limiting) centers of the pons work together to control the rate of breathing.
• The medulla sends signals to the muscles that initiate inspiration and expiration and controls nonrespiratory air movement reflexes, like coughing and sneezing.
• The motor cortex within the cerebral cortex of the brain controls voluntary respiration (the ascending respiratory pathway).
• Voluntary respiration may be overridden by aspects of involuntary respiration, such as chemoreceptor stimulus, and hypothalamus stress response.
• The phrenic nerves, vagus nerves, and posterior thoracic nerves are the major nerves involved in respiration.
• Voluntary respiration is needed to perform higher functions, such as voice control.
Key Terms
respiratory control centers: The medulla which sends signals to the muscles involved in breathing, and the pons which controls the rate of breathing.
Chemoreceptors: These are receptors in the medulla and in the aortic and carotid bodies of the blood vessels that detect changes in blood pH and signal the medulla to correct those changes.
primary motor cortex: The region in the brain that initiates all voluntary muscular movement, including those for respiration.
central chemoreceptors: located within the medulla, they are sensitive to the pH of their environment.
involuntary ventilation: breathing controlled by the brain
voluntary ventilation: conscious control of breathing
medulla oblongata: a long stem-like structure which makes up part of the brainstem
the pons: contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that deal primarily with sleep, respiration and swallowing
ventilation rate: the rate of breathing
expiratory: breathing out
inspiratory: breathing in
apneustic center: part of the medulla that signals for inspiration for long and deep breaths
pnuemotaxic center: sends signals to inhibit inspiration that allows it to control the respiratory rate finely