(USMLE topics, pulmonology) Central regulation of breathing, receptors and nerves involved, involuntary and voluntary control.

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Receptors in the body feed information about its metabolic state to the respiratory center in the brainstem, which responds by changing the firing pattern of inspiratory and expiratory neurons. The fibers of these neurons descend to the cervical and thoracic spine where they synapse with motor neurons. Motor neurons then travel in several nerves to respiratory muscles, changing the way these muscles contract, adjusting thereby the rate and depth of breathing to suit the body’s needs. Of most importance are phrenic nerves which control the diaphragm, and intercostal nerves which innervate intercostal muscles. The respiratory center is composed of several areas in the medulla: the dorsal respiratory group, DRG, mainly associated with inspiration; the ventral respiratory group, VRG, mostly concerned with expiration; and the pre-Bötzinger complex, possibly coupled with two other oscillators, thought to be the intrinsic rhythm generator, similar to the pacemaker in the heart. The medullar areas also communicate with two other areas in the pons to fine-tune the respiration control: the pneumotaxic center which seems to inhibit inspiration, while the apneustic center stimulates it.
The most important factor regulating breathing rate is the concentration of carbon dioxide. Changes in carbon dioxide leads to changes in pH, and these are detected by chemoreceptors. Central chemoreceptors located on the surface of the medulla monitor pH changes in the cerebrospinal fluid; while peripheral chemoreceptors found in the aortic and carotid bodies respond to fluctuations in pH, carbon dioxide, as well as oxygen levels in the blood. Peripheral receptors transmit signal to the respiratory center via the vagus and glossopharyngeal nerves. The respiratory center also receives input from various mechanoreceptors in the lungs, which transmit information about the mechanical status of the lungs via the vagus nerve.Pulmonary stretch receptors present in smooth muscle of the airways are activated when the lungs are excessively inflated, and trigger the inflation reflex. Other receptors respond to inhaled irritants and are responsible for defensive respiratory reflexes such as bronchoconstriction or coughing. The limbic system and hypothalamus also send information to the respiratory center and allow pain and emotional state to affect breathing. Voluntary control is also possible, for example, during singing, playing wind instruments, or holding breath under water. In this case the control originates from the primary motor cortex, which sends signals directly to the spinal cord, bypassing the respiratory center in the brainstem.
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