Breathing is spontaneously initiated in the central nervous system. Neurons in the brainstem automatically generate a cycle of inspiration and expiration. This spontaneously generated cycle of inspiration and expiration can be modified, altered, or even temporarily suppressed by a number of mechanisms. As shown in Figure 33-1, these include reflexes arising in the lungs, the airways, and the cardiovascular system; information from receptors in contact with the cerebrospinal fluid; and commands from higher centers of the brain such as the hypothalamus, the centers of speech, or other areas in the cortex. The centers that are responsible for the generation of the spontaneous rhythmicity of inspiration and expiration are, therefore, able to alter their activity to meet the increased metabolic demand on the respiratory system during exercise or may even be temporarily superseded or suppressed during speech or breath holding.
Schematic representation of the organization of the respiratory control system. A cycle of inspiration and expiration is automatically established in the medullary respiratory center. Its output represents a final common pathway to the respiratory muscles, except for some voluntary pathways that may go directly from higher centers to the respiratory muscles (dashed line). Reflex responses from chemoreceptors and other sensors may modify the cycle of inspiration and expiration established by the medullary respiratory center. (Reproduced with permission from Levitzky MG: Pulmonary Physiology, 9th ed. New York, NY: McGraw Hill; 2018.)
The respiratory centers in the brainstem control breathing via a final common pathway consisting of the spinal cord, the innervation of the muscles of respiration (such as the phrenic nerves), and the muscles of respiration themselves. The interval between successive groups of discharges of the respiratory neurons determines the breathing frequency. The frequency and duration of neural discharges to each respiratory muscle fiber and the number of respiratory muscle fibers and types of muscles activated determine the tidal volume.
GENERATION OF SPONTANEOUS RESPIRATORY RHYTHMICITY
The neurons that initiate breathing are located in an area in the reticular formation of the medulla, beneath the floor of the fourth ventricle, known as the medullary center (or medullary respiratory center). If the brainstem of an anesthetized animal is cut above this area, a pattern of inspiration and expiration continues, even if all other nerves leading to this area, including the vagi, are also cut. If the brainstem is cut below this area, breathing stops.
The medullary respiratory center consists of two dense bilateral aggregations of respiratory neurons known as the dorsal respiratory groups (DRG) and the ventral respiratory groups (VRG). Inspiratory and expiratory neurons are anatomically intermingled within these areas. The DRG consist mainly of inspiratory cells, which project their fibers primarily to the contralateral spinal cord and innervate inspiratory muscles. They probably serve as the chief initiators of the ...