The reader understands the organization and function of the respiratory control system.
- Describes the general organization of the respiratory control system.
- Localizes the centers that generate the spontaneous rhythmicity of breathing.
- Describes the groups of neurons that effect inspiration and expiration.
- Describes the other centers in the brainstem that may influence the spontaneous rhythmicity of breathing.
- Lists the cardiopulmonary and other reflexes that influence the breathing pattern.
- States the ability of the brain cortex to override the normal pattern of inspiration and expiration temporarily.
- Describes the effects of alterations in body oxygen, carbon dioxide, and hydrogen ion levels on the control of breathing.
- Describes the sensors of the respiratory system for oxygen, carbon dioxide, and hydrogen ion concentration.
Breathing is spontaneously initiated in the central nervous system. A cycle of inspiration and expiration is automatically generated by neurons located in the brainstem; in eupneic states, breathing occurs without a conscious initiation of inspiration and expiration. Normal individuals do not have to worry about forgetting to breathe while they sleep.
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 9–1, 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activity to meet the increased metabolic demand on the respiratory system during exercise or may even be temporarily superseded or suppressed during speech or breath holding.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.
The output of the respiratory control centers in the brainstem controls 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. Alveolar ventilation is therefore determined by the interval between successive groups of discharges of the respiratory neurons and the innervation of the muscles of respiration, which determines the respiratory rate or breathing frequency, and by the frequency of neural discharges transmitted by individual nerve fibers to their motor units, the duration of these discharges, and the number of motor units activated during each inspiration or expiration, ...