Breathing requires the integrity of both the central and peripheral nervous systems at multiple levels ( Bergerefa/ 1977). Peripheral receptors located in aortic and carotid bodies are activated by decreased arterial oxygen, pH, or blood pressure, and by increased carbon dioxide tension. This leads to increased central ventilatory drive. In addition, mechanical receptors along the respiratory tract can facilitate or inhibit breathing, or affect the breathing cycle in a reflex fashion (e.g. sneezing, coughing, sniffing, or brief apnea).
Centrally, ventilation is controlled by the cerebral cortex, brainstem ventilatory centers and their descending pathways, and spinal cord segments. Activation of limbic structures tends to inhibit ventilation, while stimulation of the motor and premotor areas of the frontal lobes may increase it. Such modulation of breathing allows for certain voluntary activities including speech.
The main automatic breathing centers lie in the brainstem. The pneumotaxic center in the pontine tegmentum switches off inspiration. The medullary center includes the dorsal respiratory group, the nucleus of the tractus solitarius, and the ventral respiratory group in the nuclei ambiguus and retroambiguus ( CDFiguie...?). The dorsal respiratory group contains inspiratory neurons which drive the ventral respiratory group. The ventral respiratory group contains both inspiratory and expiratory regions. Fibers from the ventral respiratory group descend in the lateral columns of the spinal cord to innervate spinal cord intercostal and phrenic neurons for inspiration and other intercostal and abdominal muscles for expiration.
CD Figure 2. A schematic representation of the brainstem structures important for the control of respiration, including the dorsal (DRG) and ventral (VRG) respiratory groups with their efferent projections: ION, inferior olivary nucleus; NA, nucleus ambiguus; NRA, nucleus retroambiguus; NTS, nucleus tractus solitarius; OB, obex; VH, ventral horn; IX, glossopharyngeal nerve; X, vagal nerve; XII, hypoglossal nucleus. (Adapted with permission from R.A. Mitchell and A.J. Berger, Neural regulation of respiration, American Review of Respiratory Diseases, 111, 206-24 (1975).)
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