The automatic control of breathing is also influenced by input from receptors sensitive to the chemical composition of the blood. There are two groups of chemoreceptors that respond to changes in blood PCo2, pH, and Po2. These are the central chemoreceptors in the medulla oblongata and the peripheral chemoreceptors. The peripheral chemoreceptors are contained within small nodules associated with the aorta and the carotid arteries, and they receive blood from these critical arteries via small arterial branches. The peripheral chemorecep-tors include the aortic bodies, located around the aortic arch, and the carotid bodies, located in each common carotid artery at the point where it branches into the internal and external carotid arteries (fig. 16.26). The aortic and carotid bodies should not be confused with the aortic and carotid sinuses (chapter 14) that are located within these arteries. The aortic and carotid sinuses contain receptors that monitor the blood pressure.
The peripheral chemoreceptors control breathing indirectly via sensory nerve fibers to the medulla. The aortic bodies send sensory information to the medulla in the vagus nerve (X); the carotid bodies stimulate sensory fibers in the glossopharyn-geal nerve (IX). The neural and sensory control of ventilation is summarized in figure 16.27.
Wfr The automatic control of breathing is regulated by nerve fibers that descend in the lateral and ventral ^ \ ^ white matter of the spinal cord from the medulla oblongata. The voluntary control of breathing is a function of the cerebral cortex and involves nerve fibers that descend in the corticospinal tracts (chapter 8). The separation of the voluntary and involuntary pathways is dramatically illustrated in the condition called Ondine's curse (the term is taken from a German fairy tale). In this condition, neurological damage abolishes the automatic but not the voluntary control of breathing. People with Ondine's curse must remind themselves to breathe and they cannot go to sleep without the aid of a mechanical respirator.
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