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NEURAL CONTROL OF RESPIRATION Skeletal muscles provide the motive force for respiration. Unlike cardiac or smooth muscle, they have no rhythmic "beat" of their own; they depend entirely on the nervous system for a stimulus to contract. Two separate neural systems control respiration: (1) Voluntary control originates in cerebral cortex neurons, which send impulses down the corticospinal nerve tracts to motor neurons located in the spinal cord, which relay excitatory impulses to the muscles of respiration, the intercostal muscles and the diaphragm. This voluntary system can interrupt or modulate the normal automatic breathing pattern; it is most apparent during speech and while playing wind instruments, where the lungs serve as air reservoirs to be emptied at controlled rates. (2) Automatic control originates in lower brain centers, in the pons and the medulla. Impulses arising in this system also descend in the spinal cord to the motor neurons controlling respiratory muscles, but they travel along nerve tracts lying in the lateral and ventral parts of the cord, separate from the corticospinal tracts. In general, motor neurons to expiratory muscles are inhibited during inspiration and vice versa. The medulla contains a diffuse network of neurons involved in respiration. Although they are collectively referred to as the respiratory "center" (or "centers"), they are not located in nice discrete packages. There are two types of these neurons: the I neurons, which fire during inspiration, and the E neurons, which fire during expiration. During inspiration, E neurons are actively inhibited; during expiration, I neurons are inhibited. The primitive rhythm for involuntary breathing is apparently generated by the I neurons. They show bursts of spontaneous activity interspersed with quiet periods about 12 to 15 times/min. In contrast, the E neurons are not self-excitatory; they are excited only by other neurons (including the I neurons) that send impulses to them. When the activity of the inspiratory neurons increases, the rate and depth of breathing increase. The primitive activity of the I neurons, like that of all pacemakers, is modulated by a number of outside influences, including nerve impulses from centers in the pons and from receptors in the lungs. These influences are dramatically revealed after injuries and are outlined in the plate. If the brainstem is transacted below the medulla (at D in the plate), all breathing stops, showing that the brain drives respiration and that communication between brain and respiratory muscles takes place via the spinal cord. But if the transaction is made lower in the cord, at E, breathing is not interrupted because the connections between brain and respiratory neurons remain intact, as do motor nerves (i.e., the phrenic nerve) that carry the impulses to the muscles of respiration. Regular breathing also continues when all the cranial nerves, including the vagi, are severed, and the brain is transacted above the pons at A. These results locate the centers for automatic breathing somewhere between the top of the pons and the lower medulla - clearly, higher brain centers like those in the cortex are not necessary. Given this localization, we can dissect the respiratory centers even further. If the vagus nerves are cut and two transactions are made, one at the top of the pons as before at A and the other in the middle at B, the I cells discharge continuously, arresting respiration in inspiration. This stopping of respiration in sustained inspiration is called apneusis, and the neurons in the lower pons, which apparently shower I neurons with excitatory impulses and keep them firing, are collectively referred to as the apneustic center. Apneusis occurs only when influences from the upper pons are removed (transaction at B). This suggests that neurons in the upper pons continually inhibit the apneustic center, holding its inspiratory drive in check. These neurons are members of another collection called the pneumotaxic center. When xll pons influence is removed by a transaction at C, respiration continues. Although it may be irregular and punctuated with gasps, it is rhythmic, and it demonstrates that the neurons of the respiratory centers themselves have a spontaneous rhythmicity. The role of the pontine centers appears to be to make these rhythmic discharges smooth and regular. All these responses depend to some extent on whether the vagus nerves are intact. Apneusis, for example, cannot be demonstrated by transaction of the mid pons (B) unless the vagi are also severed because vagus nerves carry impulses that originates in stretch receptors located in the lung airways. When the lungs expand during inspiration, these receptors initiate impulses that reflexively inhibit the inspiratory drive, reinforcing the actions of the pneumotaxic center and protecting the lungs from overexpansion. This response is called a Haring-Breuer reflex. In humans, it does not appear to be activated until the tidal volume reaches 1 L, so it plays no part in regulating ventilation during normal quiet breathing. Several additional factors influence the respiratory centers so that their activity is commensurate with the body's metabolic needs. These include reflexes originating in receptors (proprioceptors) located in muscles, tendons, and joints that are sensitive to movement. They send to the respiratory centers stimulating impulses that presumably help increase ventilation during exercise. Other important reflexes are initiated by low P02, low pH, and high PC02 in the plasma; these are taken up in detail in plate 52.