Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

impulses into motor responses of the respiratory mus- cles. The first, or dorsal, group of neurons is concerned primarily with inspiration. These neurons control the activity of the phrenic nerves that innervate the dia- phragm and drive the second, or ventral, group of respi- ratory neurons. They are thought to integrate sensory input from the lungs and airways into the ventilatory response. The second group of neurons, which contains inspiratory and expiratory neurons, controls the spinal motor neurons of the intercostal and abdominal muscles. The pacemaker properties of the respiratory center in the medulla result from the cycling of two groups of neurons: the pneumotaxic center in the upper pons and the apneustic center in the lower pons (see Fig. 21-18). The apneustic center has an excitatory effect on inspira- tion, tending to prolong inspiration. The pneumotaxic center switches inspiration off, assisting in the control of the respiratory rate and inspiratory volume. Brain injury, which damages the connections between the pneumotaxic and apneustic centers, results in an irregu- lar breathing pattern that consists of prolonged inspira- tory gasps interrupted by expiratory efforts. Axons from the neurons in the respiratory center cross in the midline and descend in the ventrolateral col- umns of the spinal cord. The tracts that control expira- tion and inspiration are spatially separated in the cord, as are the tracts that transmit specialized reflexes (i.e., coughing and hiccupping) and voluntary control of ven- tilation. Only at the level of the spinal cord are the respi- ratory impulses integrated to produce a reflex response.

Control of Breathing Unlike the heart, which has inherent rhythmic proper- ties and can beat independently of the nervous system, the muscles that control respiration require continu- ous input from the nervous system. Movement of the diaphragm, intercostal muscles, sternocleidomastoid, and other accessory muscles that control ventilation is integrated by neurons located in the pons and medulla. These neurons are collectively referred to as the respira- tory center (Fig. 21-18). Respiratory Center The respiratory center consists of two dense, bilateral aggregates of respiratory neurons involved in initiating inspiration and expiration and incorporating afferent ■■ Carbon dioxide is transported in the blood: (1) as the dissolved gas (10%), (2) attached to hemoglobin (30%), and (3) as bicarbonate (60%). The reversible action of carbon dioxide with water to form bicarbonate is catalyzed by the enzyme carbonic anhydrase within red cells and is the major pathway for generation of bicarbonate.

Impulses from higher brain centers

Nose and throat

Carotid arteries and aorta

Pons Pneumotaxic center

Trachea and bronchi

Apneustic center Inspiratory center Expiratory center

Medulla

Lungs

Cervical nerves

Diaphragm

Thoracic nerves

FIGURE 21-18. Schematic representation of activity in the respiratory center. Impulses traveling over afferent neurons (dashed lines) communicate with central neurons, which activate efferent neurons that supply the muscles of respiration. Respiratory movements can be altered by a variety of stimuli.

Intercostal muscles