Chapter 36

After studying this chapter, you should be able to:

• Locate the pre-Bötzinger complex and describe its role in producing spontaneous respiration.
• Identify the location and probable functions of the dorsal and ventral groups of respiratory neurons, the pneumotaxic center, and the apneustic center in the brain stem.
• List the specific respiratory functions of the vagus nerves and the respiratory receptors in the carotid body, the aortic body, and the ventral surface of the medulla oblongata.
• Describe and explain the ventilatory responses to increased CO2 concentrations in the inspired air.
• Describe and explain the ventilatory responses to decreased O2 concentrations in the inspired air.
• Describe the effects of each of the main nonchemical factors that influence respiration.
• Describe the effects of exercise on ventilation and O2 exchange in the tissues.
• Define periodic breathing and explain its occurrence in various disease states.

Spontaneous respiration is produced by rhythmic discharge of motor neurons that innervate the respiratory muscles. This discharge is totally dependent on nerve impulses from the brain; breathing stops if the spinal cord is transected above the origin of the phrenic nerves. The rhythmic discharges from the brain that produce spontaneous respiration are regulated by alterations in arterial Po2, Pco2, and H+ concentration, and this chemical control of breathing is supplemented by a number of nonchemical influences. The physiological bases for these phenomena are discussed in this chapter.

Control Systems

Two separate neural mechanisms regulate respiration. One is responsible for voluntary control and the other for automatic control. The voluntary system is located in the cerebral cortex and sends impulses to the respiratory motor neurons via the corticospinal tracts. The automatic system is driven by a group of pacemaker cells in the medulla. Impulses from these cells activate motor neurons in the cervical and thoracic spinal cord that innervate inspiratory muscles. Those in the cervical cord activate the diaphragm via the phrenic nerves, and those in the thoracic spinal cord activate the external intercostal muscles. However, the impulses also reach the innervation of the internal intercostal muscles and other expiratory muscles.

The motor neurons to the expiratory muscles are inhibited when those supplying the inspiratory muscles are active, and vice versa. Although spinal reflexes contribute to this reciprocal innervation, it is due primarily to activity in descending pathways. Impulses in these descending pathways excite agonists and inhibit antagonists. The one exception to the reciprocal inhibition is a small amount of activity in phrenic axons for a short period after inspiration. The function of this postinspiratory output appears to be to brake the lung's elastic recoil and make respiration smooth.

Medullary Systems

The main components of the respiratory control pattern generator responsible for automatic respiration are located in the medulla. Rhythmic respiration is initiated by a small group of synaptically coupled pacemaker cells in the pre-Bötzinger complex...

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