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INTRODUCTION TO THE CONTROL OF RESPIRATION

Learning Objectives

  • The student will be able to summarize the neuroanatomical basis for the rhythm and pattern of simple respiration at rest.

  • The student will be able to describe the anatomy and physiology of both peripheral and central chemoreceptors.

  • The student will be able to distinguish the role that sensory afferent fibers sensitive to chemical/mechanical stimulation of the respiratory tract or muscle metabolism may have on ventilation.

  • The student will be able to define periodic breathing and discuss it in terms of abnormalities of either central or peripheral chemoreception.

As demonstrated in earlier chapters, ventilation is simple in concept but complex in execution. The brain controls the basic pattern of breathing, integrating multiple influences within lower motor neurons of the brainstem and spinal cord to drive pharyngeal, laryngeal, diaphragmatic, intercostal, and other respiratory muscles. Recall that V̇E (L/min) = VT · f. The central nervous system regulates respiration by controlling the rhythm and pattern of its output to respiratory muscles, adjusting f, VT or both, depending on overall ventilatory needs for a greater or lesser V̇E (Fig. 11.1).

FIGURE 11.1

Relationship between V̇E and VT in normal awake subjects. With stimuli such as hypercapnia, V̇E increases linearly with VT up to about 50% of vital capacity (VC). Above that volume, V̇E increases primarily by increasing f with little change in VT.

THE CENTRAL RHYTHM GENERATOR

The frequency of respiration, or its rhythm, is intrinsic to the brainstem. All vertebrates that use tidal oscillations for the exchange of O2 and CO2 in their lungs have such movements generated in their medulla oblongata. Indeed surgically isolated brainstem preparations, lacking afferent inputs from chemoreceptors and mechanoreceptors, still produce rhythmic outputs along the same cranial nerves as during normal respiration. One critical rhythm generator within a small area of the medulla and rostral to the obex is called the pre-Bötzinger complex. Bilateral lesions in this portion of the medulla induce complete respiratory arrest in humans. Whether this rhythmic discharge initiates within individual pacemaker cells, or from a network of such cells, remains a matter of debate. At this point, it is important to understand that a rhythm is constantly generated by the medulla, a rhythm that is modifiable by afferent input from sensory receptors.

THE CENTRAL PATTERN GENERATOR

The central pattern generator, or the brainstem output controlling all muscles involved in respiration, is much more complex. This pattern generator algebraically sums all the afferent inputs to produce well-coordinated activations of the diaphragm, intercostal muscles, and abdominal muscles, and if needed, the accessory muscles of respiration. Like respiratory rhythm, the goal of such pattern generation is maintenance of normal ...

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