Control Centers in the Brainstem

The medulla oblongata contains inspiratory (I) neurons, which fire during inspiration, and expiratory (E) neurons, which fire during forced expiration (but not during eup-nea). Fibers from these neurons travel down the spinal cord and synapse with lower motor neurons in the cervical to thoracic regions. From here, nerve fibers travel in the phrenic nerves to the diaphragm and intercostal nerves to the intercostal muscles. No pacemaker neurons have been found that are analogous to the autorhythmic cells of the heart, and the exact mechanism for setting the rhythm of respiration remains unknown despite intensive research.

The medulla has two respiratory nuclei (fig. 22.15). One of them, called the inspiratory center, or dorsal respiratory group (DRG), is composed primarily of I neurons, which stimulate the muscles of inspiration. The more frequently they fire, the more motor units are recruited and the more deeply you inhale. If they fire longer than usual, each breath is prolonged and the respiratory rate is slower. When they stop firing, elastic recoil of the lungs and thoracic cage produces passive expiration.

The other nucleus is the expiratory center, or ventral respiratory group (VRG). It has I neurons in its midregion and E neurons at its rostral and caudal ends. It is not involved in eupnea, but its E neurons inhibit the inspira-tory center when deeper expiration is needed. Conversely, the inspiratory center inhibits the expiratory center when an unusually deep inspiration is needed.

The pons regulates ventilation by means of a pneu-motaxic center in the upper pons and an apneustic (ap-NEW-stic) center in the lower pons. The role of the apneustic center is still unclear, but it seems to prolong inspiration. The pneumotaxic (NEW-mo-TAX-ic) center sends a continual stream of inhibitory impulses to the inspiratory center of the medulla. When impulse frequency rises, inspiration lasts as little as 0.5 second and the breathing becomes faster and shallower. Conversely, when impulse frequency declines, breathing is slower and deeper, with inspiration lasting as long as 5 seconds.

_Think About It_

Do you think the fibers from the pneumotaxic center produce EPSPs or IPSPs at their synapses in the inspiratory center? Explain.

Pneumotaxic Centre

Figure 22.15 Respiratory Control Centers. Functions of the apneustic center are hypothetical and its connections are therefore indicated by broken lines. As indicated by the plus and minus signs, the apneustic center stimulates the inspiratory center, while the pneumotaxic center inhibits it. The inspiratory and expiratory centers inhibit each other.

Figure 22.15 Respiratory Control Centers. Functions of the apneustic center are hypothetical and its connections are therefore indicated by broken lines. As indicated by the plus and minus signs, the apneustic center stimulates the inspiratory center, while the pneumotaxic center inhibits it. The inspiratory and expiratory centers inhibit each other.

Saladin: Anatomy & I 22. The Respiratory System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 22 The Respiratory System 859

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • marko
    Where is pneumotaxic centre?
    3 years ago

Post a comment