A loose aggregation of neurons in the reticular formation of the medulla oblongata forms the rhythmicity center that controls automatic breathing. The rhythmicity center consists of interacting pools of neurons that fire either during inspiration (inspira-tory, or I, neurons) or expiration (expiratory, or E, neurons). The I neurons project to and stimulate spinal motoneurons that innervate the respiratory muscles. Expiration is a passive process that occurs when the I neurons are inhibited, presumably by the activity of the E neurons.
The inspiratory neurons are located primarily in the dorsal respiratory group, and the expiratory neurons in the ventral respiratory group. These form two parallel columns within the medulla oblongata. The dorsal group of neurons regulates the activity of the phrenic nerves to the diaphragm, and the ventral group controls the motor neurons to the internal intercostal muscles.
The activity of the I and E neurons varies in a reciprocal way to produce a rhythmic pattern of breathing. There is evidence that the rhythmicity of I and E neurons may be driven by the cyclic activity of particular pacemaker neurons within the medulla. These pacemaker neurons display spontaneous, cyclic changes in the membrane potential, somewhat like the pacemaker cells of the heart (chapter 13).
The activity of the medullary rhythmicity center is influenced by centers in the pons. As a result of research in which the brain stem is destroyed at different levels, two respiratory control centers have been identified in the pons. One area—the ap-neustic center—appears to promote inspiration by stimulating the I neurons in the medulla. The other area—the pneumotaxic center—seems to antagonize the apneustic center and inhibit inspiration (fig. 16.25).
■ Figure 16.25 Approximate locations of the brain stem respiratory centers. The rhythmicity center in the medulla oblongata directly controls breathing, but it receives input from the control centers in the pons and from chemoreceptors.
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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.