12.8: Control of Breathing

Respiratory Rate and Control of Ventilation

Breathing typically occurs involuntarily, but there are instances where one can consciously control it, like when swimming underwater, singing, or blowing bubbles. The rate of breathing is governed by respiratory centers situated in the pons and medulla oblongata, primarily responding to alterations in carbon dioxide, oxygen, and blood pH levels. 

A child's normal breathing rate undergoes changes from birth to adolescence. An infant might breathe 30 to 60 times per minute, while a 10-year-old might average 18 to 30 breaths per minute. As they reach adolescence, their normal breathing rate aligns more closely with that of adults, around 12 to 18 breaths per minute.

Neural Control of Ventilation

The regulation of ventilation involves an intricate coordination among various brain regions, which signal the muscles responsible for pulmonary ventilation to contract (Figure 36, Table 6). This coordination typically leads to a regular and rhythmic pattern of breathing, ensuring adequate intake of oxygen and effective removal of carbon dioxide from the body.

Figure 35 | Neural Control of Respiration |⁶⁰ This illustration depicts the hierarchical neural control of respiration, highlighting the key brainstem centers and their connections to respiratory muscles. At the top, a simplified brain diagram shows the location of the pons and medulla oblongata, the primary regions involved in respiratory regulation. An inset image provides a magnified view of these brainstem structures. Within the pons, the pneumotaxic center and apneustic center, collectively known as the pontine respiratory group (PRG), are shown. The PRG modulates respiratory rhythm and depth. In the medulla oblongata, the dorsal respiratory group (DRG) and the ventral respiratory group (VRG) are depicted. The DRG primarily controls inspiration, while the VRG is involved in both inspiration and expiration, especially during forceful breathing. Descending pathways from these brainstem centers are illustrated, showing their connections to the respiratory muscles. These include the diaphragm, the primary muscle of inspiration, as well as the external and internal intercostal muscles, which assist in breathing by expanding and contracting the rib cage. Accessory respiratory muscles, shown on the left, are recruited during increased respiratory demand. The red lines indicate the neural pathways, with (+) symbols denoting excitatory signals, indicating that these pathways stimulate muscle contraction. The inset shows inhibitory signals (-) indicating that the pontine respiratory group modulates the medullary respiratory centers.

Neurons that innervate the respiratory muscles play a crucial role in controlling and regulating pulmonary ventilation. The primary brain centers responsible for this process are located in the medulla oblongata and the pontine respiratory group (Figure 35). The medulla oblongata contains the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). The DRG helps maintain a steady breathing rhythm by stimulating the diaphragm and intercostal muscles to contract during inhalation. When DRG activity decreases, it ceases to prompt these muscles, allowing them to relax, thereby causing exhalation. DRG initiates breathing through intrinsic nerve activity, acting like a pacemaker for respiration. It generates a ramp signal, which begins with a weak impulse that gradually strengthens in a ramp-like manner over approximately 2 seconds. This steady increase in nerve activity stimulates the diaphragm to contract and initiate inhalation. After 2 seconds, the ramp signal ceases abruptly for about 3 seconds, which stops diaphragm excitation and allows the lungs to undergo passive elastic recoil, leading to exhalation. Following this period, a new inspiratory cycle begins as the DRG reactivates, maintaining a rhythmic breathing pattern. In contrast, the VRG is involved in forced breathing. Neurons within the VRG stimulate the accessory muscles for forced inhalation and exhalation, facilitating these specific breathing actions.

The pontine respiratory group includes the apneustic and pneumotaxic centers, essential parts of the breathing control mechanism. The apneustic center, a dual cluster of neuronal cell bodies, stimulates the neurons in the DRG, regulating the depth of inspiration, particularly for deeper breaths. On the other hand, the pneumotaxic center, comprising a network of neurons, suppresses the activity of the neurons in the DRG. It facilitates the relaxation phase after inspiration, thereby managing the overall rate of breathing.

The respiratory system’s neural connections are organized into afferent and efferent pathways that facilitate breathing regulation. Afferent pathways carry sensory signals to the respiratory centers from various receptors. Chemoreceptor and baroreceptor signals travel through cranial nerves IX and X, while signals from thermoreceptors, pain receptors, and proprioceptors also contribute to respiratory adjustments based on temperature, pain, and body position. Efferent pathways carry motor signals from the brain to the respiratory muscles. These signals descend through the anterior portion of the lateral column of the spinal cord, reaching the cervical and thoracic segments. The phrenic nerve, originating from spinal levels C3 to C5, innervates the diaphragm, and the intercostal nerves, from T1 to T11, supply the intercostal muscles. Together, these pathways coordinate respiratory muscle activity to maintain effective breathing.¹⁰

The brain centers respond to systemic stimuli like carbon dioxide, oxygen, and pH levels in the blood. The primary stimulant for the medulla oblongata and pons is carbon dioxide concentration, not oxygen. Chemoreceptors, namely the central and peripheral receptors, sense these chemical changes. Central chemoreceptors are located on the ventrolateral surface of the medulla oblongata. They are sensitive primarily to changes in the pH of the cerebrospinal fluid, which reflects carbon dioxide levels in the blood. When blood carbon dioxide levels rise, it diffuses across the blood-brain barrier into the CSF, where it forms hydrogen ions and lowers the pH. This decrease in pH stimulates the central chemoreceptors to increase the respiratory rate and depth, helping to expel excess carbon dioxide and restore pH balance. Conversely, decreased carbon dioxide in the blood causes decreased hydrogen ions, resulting in shallower and slower breathing. Furthermore, peripheral chemoreceptors located in the aortic arch and carotid arteries are sensitive to hydrogen ion levels in the blood, adjusting ventilation to reduce carbon dioxide levels and balance hydrogen ions, thus regulating pH. They also sense drastic changes in dissolved oxygen. A significant drop in dissolved oxygen triggers these receptors to increase the respiratory rate. However, most oxygen is carried by hemoglobin, and these receptors sense dissolved oxygen only.

The peripheral chemoreceptors that are situated in the carotid bodies and aortic bodies, respond differently to changes in blood gases. They respond to decreased PaO2 and changes in pH. Moreover, the hypoxic drive originating from the peripheral chemoreceptors is crucial in advanced hypoxic states, where these receptors sustain respiration when other respiratory centers are unresponsive to direct chemical stimulation. Additionally, various brain regions, such as the hypothalamus and the limbic system, interact with the respiratory centers. The hypothalamus and other limbic regions influence breathing rates in response to emotions, pain, and changes in body temperature. For example, increased body temperature often leads to heightened respiratory rates. Feelings of excitement or fight-or-flight responses also prompt increased breathing rates. In chronic acidosis, the pH of cerebrospinal fluid is maintained close to normal through adjustments in bicarbonate ion concentrations, regulated by an ion pump at the blood-brain barrier.