The respiratory areas of the medulla oblongata and pons are influenced by a number of factors that cause modifications in the rate and depth of breathing. Factors involved in involuntary control are detected by sensory receptors, which forward nerve impulses to the DRG. Higher brain centers involved in voluntary control send nerve impulses to the pontine respiratory group, which then transmits nerve impulses to the respiratory rhythmicity center.
The most important chemical factors affecting respiration are the concentrations of CO2, H+, and O2 in the blood or cerebrospinal fluid. Sensory receptors that are sensitive to these factors are called chemoreceptors. The chemoreceptors in the medulla oblongata are sensitive to increases in H+ and CO2 in the cerebrospinal fluid. The chemoreceptors in the carotid bodies and aortic bodies are sensitive to changes in CO2, H+, and O2 , The carotid bodies are located in the walls of external carotid arteries, while aortic bodies are located in the aortic arch. You can see that they are strategically located, especially to monitor blood going to the brain.
You may wonder why the concentration of H+ is involved in respiratory control. The mechanism for transporting CO2 in the blood releases H+ as a by-product. Therefore, an increase in CO2 concentration produces an increase in the H+ concentration.
If the concentrations of CO2 and H+ in the blood or CSF increase, the DRG relays the information so that the VRG is stimulated to increase the rate and depth of breathing, which increases the rate of CO2 and H+ removal and returns their concentrations to normal resting levels. Once homeostasis is restored, the rate and depth of breathing also return to normal quiet levels.
If the CO2 and H+ concentrations in the blood or cerebrospinal fluid are abnormally low, breathing is slow and shallow until their concentrations increase to normal levels.
As mentioned previously, only the chemoreceptors in the carotid and aortic bodies are sensitive to changes in blood O2 levels, specifically to a decline in blood O2 concentration. Usually, a drop in O2 concentration is not a strong stimulus for increasing the rate and depth of breathing, and its main effect seems to be to increase the sensitivity of chemoreceptors to changes in the CO2 concentration.
Baroreceptors (stretch receptors) in the bronchi, bronchioles, and visceral pleurae are sensitive to inflation of the lungs. During inspiration, nerve impulses from the stretch receptors are sent to the DRG via the vagus nerves (CN X), where they inhibit the formation of nerve impulses causing inspiration. This promotes expiration and prevents excessively deep inspirations that may damage the lungs.
The respiratory tract contains irritant receptors that are sensitive to various chemical and physical irritants, such as smoke, dust, and excess amounts of mucus. When stimulated by irritants, these receptors send sensory nerve impulses to the DRG via the vagus nerves. The DRG then alters the function of the VRG, which triggers a reflex contraction of the respiratory muscles that leads to a sneeze or a cough in order to expel the irritants from the respiratory tract.
Nerve impulses from higher brain centers also alter the rhythmic cycle of breathing. These nerve impulses may be voluntarily generated in the cerebrum, as when a person chooses to alter the normal pattern of quiet breathing. However, these voluntary controls are limited. For example, if a little child tries to “punish” his mother by holding his breath, the nerve impulses from higher brain centers are ignored and involuntary breathing resumes once CO2 level in his blood increases to a critical point.
Involuntary nerve impulses may be formed by higher brain centers in the cerebral cortex and the hypothalamus during emotional experiences, such as anxiety, fear, and excitement, which activate the autonomic nervous system. At such times, the breathing rate is increased. Similarly, a sudden emotional experience, or a sharp pain tends to momentarily stop breathing, a condition called apnea.
An increase in body temperature, such as occurs during strenuous exercise or a fever, increases the breathing rate. Conversely, a decrease in body temperature decreases the breathing rate.