Physiology of Respiration II: Neurogenic Control of Respiration

The brainstem contains two primary respiratory centers: the medullary respiratory center in the medulla oblongata and the pontine respiratory center in the pons. The medullary center comprises the dorsal respiratory group (DRG) and ventral respiratory group (VRG), which initiate the basic breathing rhythm. The pontine center modulates medullary activity to refine respiratory rate and depth during vocalization, sleep, and exercise.

The dorsal respiratory group transmits impulses through the phrenic and intercostal nerves, stimulating the diaphragm and external intercostal muscles to contract and induce inhalation. After approximately two seconds of DRG activity, inactivity of these neurons relaxes the muscles, leading to exhalation and completing the basic breathing cycle.

The ventral respiratory group generates the breathing rhythm through its pre-Bötzinger complex, regulating the rate at which dorsal respiratory group neurons fire action potentials. Additional VRG neurons control forceful breathing by managing accessory breathing muscle movements during high respiratory demand, such as exercise or increased metabolic activity.

Chemoreceptors monitor blood chemistry changes, particularly carbon dioxide, oxygen, and pH levels. Central chemoreceptors in the medulla respond to CO2 and cerebrospinal fluid pH, while peripheral chemoreceptors in the aortic and carotid bodies adjust breathing rate and depth to maintain chemical balance and support optimal gas exchange.

The Hering-Breuer reflex is activated by stretch receptors located in the lungs, airways, chest wall, and diaphragm. These mechanoreceptors prevent lung over-expansion by signaling the respiratory centers when lungs reach maximum inflation, triggering exhalation and protecting respiratory tissue from damage during breathing.

Mechanoreceptors throughout the respiratory system detect airway irritation, constriction, muscular effort, and posture changes, then relay this information to respiratory centers. These sensory inputs influence breathing patterns and help the respiratory system adapt to environmental conditions and physical demands, ensuring appropriate ventilation responses.

During exercise, chemoreceptors detect increased carbon dioxide and decreased oxygen in the blood, signaling respiratory centers to enhance ventilation. The integration of central and peripheral receptor signals allows respiratory control centers to optimize gas exchange in response to metabolic demand variations, maintaining internal balance.