Breathing is an automatic process controlled deep within the brainstem. The rhythm of inhalation and exhalation is set by a collection of nerve cells called the Dorsal Respiratory Group (DRG). The DRG acts as the body’s primary control center for the automatic initiation of breathing. Its activity ensures a reliable breathing pattern, meeting the body’s constant need for oxygen.
Where the DRG is Located
The Dorsal Respiratory Group is situated within the medulla oblongata, the lowest part of the brainstem. It forms an elongated mass of neurons located near the nucleus of the solitary tract, a major receiving center for sensory information from the body’s organs. The DRG is positioned to integrate incoming signals related to respiratory needs.
The DRG is composed primarily of inspiratory neurons. These cells send their projections down the spinal cord to the muscles responsible for expanding the chest cavity. This action makes the DRG the command center for the mechanical action of taking a breath.
Generating the Basic Breathing Rhythm
The core function of the DRG is to establish the rhythmic pattern of breathing, known as eupnea. It achieves this by sending electrical impulses to the inspiratory muscles. In a typical resting breath, DRG neurons fire signals for approximately two seconds, followed by three seconds of inactivity.
The intensity of the DRG signal increases gradually over the two-second period, a pattern known as the “ramp signal.” This ramp-up of activity is transmitted down the phrenic and intercostal nerves to the diaphragm and external intercostal muscles. The smooth, increasing signal causes a slow, steady contraction, allowing the lungs to fill with air gradually instead of with sudden, jerky gasps.
When the DRG neurons abruptly stop firing, the inspiratory muscles immediately relax. This cessation of the signal marks the end of inhalation and initiates expiration. Under normal, quiet breathing conditions, exhalation is a passive process resulting from the elastic recoil of the lungs and chest wall.
Receiving and Interpreting Body Signals
The DRG acts as a processing center that modifies the breathing pattern based on information from the body. It receives sensory input via the glossopharyngeal (Cranial Nerve IX) and vagus (Cranial Nerve X) nerves. These nerves carry information about the body’s chemical and physical state.
Chemoreceptors in the carotid and aortic bodies monitor blood chemistry, including oxygen, carbon dioxide, and pH levels. If carbon dioxide levels rise, these receptors signal the DRG. The DRG responds by increasing the frequency and intensity of the ramp signal, leading to deeper and faster breaths to expel the excess carbon dioxide.
The DRG also receives input from stretch receptors within the walls of the lungs. These mechanoreceptors send signals to the DRG as the lungs inflate. When the lungs are sufficiently stretched, the DRG signal is prematurely inhibited, immediately ending inhalation and preventing over-inflation.