The medulla oblongata, located in the brainstem, acts as the body’s primary, involuntary control center for respiration. It ensures that breathing occurs automatically, maintaining the steady rhythm that continues even when we are asleep or unconscious. The respiratory control system constantly regulates the exchange of oxygen and carbon dioxide, allowing the body to meet its metabolic demands. This automatic regulation fine-tunes the breathing pattern based on various internal and external signals.
Identifying the Respiratory Control Groups
The medulla oblongata contains two bilaterally paired clusters of neurons, known as the medullary respiratory centers, which organize the breathing process. The Dorsal Respiratory Group (DRG) is located near the central part of the medulla. Its neurons are mainly active during inspiration, initiating the breathing cycle. The DRG acts as the main receiving center for sensory information, taking input from chemoreceptors and mechanoreceptors throughout the body.
The Ventral Respiratory Group (VRG) is an elongated column of neurons positioned in the ventrolateral portion of the medulla. The VRG contains a mix of both inspiratory and expiratory neurons, but it is largely inactive during quiet breathing. Its primary function becomes apparent during periods of increased demand, such as exercise, when forced inhalation and exhalation are required. Within the VRG lies the Pre-Bötzinger Complex, a collection of cells considered the kernel of the respiratory rhythm.
Generating the Involuntary Breathing Rhythm
The spontaneous, inherent rhythm of breathing originates within the Pre-Bötzinger Complex (Pre-BötC) of the VRG, which acts as the central pattern generator for respiration. This region is composed of a network of neurons that possess intrinsic pacemaker properties. They fire rhythmic bursts of electrical activity without external stimulation, creating the basic template for the inspiratory drive.
The cyclical pattern begins when the Pre-BötC neurons fire, sending signals to activate the inspiratory neurons in the DRG. The DRG neurons then transmit a signal that steadily increases in intensity, known as the inspiratory ramp signal. This signal causes the inspiratory muscles to contract gradually, leading to smooth inhalation. Once the active phase of inspiration is complete, the DRG neurons abruptly stop firing, known as the “inspiratory off-switch.”
During quiet breathing, the cessation of the inspiratory signal leads to a passive process of exhalation. The diaphragm and external intercostal muscles relax, and the elastic recoil of the lungs and chest wall pushes the air out. If the body requires more air movement, the VRG’s expiratory neurons become active. These neurons send signals to accessory muscles to force air out, transitioning the breath from passive to active expiration.
Adjusting Breathing Based on Body Needs
While the Pre-BötC generates the fundamental rhythm, the medulla constantly modifies it to ensure the body’s gases remain balanced. The most important sensors for this adjustment are the chemoreceptors, which monitor the chemical composition of the blood and cerebrospinal fluid. Central chemoreceptors are located on the ventrolateral surface of the medulla, making them sensitive to changes in the acidity (pH) of the cerebrospinal fluid.
Carbon dioxide (\(\text{CO}_2\)) easily diffuses from the blood into the cerebrospinal fluid, where it converts to carbonic acid and releases hydrogen ions (\(\text{H}^+\)). An increase in \(\text{CO}_2\) concentration causes a drop in pH near the central chemoreceptors, which strongly stimulates the medullary respiratory centers. This stimulation immediately increases both the rate and depth of breathing, blowing off the excess \(\text{CO}_2\) to restore balance.
Peripheral chemoreceptors, located in the carotid bodies and aortic bodies, also play a modifying role. These receptors monitor oxygen (\(\text{O}_2\)) levels in the arterial blood, making them important during severe oxygen deprivation (hypoxia). They also respond to changes in \(\text{CO}_2\) and pH, sending their signals via the glossopharyngeal and vagus nerves directly to the DRG for processing.
Further fine-tuning is accomplished by the Pontine Respiratory Group, located superior to the medulla in the pons. This group, which includes the Pneumotaxic and Apneustic centers, acts to smooth the transition between inspiration and expiration. The Pneumotaxic center sends inhibitory signals to the inspiratory neurons, limiting the duration of inhalation. The medulla also receives inputs from stretch receptors in the lungs, which signal when the lungs are sufficiently inflated.
Delivering the Signal to Breathing Muscles
The final step in the medulla’s control of breathing is the transmission of the generated rhythm to the muscles that execute the action. Inspiratory signals originating from the DRG and VRG are sent down the spinal cord to the motor neurons that control the primary muscles of respiration. The efferent pathway is the phrenic nerve, which originates from the cervical spinal segments C3, C4, and C5.
The phrenic nerve provides the sole motor innervation to the diaphragm, which is the dome-shaped muscle responsible for about 75 percent of the air moved during quiet breathing. When the medullary signal reaches the diaphragm, the muscle contracts and flattens, increasing the volume of the chest cavity and drawing air into the lungs.
Signals are also sent via intercostal nerves to the external intercostal muscles, which assist by lifting the ribs and expanding the chest. This process completes the circuit, transforming the rhythmic electrical pulses of the medulla into the physical act of breathing.