How does the medulla control respiration?
The medulla plays a vital role in controlling respiration, specifically maintaining a regular breathing rhythm and adjusting breathing patterns in response to various conditions. Here's an overview of how the medulla controls respiration:
1. Respiratory Rhythm Generation: The medulla contains a group of neurons known as the respiratory center or respiratory rhythm generator. These neurons generate the basic rhythm of breathing. The respiratory center consists of two clusters of neurons:
- Dorsal Respiratory Group (DRG): Located in the dorsal part of the medulla, the DRG is responsible for inspiration (inhaling). It sends signals to the diaphragm and intercostal muscles, causing them to contract and draw air into the lungs.
- Ventral Respiratory Group (VRG): Situated in the ventral part of the medulla, the VRG controls expiration (exhaling). When active, the VRG inhibits inspiration and promotes relaxation of the diaphragm and intercostal muscles, allowing air to be expelled from the lungs.
2. Inspiration: During normal quiet breathing, inspiration is primarily driven by the DRG. It sends signals to the phrenic nerves, which innervate the diaphragm, the main muscle of respiration. These signals cause the diaphragm to contract and move downward, increasing the volume of the thoracic cavity. Simultaneously, the intercostal muscles between the ribs also contract, further expanding the chest. As a result, air is drawn into the lungs.
3. Expiration: At rest, expiration is a passive process that occurs due to the recoil of the lungs and chest wall. As the diaphragm and intercostal muscles relax, the elastic recoil of the lungs pulls them back to their original shape, pushing air out. The VRG plays a role in modulating expiration during deeper breaths or increased respiratory demand by actively inhibiting inspiration.
4. Central Chemoreception: The medulla contains chemoreceptors that monitor the levels of carbon dioxide and hydrogen ions (pH) in the cerebrospinal fluid (CSF). These chemoreceptors detect changes in the chemical environment and send signals to the respiratory center. An increase in carbon dioxide or hydrogen ions stimulates the respiratory center, leading to an increased rate and depth of breathing.
5. Reflex Control: The medulla also integrates signals from various receptors and reflexes involved in the regulation of respiration, such as:
- Baroreceptors: Located in the carotid arteries and aortic arch, baroreceptors sense changes in blood pressure. Increased blood pressure can lead to a reflex decrease in heart rate and respiratory rate.
- Lung Stretch Receptors: Located in the airways, these receptors sense lung inflation and initiate reflexes that inhibit further inspiration and promote expiration.
- Hering-Breuer Reflex: This reflex is triggered by excessive lung inflation. When the lungs are overstretched, it activates vagal sensory receptors, sending signals to the medulla that inhibit inspiration and cause expiration.
6. Integration with Higher Brain Centers: The medulla receives signals from higher brain centers, such as the pons and cerebral cortex. These signals can modify the rhythm and pattern of breathing in response to voluntary control, emotions, and behavioral state.
Overall, the medulla acts as a central control center for respiration, generating the basic breathing rhythm, responding to chemical changes in the body, integrating reflex inputs, and coordinating with higher brain centers to ensure proper ventilation.