Final answer:
Negative-pressure ventilation occurs through the contraction of the diaphragm, controlled by the phrenic nerves. Without signals from these nerves, the diaphragm cannot contract, preventing the creation of negative pressure needed for inhalation.
Step-by-step explanation:
Negative-pressure ventilation occurs when there is a mechanical expansion of the thoracic cavity, which reduces the pressure in the chest cavity, allowing air to flow into the lungs during inspiration. The diaphragm plays a critical role in this process as it contracts and flattens, creating more space in the thoracic cavity. However, if the phrenic nerves stop sending messages to the diaphragm, this contraction will not occur, and negative pressure cannot be generated effectively. The phrenic nerves originate from the spinal cord at cervical levels 3 to 5 and control the rhythmic movements required for ventilation. Without these nerve impulses, the diaphragm cannot contract, and quiet breathing or eupnea—breathing at rest without active thought—is disrupted.
In forced breathing, or hyperpnea, which is often necessary during exercise or heavy physical activity, accessory muscles in addition to the diaphragm are used to facilitate both inspiration and expiration. But the principal mechanism of generating negative pressure within the thorax for quiet breathing is the contraction of the diaphragm stimulated by the phrenic nerves. Control of Breathing relies on this autonomic process, where the brain stem adjusts the rate of diaphragm contraction in response to levels of carbon dioxide in the blood.