What triggers the medullary respiratory center to initiate expiration?

The initiation of expiration is primarily triggered by the activation of the ventral respiratory group (VRG) in the medullary respiratory center. The VRG is crucial for generating rhythmic patterns of breathing, and during expiration, it plays a key role in inhibiting inspiratory neurons and promoting the relaxation of respiratory muscles.

When the VRG is activated, it signals the diaphragm and intercostal muscles to cease contraction, allowing for passive expiration due to the elastic recoil of the lungs. This process is essential for normal breathing cycles where expiration does not require significant muscular effort under resting conditions.

Other factors that can influence the respiratory centers include hypoxemia or high arterial CO2 levels, which can stimulate ventilation but do not directly trigger the medullary respiratory center to start the process of expiration itself. Instead, these stimuli are more related to the overall drive to breathe and adjust the rate and depth of breathing rather than the specific initiation of the expiratory phase.