Generation of Rhythmic Breathing
The medullary respiratory center generates the basic pattern of spontaneous, rhythmic breathing. Although the precise mecha-nism is not well understood, the generation of rhythmic breathing involves the integration of stimuli that start and stop inspiration.
1. Starting inspiration. The neurons in the medullary respiratorycenter that promote inspiration are continuously active. The medullary respiratory center constantly receives stimulation from many sources, such as receptors that monitor blood gas levels and the movements of muscles and joints. In addition, stimulation can come from parts of the brain concerned with voluntary respiratory movements and emotions. When the inputs from all these sources reach a threshold level, somatic nervous system neurons stimulate respiratory muscles via action potentials, and inspiration starts.
2. Increasing inspiration. Once inspiration begins, more andmore neurons are activated. The result is progressively stronger stimulation of the respiratory muscles, which lasts for approximately 2 seconds (s).
3. Stopping inspiration. The neurons stimulating the musclesof respiration also stimulate the neurons in the medullary respiratory center that are responsible for stopping inspiration. The neurons responsible for stopping inspiration also receive input from the pontine respiratory neurons, stretch receptors in the lungs, and probably other sources. When the inputs to these neurons exceed a threshold level, they cause the neurons stimulating respiratory muscles to be inhibited. Relaxation of respiratory muscles results in expiration, which lasts approximately 3 s. The next inspiration begins with step 1.
Although the medullary neurons establish the basic rate and depth of breathing, their activities can be influenced by input from other parts of the brain and from peripherally located receptors.