Respiratory rhythm

Ventilation of the lungs is controlled by a network of medullary respiratory neurons within the lower brainstem ( Richter.eiaf 1992). In simple terms, this network is an active center that augments and diminishes inspiratory activity which is periodically interrupted during active expiration. The two phases of ventilatory mechanics are controlled by three neural phases: inspiration (I phase), postinspiration (PI phase, passive expiration), and stage 2 expiration (E2 phase, active expiration).

Rhythmic nervous output from the respiratory center is fed to spinal motoneurons and leads to periodic contraction of thoracic, abdominal, and diaphragmatic muscles. Breathing can be voluntarily interrupted (for speech, singing, sniffing, coughing, etc.); reflex interferences induce sneezing, swallowing, and reflex coughing. Changes in arterial blood gases may alter the pattern of respiration; for example voluntary hyperventilation is followed by periods of apnea.

Active expiratory phase activities can be totally blocked during rapid shallow breathing or panting. Respiratory center

The bilateral respiratory center is localized within the pre-Botzinger complex, which is located in the ventral medulla ( BianchLef a/; 1995). This center produces rhythmic activity which to some extent is independent of peripheral feedback. The same region also contains the presumed central chemosensitivity. Medullary respiratory neurons are apparently not endogenously active in humans; their rhythmic activity seems to originate from synaptic interaction between neurons. Obviously, there is a separate gasping center within the caudal medulla.

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