Emerging concepts in lower vertebrate respiratory rhythm generation

Respiratory rhythm generators appear both evolutionarily and developmentally as paired segmental rhythm generators in the reticular formation, associated with the motor nuclei of cranial nerves V, VII, IX, X, and XII. At least two pairs of respiratory rhythm generators appear to remain in adult lampreys with the most rostral pair in the trigeminal region producing normal respiration. Cartilaginous and bony fishes possess a dual pump with rhythmic motor control of the buccal and opercular cavities by trigeminal and facial nerves and there is evidence both for multiple respiratory rhythm generators within the medulla, as well as a diffuse rostral‐caudal distribution of respiratory rhythm generating networks. In amphibians, while the basic respiratory pump remains the same, the dominant site of respiratory rhythm generation has been assumed by neurons in the vicinity of the glossopharyngeal/vagal and hypoglossal motor nuclei. In reptiles, birds and mammals, there is a switch to an aspiration pump driven by thoraco‐lumbar muscles innervated by spinal nerves and the critical sites necessary for respiratory rhythmogenesis now sit near the ponto‐medullary border, in the parafacial region (which may underlie expiratory‐dominated, intercostal‐abdominal breathing in non‐mammalian tetrapods) and in the PreBötzinger Complex (which may underlie inspiratory‐dominated diaphragmatic breathing in mammals). Supported by the NSERC of Canada.