Evolutionary trends in respiratory mechanisms.

As we progress through the vertebrate phyla we see a number of changes in the functional morphology of the respiratory system that offer insights into the physiological systems that control them. We see a switch from a buccal pump powered by branchiomeric and hypobranchial muscles innervated by cranial nerves to a thoraco-abdominal aspiration pump powered by axial muscles innervated by spinal nerves with pre-motor neurons situated in the ventral respiratory column. The initial steps in the evolution of air breathing were a behavioural commitment to surface and changes in valving of the mouth/spiracle/nares and the operculum and glottis (or their equivalents) (i.e., changes in the activation of the muscles dilating and/or constricting various openings). These allowed the production of water breaths versus deflation or inflation of the air-breathing organ. Changes in the respiratory pump muscles evolved later. While highly speculative, it is suggested that these three independent valving circuits may have arisen in association with different pairs of segmental rhythm generators, and that all circuits continue to work together in a coordinated fashion to produce all types of breaths (including eupneic breaths and gasps).