Expiration: breathing's other face

Area of Science:

• Evolutionary biology
• Comparative physiology
• Neuroscience

Background:

• The tetrapod aspiration pump evolved from simpler buccal-pharyngeal pumps in fish and amphibians.
• Early evolution likely involved the development of active expiration before active inspiration.

Purpose of the Study:

• To investigate the evolutionary sequence of active expiration and inspiration in vertebrate respiration.
• To understand the neural mechanisms and evolutionary origins of the respiratory control system.

Main Methods:

• Comparative analysis of respiratory control mechanisms across vertebrate groups.
• Examination of neural oscillator reconfiguration (parafacial oscillator/pFRG/RTN and paravagal oscillator/preBötC).

Main Results:

• The parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN) was reconfigured for active expiration, followed by the pre-BötC for active inspiration.
• Reptiles and birds exhibit a four-phase cycle initiated by the pFRG/RTN: active inspiration, glottal closure, pause, and active expiration.
• Mammalian resting breathing suppresses active expiration, featuring airway constriction and diaphragmatic braking, but active expiration emerges with increased respiratory drive.

Conclusions:

• The evolution of vertebrate breathing involved a sequential development of neural circuits for active expiration and inspiration.
• The pFRG/RTN plays a key role in initiating the respiratory cycle, particularly in reptiles and during elevated drive in mammals.
• The recruitment of active expiration in mammals can serve to increase tidal volume or breathing frequency, with mechanisms for its placement in the cycle still under investigation.