Distinct generators for aspiration and expiration reflexes: localization, mechanisms and effects

Re-evaluation of our earlier c-Fos-like immuno-reactive studies and brainstem transection/lesion experiments in over 40 anaesthetized, non-paralyzed cats allowed comparison of two distinct airway defensive reflexes with the distinct generators for inspiration (I) and expiration (E), described recently in juvenile rats. The spiration reflex (AspR) is characterized by solitary rapid and strong inspiratory effort with a reciprocal inhibition, preventing a subsequent active expiration, while the expiration reflex (ExpR) manifests by rapid and strong expiratory effort, starting without a preceding, inspiration, or reciprocal inhibition of occasional spontaneous inspiration. The retro-trapezoid nucleus/parafacial respiratory group neurones described as the distinct generator for active E in rats, are activated also during the ExpR in adult cats. Brainstem transection 5 mm above the obex eliminates the E generator and the ExpR, but preserves the I generator located in the pre-Bötzinger Complex, and also the AspR. This suggests the existence of a distinct I generator in cats as well as rats, and its contribution to the generation of the AspR. Persistence of the AspR in adult cats during asphyxic gasping, their similar character and the strong activation of I neurones at many places in the medulla and pons, suggest a common brainstem neuronal circuit contributing to generation of both the gasping and the gasp-like AspR. That the AspR and ExpR have distinct multilevel brainstem control mechanisms supports the dual theory of control and provides unique models for testing respiratory rhythm and pattern generation. The AspR may be compared with the powerful "auto-resuscitation effects of asphyxic gasping"; the ExpR may underly the effectiveness of the laryngeal chemoreflexes in prevention of lung diseases.