Vagal afferent activity increases dynamical dimension of respiration in rats

Geometric and numerical techniques from nonlinear dynamics are employed to characterize the respiratory patterns of adult rats. In phase space constructions from volume measurements, the respiratory behavior of the vagotomized rat is seen to be unimodal and periodic, whereas that of the vagi-intact animal is bimodal and sensitive to initial conditions and displays stretching and folding of trajectories. By use of the correlation integral, the dimension and entropy were estimated for the respiratory patterns of five urethan-anesthetized rats (before and after vagotomy) and two awake animals (resting, vagi intact only). The results indicate that the central respiratory pattern of the anesthetized vagotomized rat behaves primarily as a single degree-of-freedom oscillator, which can be moved into a regime of low-order chaos in the presence of feedback from pulmonary stretch receptors, contributing significantly to the variability observed in the respiratory pattern of the vagi-intact animal. A model designed to emulate the Hering-Breuer inflation reflex demonstrates that the inspiration-inhibiting action of the reflex interacting with the oscillator at a critical phase transition is sufficient to drive the central pattern generator output into low-order chaos.