Stimulation of the carotid sinus nerve causes an increase in inspiratory (I) and expiratory (E) neural activities. If central respiratory oscillation is generated by an attractor-cycle process, an increase in its activity can be caused by a centrifugal perturbation of state. We evaluated this hypothesis by comparing the respiratory oscillator's phase responses to carotid sinus nerve stimulations in cats to the phase responses of an attractor-cycle oscillator, the Bonhoeffer-van der Pol (BvP) equations, subjected to centrifugal perturbations. 2. We recorded phrenic activity in seven anaesthetized, vagotomized, glomectomized, paralysed and servo-ventilated cats. Carotid sinus nerve (CSN) stimulation with 0.5-0.8 s electrical pulse trains increased the immediate cycle period and delayed the onset of breaths after stimulation in a highly predictable manner, with the exception that strong stimuli (25 Hz, 0.25-0.90 V) caused unpredictable responses when given at the I-E or the E-I transitions. The resetting plots exhibited focal gaps corresponding to these unpredictable responses, and the size of the gaps increased with increases in the strength of CSN stimulation. Type 0 resetting was not achieved despite the large perturbations in rhythm induced by CSN stimulation. 3. Centrifugal perturbations of the BvP oscillator resulted in phase responses which were similar to those found in the animal experiments. The BvP cycle had two critical phases at which phase resetting was highly irregular and neighbouring state trajectories were highly divergent. The resetting plots had focal gaps that increased in size with increases in the strength of perturbation. The gaps did not represent true discontinuity because at higher computational resolution the resetting plots appeared to be steep but smooth portions of topological Type 1 resetting curves. 4. These studies support the concept that brief carotid sinus nerve stimulations cause a transient outward displacement of the central respiratory state away from its attractor cycle, in contrast to the unidirectional displacements that accompany midbrain reticular or superior laryngeal nerve stimulations. The findings define particular geometrical relationships between oscillatory state trajectories of the rhythm generator and perturbed state trajectories induced by inputs to the oscillator. These relationships provide a framework for developing and testing the validity of neural models of the respiratory oscillator.
