PHASE RESETTING OF THE RESPIRATORY OSCILLATOR BY CAROTID-SINUS NERVE-STIMULATION IN CATS

1. Stimulation of the carotid sinus nerve causes an increase in inspir atory (I) and expiratory (E) neural activities. If central respiratory oscillation is generated by an attractor-cycle process, an increase i n its activity can be caused by a centrifugal perturbation of state. W e evaluated this hypothesis by comparing the respiratory oscillator's phase responses to carotid sinus nerve stimulations in cats to the pha se responses of an attractor-cycle oscillator, the Bonhoeffer-van der Pol (BvP) equations, subjected to centrifugal perturbations. 2. We rec orded phrenic activity in seven anaesthetized, vagotomized, glomectomi zed, paralysed and servo-ventilated cats. Carotid sinus nerve (CSN) st imulation with 0.5-0.8 s electrical pulse trains increased the immedia te cycle period and delayed the onset of breaths after stimulation in a highly predictable manner, with the exception that strong stimuli (2 5 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 cor responding to these unpredictable responses, and the size of the gaps increased with increases in the strength of CSN stimulation. Type 0 re setting was not achieved despite the large perturbations in rhythm ind uced by CSN stimulation. 3. Centrifugal perturbations of the BvP oscil lator resulted in phase responses which were similar to those found in the animal experiments. The BvP cycle had two critical phases at whic h phase resetting was highly irregular and neighbouring state trajecto ries were highly divergent. The resetting plots had focal gaps that in creased in size with increases in the strength of perturbation. The ga ps did not represent true discontinuity because at higher computationa l resolution the resetting plots appeared to be steep but smooth porti ons of topological Type 1. resetting curves. 4. These studies support the concept that brief carotid sinus nerve stimulations cause a transi ent outward displacement of the central respiratory state away from it s attractor cycle, in contrast to the unidirectional displacements tha t accompany midbrain reticular or superior laryngeal nerve stimulation s. The findings define particular geometrical relationships between os cillatory state trajectories of the rhythm generator and perturbed sta te trajectories induced by inputs to the oscillator. These relationshi ps provide a framework for developing and testing the validity of neur al models of the respiratory oscillator.