Aortic and inferior vena caval balloons were used to alter mean arterial blood pressure, pulse pressure (PP), and right atrial pressure (RAP) in unanesthetized rabbits and to reflexly evoke changes in heart period (pulse interval). Curves relating mean arterial blood pressure to heart period were compared in different groups of rabbits at similar APP and ARAP. Median blood pressure (BP 00 ), average gain (G), and heart period range (maximum to minimum heart period) were calculated from the

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... ped curves. The reflex was evoked from arterial baroreceptors and probably from cardiac and pulmonary baroreceptors. Curves relating mean arterial blood pressure and heart period differed with regard to BP S0 and G in sham-operated, thalamic, and pontine rabbits, indicating that suprabulbar centers normally play a role in the reflex. Curves from sham-operated and pontine rabbits treated with atropine also differed, suggesting suprabulbar control of sympathetic effectors. In intact rabbits, forebrain and diencephalic centers caused vagal and sympathetic effectors to respond over the same arterial blood pressure range, but, in pontine rabbits, the effectors responded over dissimilar ranges. In intact rabbits, changes in mean arterial blood pressure evoked reciprocal and nearly equal changes in vagal and sympathetic effectors, but, in pontine rabbits, a given pressure change altered heart period predominantly through one effector. In sham-operated rabbits, vagal effects on heart period were lower by a constant amount at every level of mean arterial blood pressure than they were in pontine rabbits, suggesting that suprabulbar centers exerted a tonic inhibitory effect on vagal motoneurons not involved in the reflex. KEY WORDS cardiac sympathetic receptors aortic nerves suprabulbar circulatory centers carotid sinus nerves vagal efferents pontomedullary centers vagal afferents closed-loop analysis From the