Central Nervous Resetting of Baroreceptor Reflexes

Summary Stimulus‐response curves were derived in unanaesthetized rabbits relating menu arterial pressure to heart period (pulse interval). Arterial and atrial pressure were altered by graded inflation of balloons placed around the upper abdominal aorta and inferior vena cava. The stimulus‐response curves differed significantly between sham‐operated, thalamic and pontine rabbits, indicating that suprapontine as well as bulbar heart rate centres normally participated in reflexes arising from circulatory baroreceptors. The stimulus‐response curves were also used to determine whether generalized disturbances acted independently of the input from the baroreceptors on a distinctive part of the heart rate motoneurone pool, or whether they interacted with the baroreceptor input because of convergence on to common autonomic motoneurones. Vagal afferents from lung inflation receptors acted independently of the baroreceptor input on cardiac sympathetic motoneurones. Chemoreceptor stimulation produced by severe arterial hypoxia acted on a distinctive section of the motoneurone pool, but also interacted with the baroreceptor input on common motoneurone sites. This interaction increased the threshold to baroreceptor stimuli in both sham‐operated and pontine animals, thus altering baroreceptor reflex performance from regulator to servomechanism action. Because of this type of interaction, the reflex response to multiple inputs is not the same as the sum of individual reflex effects studied under controlled conditions.