Noise-enhanced heart rate and sympathetic nerve responses to oscillatory lower body negative pressure in humans

By injecting noise into the carotid sinus baroreceptors, we previously show ed that heart rate (HR) responses to weak oscillatory tilt were enhanced vi a a mechanism known as "stochastic resonance." It remains unclear, however, whether the same responses would be observed when using oscillatory lower body negative pressure (LBNP), which would unload the cardiopulmonary baror eceptors with physically negligible effects on the arterial system. Also, t he vasomotor sympathetic activity directly controlling peripheral resistanc e against hypotensive stimuli was not observed. We therefore investigated t he effects of weak (0 to approximately -10 mmHg) oscillatory (0.03 Hz) LBNP on HR and muscle sympathetic nerve activity (MSNA) while adding incrementa l noise to the carotid sinus baroreceptors via a pneumatic neck chamber. Th e signal-to-noise ratio of HR, cardiac interbeat interval, and total MSNA w ere all significantly improved by increasing noise intensity, while there w as no significant change in the arterial blood pressure in synchronized wit h the oscillatory LBNP. We conclude that the stochastic resonance, affectin g both HR and MSNA, results from the interaction of noise with the signal i n the brain stem, where the neuronal inputs from the arterial and cardiopul monary baroreceptors first come together in the nucleus tractus solitarius. Also, these results indicate that the noise could induce functional improv ement in human blood pressure regulatory system in overcoming given hypoten sive stimuli.