Competitive interactions between vestibular and cardiac rhythms in the modulation of muscle sympathetic nerve activity.

We tested the hypothesis that vestibular and cardiac rhythms compete to modulate muscle sympathetic nerve activity (MSNA) in human subjects. Sinusoidal galvanic vestibular stimulation was applied across the mastoid processes at each subject's cardiac frequency and at ±0.1, ±0.2, ±0.3 and ±0.6 Hz. Cyclic modulation of MSNA was weakest at this central frequency (44.8±2.3%; n=8); significantly lower than when delivered 0.1 Hz lower (57.7±3.3%) or 0.1 Hz higher (56.3±3.3%) than this frequency. We conclude that vestibular inputs compete with baroreceptor inputs operating at the cardiac rhythm, with vestibular modulation of MSNA being lowest when competition with the baroreceptors is highest.