Vestibular modulation of muscle sympathetic nerve activity assessed over a 100-fold frequency range of sinusoidal galvanic vestibular stimulation

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered at 0.2-2.0 Hz, evokes a partial entrainment of muscle sympathetic nerve activity (MSNA). Moreover, at lower frequencies of stimulation (0.08-0.18 Hz) sGVS produces two peaks of modulation: one (primary) peak associated with the positive peak of the sinusoidal stimulus and a smaller (secondary) peak associated with the trough. Here, we assessed whether sGVS delivered at 0.05 Hz causes a more marked modulation of MSNA than at higher frequencies, and tested the hypothesis that the primary and secondary peaks are of identical amplitude because of the longer cycle length. MSNA was recorded via tungsten microelectrodes inserted into the left peroneal nerve in 11 seated subjects. Bipolar binaural sGVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.05, 0.5 and 5.0 Hz (500 cycles). Cross-correlation analysis revealed two bursts of modulation of MSNA for each cycle at 0.05 and 0.5 Hz, but only one at 5 Hz. There was a significant inverse linear relationship between vestibular modulation (primary peak) and frequency (p<0.0001), with the amplitudes of the peaks being highest at 0.05 Hz. Moreover, the secondary peak at this frequency was not significantly different from that of the primary peak. These results indicate that vestibular modulation of MSNA operates over a large range of frequencies, but is greater at lower frequencies of sGVS. We conclude that the vestibular apparatus, through its influence on muscle sympathetic outflow via, preferentially contributes to the control of blood pressure at low frequencies.