Evidence from bilateral recordings of sympathetic nerve activity for lateralisation of vestibular contributions to cardiovascular control.

Using low-frequency (0.08-0.18 Hz) sinusoidal galvanic vestibular stimulation (sGVS), we recently showed that two peaks of modulation of muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA) occurred for each cycle of stimulation: a large peak associated with the positive peak of the sinusoid (defined as the primary peak) and a smaller peak (defined as the secondary peak) related to the negative peak of the sinusoid. However, these recordings were only made from the left common peroneal nerve, so to investigate lateralisation of vestibulosympathetic reflexes, concurrent recordings were made from both sides of the body. Tungsten microelectrodes were inserted into muscle or cutaneous fascicles of the left and right common peroneal nerves in 17 healthy individuals. Bipolar binaural sinusoidal GVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08 Hz. Cross-correlation analysis revealed that vestibular modulation of MSNA (10 bilateral recordings) and SSNA (6 bilateral recordings) on the left side was expressed as a primary peak related to the positive phase of the sinusoid and a secondary peak related to the negative phase of the sinusoid. Conversely, on the right side, the primary and secondary peaks were reversed: the secondary peak on the right coincided with the primary peak on the left and vice versa. Moreover, differences in pattern of outflow were apparent across sides. We believe the results support the conclusion that the left and right vestibular nuclei send both an ipsilateral and contralateral projection to the left and right medullary output nuclei from which MSNA and SSNA originate. This causes a "flip-flop" patterning between the two sympathetic outflows: when vestibular modulation of a burst is high on the left, it is low on the right, and when modulation is low on the left, it is high on the right.