Interaction between vestibulosympathetic and skeletal muscle reflexes on sympathetic activity in humans

Evidence from animalsindicates that skeletal muscle afferents activate the vestibular nucleiand that both vestibular and skeletal muscle afferents have inputs tothe ventrolateral medulla. The purpose of the present study was toinvestigate the interaction between the vestibulosympathetic andskeletal muscle reflexes on muscle sympathetic nerve activity (MSNA)and arterial pressure in humans. MSNA, arterial pressure, and heartrate were measured in 17 healthy subjects in the prone position duringthree experimental trials. The three trials were 2 min of 1)head-down rotation (HDR) to engage the vestibulosympathetic reflex,2) isometric handgrip (IHG) at 30% maximal voluntarycontraction to activate skeletal muscle afferents, and 3)HDR and IHG performed simultaneously. The order of the three trials wasrandomized. HDR and IHG performed alone increased total MSNA by 46 ± 16 and 77 ± 24 units, respectively (P < 0.01). During the HDR plus IHG trial, MSNA increased 142 ± 38 units (P < 0.01). This increase was not significantlydifferent from the sum of the individual trials (130 ± 41 units).This finding was also observed with mean arterial pressure (sum = 21 ± 2 mmHg and HDR + IHG = 22 ± 2 mmHg). Thesefindings suggest that there is an additive interaction for MSNA andarterial pressure when the vestibulosympathetic and skeletal musclereflexes are engaged simultaneously in humans. Therefore, no centralmodulation exists between these two reflexes with regard to MSNA outputin humans.