Heterogeneous baroreflex control of sympathetic action potential subpopulations in humans

Abstract

Emission patterns in muscle sympathetic nerve activity stem from differently sized action potential (AP) subpopulations that express varying discharge probabilities. The mechanisms governing these firing behaviours are unclear. This study investigated the hypothesis that the arterial baroreflex exerts varying control over the different AP subpopulations. r During baseline, medium APs expressed the greatest baroreflex slopes, while small and large APs exhibited weaker slopes. On going from baseline to lower body negative pressure (LBNP; simulated orthostatic stress), baroreflex slopes for some clusters of medium APs expressed the greatest increase, while slopes for large APs also increased but to a lesser degree. A subpopulation of previously silent larger APs was recruited with LBNP but these APs expressed weak baroreflex slopes. r The arterial baroreflex heterogeneously regulates sympathetic AP subpopulations, exerting its strongest effect over medium APs. Weak baroreflex mechanisms govern the recruitment of latent larger AP subpopulations during orthostatic stress.

Fine‐tuning our understanding of the baroreflex: size‐dependent regulation of action potential subpopulations

“…A significant finding from Klassen et al . (2020) was the robust changes in baroreflex gain of varying AP subpopulations despite no change in baroreflex gain of the integrated MSNA signal. The authors speculate that, although inconsequential to the integrated baroreflex threshold slope, these changes in AP subpopulation baroreflex gain may have a vasomotor impact (i.e.…”

“…These findings beg the question – how is size‐dependent baroreflex control of AP subpopulations altered during baroreceptor unloading? This matter was addressed in a recent article published in The Journal of Physiology , where Klassen and colleagues (2020) used lower body negative pressure (LBNP) to test the hypothesis that the baroreflex control of efferent MSNA APs varies with the size of AP subpopulations.…”

“…Klassen et al . (2020) extracted sympathetic APs from MSNA recordings in 12 (6 female) healthy young participants, at rest and during varying degrees of LBNP. Baroreflex threshold slopes (AP firing probability [%] vs .…”

“…Lastly, Klassen and colleagues (2020) allude to a potential influence of supramedullary regions on baroreflex control of small and large AP subpopulations. Several cortical and subcortical regions outside of the medulla are now known to be involved in the central regulation of sympathetic outflow.…”

“…However, the influence that these discrete areas exert over the sensitivity of the sympathetic baroreflex response to changes in arterial pressure is incompletely understood and an intriguing avenue for future study. Klassen and colleagues (2020) outline previous research implicating a role for several possible supramedullary regions in the generation of larger APs. Therefore, the findings of the present study suggest that considering heterogenous baroreflex control of AP subpopulations may give added context to future work investigating supramedullary influences on baroreflex function.…”