Baroreflex mechanisms under weightlessness and its development

Abstract

It is well known that the systemic blood pressure(BP)is regulated by the baroreflexes of negative feedback mechanism, and its feedback loops consist of receptor regions, afferent pathways, coordinating center, efferent pathway and effectors (ie, heart and vessels). General understanding is that baroreceptor afferents firings increase in response to the increases in BP to mediate the short-term regulation of BP, and the receptors are mainly stretch sensitive. The aortic nerve fibers (ANs), which are located in the aortic arch, sense hemodynamic changes and send this information to the brain. In other words, the aortic baroreflex works in accordance with the alterations of several factors (eg, heart rate, stroke volume, intramural pressures), to maintain adequate BP and blood distribution. The hydrostatic pressure gradient due to the gravitational force in vessels disappear under weightlessness. We hypothesized that the range of alterations of BP caused by individual behavior narrows under microgravity in space, and it reduces the proportion of high-threshold unmyelinated ANs, which discharge at the higher BP level, and leads the lower baroreflex sensitivities. We studied the baroreflex mechanisms and its development in the young rats with dam, which flew on space shuttle for 16 days (Neurolab, 98, Shimizu et al.), and verified that these hypotheses were collect and reported the several findings. I would like to summarize these findings including the results from ground-based studies and discuss the future development of research for the regulation of cardiovascular system from the gravitational point of view. [J Physiol Sci. 2007;57 Suppl:S6]