Dr Rachel Lord, Cardiff Metropolitan University, UK; Twitter: LordyLaa and Dr Jonathan Moore, Bangor University, UK; Twitter: JonzoMoore
One of the most studied issues in integrative physiology is neural control and autonomic regulation of the circulation.
Recently, we had the pleasure of organising a Scientific Theme webinar, hosted by The Physiological Society, at which three speakers provided some fascinating insights into vascular sympathetic baroreflex function.
From a classical perspective, arterial blood pressure is the regulated (sensed) variable of the circulation.
Vascular stretch receptors, located mainly in the walls of the aortic arch and carotid sinuses, become active during spontaneous rises in arterial blood pressure.
The baroreflex then corrects pressure through two distinct pathways, the cardiovagal and vascular sympathetic, through which heart rate and sympathetic neural control of resistance arteries and arterioles respectively are modulated.
In the setting of increased blood pressure, reflex depressor responses, including bradycardia and vasodilatation are engaged via a negative feedback loop to restore blood pressure to the baroreflex operating setpoint.
These arterial baroreceptor reflexes constitute an important first line of defence for maintaining arterial pressure; mainly through modulation of central sympathetic outflow, particularly to arterial resistance vessels.
Notably, arterial pressure is not a static variable. In other words, it varies from moment-to-moment, and some functional characteristics of the vascular sympathetic baroreflex change according to mental and physical activity, upright posture, and environmental or experienced stress (Dampney, 2017).
Furthermore, baroreflex dysfunction and augmented sympathetic nervous system activity have been identified across a spectrum of conditions including diabetes, heart failure, post-traumatic stress and Parkinsonism.
We were very fortunate to have three fantastic presenters at this webinar.
Dr Chloe Taylor (Western Sydney University, Penrith, Australia), the first speaker, delivered a clear and concise overview of microneurography, the method for direct recording of muscle sympathetic nerve activity (MSNA) in conscious humans. Using examples from her studies, Chloe addressed some of the approaches that she, and others in the field, use to quantify vascular sympathetic baroreflex function (Taylor et al. 2015; Hissen et al. 2018). Chloe introduced some important considerations, such as hysteresis, central sites of modulation, as well as the concept of resetting. Chloe’s assessment of ‘pros and cons’ of spontaneous and pharmacological (modified Oxford) approaches to examine arterial baroreceptor reflex control of MSNA was particularly enlightening.
This was followed by two outstanding presentations about vascular sympathetic baroreflex function in pregnancy (Dr Craig Steinback, University of Alberta, Canada) and during sustained high altitude (HA) hypoxia (Dr Lydia Simpson, University of Innsbruck, Austria). In addition to providing great insight into microneurographic studies in these unique physiological conditions, a common thread in Craig’s and Lydia’s talks was upward resetting of the vascular sympathetic baroreflex. In other words, baroreflex control of MSNA is adjusted to enable higher probability of a burst for a given arterial pressure (Simpson et al. 2019; Usselman et al. 2015). This resetting may reflect compensation for systemic vasodilation that occur in pregnancy and under HA hypoxia. It is also possible that there is some form of balancing act between increased basal MSNA and reduced neurovascular transduction (Berthelsen et al. 2020; Steinback et al. 2019).
Understandably some differences were evident; for example, baroreflex sensitivity, or gain, appears to be unaffected at HA, and in early pregnancy, but it may be reduced during late gestation. Intriguingly, Craig highlighted some differences between uncomplicated and hypertensive pregnancy (Reyes et al. 2018). In her talk, Lydia presented some data to support the notion that Himalayan Sherpa may be better adapted than Andeans to life in thin air (Simpson et al. 2021). Furthermore, she introduced the possibility that afferent input originating from vascular stretch receptors in the pulmonary artery may contribute to HA sympathoexcitation and vascular sympathetic baroreflex resetting (Simpson et al. 2020).
Taken together, the three talks offered everything that we had anticipated, and much more. Indeed, we hope that by the end of the webinar, the audience’s appreciation of the function and complexity the arterial baroreceptor reflex had grown.
We thoroughly enjoyed the opportunity to organize this event and we are grateful to Caitlin Oates and the team at The Physiological Society for their support. It goes without saying that this webinar would not have be possible without the enthusiasm of our speakers, whose locations spanned a time difference of 17 hours!
Also, we are grateful to the audience that joined in real time, and those who have subsequently watched the recording. Perhaps one of the few upsides of the ongoing pandemic is that physiologists across the globe have been able to connect in way that was not appreciated fully prior to March 2020.
There is no doubt that more opportunities will be available in the next weeks and months. But, the sooner it is possible to discuss reflex control of the circulation over a glass of wine, pint of beer or of course soft drink, the better.
References
- Berthelsen LF et al. (2020) Highs and lows of sympathetic neurocardiovascular transduction: influence of altitude acclimatization and adaptation. American Journal of Physiology: Heart and Circulatory Physiology 319 (6):H1240-H1252. https://doi.org/10.1152/ajpheart.00364.2020
- Dampney RAL (2017) Resetting of the Baroreflex Control of Sympathetic Vasomotor Activity during Natural Behaviors: Description and Conceptual Model of Central Mechanisms. Frontiers in Neuroscience 11:461. https://doi.org/10.3389/fnins.2017.00461
- Hissen SL et al. (2018) The Stability and Repeatability of Spontaneous Sympathetic Baroreflex Sensitivity in Healthy Young Individuals. Frontiers in Neuroscience 12:403. https://doi.org/10.3389/fnins.2018.00403
- Reyes LM et al. (2018) Sympathetic Nervous System Regulation in Human Normotensive and Hypertensive Pregnancies. Hypertension 71 (5):793-803. doi:10.1161/HYPERTENSIONAHA.117.10766
- Simpson LL et al. (2019) Baroreflex control of sympathetic vasomotor activity and resting arterial pressure at high altitude: insight from Lowlanders and Sherpa. The Journal of Physiology 597 (9):2379-2390. https://doi.org/10.1113/JP277663
- Simpson LL et al. (2020) Evidence for a physiological role of pulmonary arterial baroreceptors in sympathetic neural activation in healthy humans. The Journal of Physiology 598 (5):955-965. https://doi.org/10.1113/JP278731
- Simpson LL et al. (2021) Global REACH 2018: Andean highlanders, chronic mountain sickness and the integrative regulation of resting blood pressure. Experimental Physiology 106 (1):104-116. https://doi.org/10.1113/EP088473
- Steinback CD et al. (2019) Blunted sympathetic neurovascular transduction during normotensive pregnancy. The Journal of Physiology 597 (14):3687-3696. https://doi.org/10.1113/JP277714
- Taylor CE et al. (2015) Relationship between spontaneous sympathetic baroreflex sensitivity and cardiac baroreflex sensitivity in healthy young individuals. Physiological Reports 3 (11). https://doi.org/10.14814/phy2.12536
- Usselman CW et al. (2015) Sympathetic baroreflex gain in normotensive pregnant women. Journal of Applied Physiology 119 (5):468-474. https://doi.org/10.1152/japplphysiol.00131.2015