Elsevier

Mayo Clinic Proceedings

Volume 84, Issue 9, September 2009, Pages 822-830
Mayo Clinic Proceedings

REVIEW
Sympathetic Neural Mechanisms in Human Cardiovascular Health and Disease

https://doi.org/10.4065/84.9.822Get rights and content

The sympathetic nervous system plays a key role in regulating arterial blood pressure in humans. This review provides an overview of sympathetic neural control of the circulation and discusses the changes that occur in various disease states, including hypertension, heart failure, and obstructive sleep apnea. It focuses on measurements of sympathetic neural activity (SNA) obtained by microneurography, a technique that allows direct assessment of the electrical activity of sympathetic nerves in conscious human beings. Sympathetic neural activity is tightly linked to blood pressure via the baroreflex for each individual person. However, SNA can vary greatly among individuals and that variability is not related to resting blood pressure; that is, the blood pressure of a person with high SNA can be similar to that of a person with much lower SNA. In healthy normotensive persons, this finding appears to be related to a set of factors that balance the variability in SNA, including cardiac output and vascular adrenergic responsiveness. Measurements of SNA are very reproducible in a given person over a period of several months to a few years, but SNA increases progressively with healthy aging. Cardiovascular disease can be associated with substantial increases in SNA, as seen for example in patients with hypertension, obstructive sleep apnea, or heart failure. Obesity is also associated with an increase in SNA, but the increase in SNA among patients with obstructive sleep apnea appears to be independent of obesity per se. For several disease states, successful treatment is associated with both a decrease in sympathoexcitation and an improvement in prognosis. This finding points to an important link between altered sympathetic neural mechanisms and the fundamental processes of cardiovascular disease.

Section snippets

SYMPATHETIC NEURAL MECHANISMS IN THE REGULATION OF BLOOD PRESSURE

Sympathetic neural influences on cardiovascular function can be divided into 4 main categories: the influences of cardiac sympathetic nerves, the influences of vascular sympathetic nerves, adrenal medullary influences caused by circulating epinephrine and norepinephrine, and the sympathetic stimulation of renal juxtaglomerular cells that activates the renin-angiotensin-aldosterone axis. Most sympathetic innervation in the human cardiovascular system is noradrenergic. Norepinephrine is the

Direct Measurement

Because sympathetic postganglionic neurons are small, unmyelinated C fibers, it was originally thought that their activity could not be directly measured in humans. In the late 1960s, a group of Swedish investigators pioneered the method of microneurography for measurement of human neural activity and somewhat accidentally came upon the sympathetic vasoconstrictor nerve signal.14 Since the 1970s, microneurography has been used successfully to elucidate the complex integrative physiology related

INTERINDIVIDUAL VARIABILITY: ROLE IN REGULATION OF BLOOD PRESSURE

Two characteristics of MSNA have perplexed both clinicians and scientists for several decades. The first is the striking interindividual variability in the amount of neuralactivity in resting humans. Muscle sympathetic neural activity can exhibit a 7- to 10-fold variation in healthy humans, a fact that was initially disappointing to investigators who thought that a narrower range of “normal values” would allow the use of MSNA measurements for clinical diagnostic purposes. However, the more

Physiology

Aging. Even in the absence of disease, MSNA increases with age, on average by approximately 1 burst per minute each year.26 This age-related increase in sympathetic activity may contribute to the increase in the risk of hypertension with aging. Indeed, in persons older than 40 years, a strong association exists between resting levels of MSNA and resting arterial blood pressure.28 This association does not exist in younger persons, suggesting that mechanismsthat buffer or balance the

CONCLUSION

The sympathetic nervous system plays a vital role in maintaining cardiovascular health because of its key effects on both short- and long-term regulation of blood pressure and blood flow to organs. In the past several decades, direct measurement of vasoconstrictor SNA by microneurography has provided great insight into sympathetic neural mechanisms in both health and disease. Because microneurography is both technically challenging and time-consuming, its widespread use in the clinical setting

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