Sympathetic control of the brain circulation: Appreciating the complexities to better understand the controversy

Although the human cerebral circulation is richly innervated with sympathetic nerve fibers, the role of sympathetic nerve activity (SNA) on the regulation of cerebral blood flow (CBF) remains debated. Several issues may be responsible for the conflicting conclusions reported in the animal vs. human literature in regards to the sympathetic control of the brain circulation. Furthermore, due to the physiological consequences associated with SNA blockade (e.g. changes in blood pressure and cardiac output), and differences in methodology (e.g. assessment of CBF), interpretation of the role of SNA in CBF regulation in humans is challenging. The goals of this brief review are to provide an overview of the role of neural control in the regulation of CBF, with a focus on SNA and discuss the likely reasons behind the controversial influence of SNA on CBF regulation. A final objective of this article is to critically review the various methods available to measure CBF and highlight their strengths and weaknesses to provide insight in SNA regulation of CBF.