Anatomy and Physiology of Vascular Sphincters in the Brain

Brain precapillary sphincters (PSs) are located at the point where the penetrating arteriole meets the first-order capillary. Sphincters link blood flow in the capillaries to arteriolar inflow. The sphincters are encircled by contractile mural cells that can control the length and width of the enclosed vessel segment in both directions. Hemodynamic consequences include the ability of PSs to generate the largest changes in cerebrovascular flow resistance of all brain vessel segments. Using two-photon microscopy and genetically encoded reporters, we have revealed the physiology of sphincters. Our work has shown that sphincters can increase their diameter by approximately 30% within a few seconds, enabling rapid control of capillary blood flow over a wide dynamic range. In this article, we discuss the morphology and functions of sphincters, outlining their central role in rapidly adjusting blood flow in the microcirculation and protecting brain tissue against blood pressure variations in the cardiac cycle.