Circulating vasoconstrictor and vasodilator hormones and the release of norepinephrine by sympathetic nerve terminals on cerebral blood vessels do not play much of a role in moment-to-moment regulation of cerebral blood flow. The blood-brain barrier effectively prevents constrictor and dilator agents in blood plasma from reaching the vascular smooth muscle. Though the cerebral arteries and arterioles are fully innervated by sympathetic nerves, stimulation of these nerves produces only mild vasoconstriction in the majority of cerebral vessels. If, however, sympathetic activity to the cerebral vasculature is permanently interrupted, the cerebral vasculature has a decreased ability to autoreg-ulate blood flow at high arterial pressures, and the integrity of the blood-brain barrier is more easily disrupted. Therefore, some aspect of sympathetic nerve activity other than the routine regulation of vascular resistance is important for the maintenance of normal cerebral vascular function. This may occur because of a trophic factor that promotes the health of endothelial and smooth muscle cells in the cerebral microvessels.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.