Q. Nervous System control of blood pressure?

Most nervous controls are achieved via involuntary reflex arcs involving pressoreceptors, the vasomotor centers of the medulla, and vascular smooth muscle. Note: occasionally input from chemoreceptors and higher brain centers may also affect BP regulation.

• Vasomotor fibers: these are sympathetic nerves that target the smooth muscles found in systemic arterioles. Most vasomotor fibers release norepinephrine, a potent vasoconstrictor (however, some may release ACh which cause vasodilation arterioles reaching skeletal muscles).

• Vasomotor centers: clusters of nuclei in medulla belonging to the sympathetic system. This center transmits signals via the vasomotor fibers to maintain a continuous state of vascular smooth muscle tone. Increased activity -> vessel constriction and ↑ BP, while decreased output from vasomotor centers -> ↓ BP (due to dilation). Again, activity in these nuclei of medulla affected by pressoreceptors, chemoreceptors, certain hormones/chemicals, and higher CNS centers.

• Pressoreceptors: located not only in aortic arch (aortic sinuses) and in carotids, but also in nearly every large artery in neck and thorax. If ↑ BP -> vasomotor center (-) inhibited. These are important in regulation of BP short-term, but cannot adjust for hypertensive states.

• Chemoreceptors: If O2 drops, or H+ increases -> chemoreceptors in aortic arch / neck vessels result in an increased BP to fasten return of blood to lungs for gas exchange.

• Higher CNS centers: nuclei from hypothalamus, lymbic system, and other parts of brain involved with emotional states (anxiety/nervousness) can have tremendous effects on BP (usually elevations).