However, this autoregulatory system breaks down when the pressure falls very low. The main factor influencing cerebrovascular resistance is the diameter of the cerebral blood vessels. The vessels are innervated by postganglionic sympathetic fibers. They will respond to norepinephrine, but apparently, this does not play a major part in controlling the vascular resistance. The two most important controlling substances are oxygen and carbon dioxide.
Carbon dioxide is the most powerful vasodilator for cerebral blood vessels.
Oxygen has strong vasoconstrictor effects on these vessels. There are a number of things that produce cerebral ischemia, as was mentioned above. The first category includes those diseases that cause a change in blood pressure. First, an interruption of cerebral circulation. The brain is irreversibly damaged quickly following occlusion of the blood flow.