The cerebral vasculature dilates in response to increased CO2 and H + and constricts if either substance is decreased. Both of these substances are formed when cerebral metabolism is increased by nerve action potentials, such as during normal brain activation. In addition, interstitial K+ is elevated when a large number of action potentials are fired. The cause of dilation in response to both K+ and CO2 involves the formation of nitric oxide (NO). However, the mechanism is not necessarily the typical endothelial formation of NO. The source of NO appears to be from nitric oxide synthase in neurons, as well as endothelial cells. The H+ formed by the interaction of carbon dioxide and water or from acids formed by metabolism does not appear to cause dilation through a NO-dependent mechanism, but additional data are needed on this topic.
Reactions of cerebral blood flow to chemicals released by increased brain activity, such as CO2, H + , and K+, are part of the overall process of matching the brain's metabolic needs to the blood supply of nutrients and oxygen. The 10 to 30% increase in blood flow in brain areas excited by peripheral nerve stimulation, mental activity, or visual activity may be related to these three substances released from active nerve cells. The cerebral vasculature also dilates when the oxygen content of arterial blood is reduced, but the vasodilatory effect of elevated CO2 is much more powerful.
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