Regulation of Cerebral Blood Flow — A Brief Review

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Abstract
Cerebral blood flow is largely independent of perfusion pressure when autoregulation is intact. Cerebral circulation is regulated mainly by changes of vascular resistance. Resistance can be modulated by local-chemical and endothelial factors, by autacoids, and by release of transmitters from perivascular nerves. Local-chemical factors such as H+-, K+-, Ca2+-ions, adenosine, and osmolarity are involved in the regulation of cerebrovascular resistance during cortical activation and under pathological conditions such as hypoxia or ischaemia. Endothelial factors such as thromboxane A2, endothelin (ET), endothelium derived constrictor factor and endothelium derived relaxing (EDRF, identified as nitric oxide, NO) or hyperpolarizing (EDHF) factor, and prostacyclin (PGI2), can be released by physical stimuli such as shear stress or haemorrhage, by autacoids, by neurotransmitters, and by cytokines. Several of these factors (NO, PGI2, ET) can also be released from neurons and astrocytes thus enabling a coupling between parenchymal function and flow. Autacoids like histamine, bradykinin, eicosanoids, and free radicals influence cerebrovascular resistance, capacitance vessels and the permeability of the blood-brain barrier under pathological conditions. They are released by trauma, ischaemia, seizures and inflammation. Cerebral arteries are innervated by several systems. The sympathetic-noradrenergic fibres originate from the superior cervical ganglion. By releasing the constricting transmitters norepinephrine and neuropeptide Y this system extends the range of autoregulation. The parasympathetic cholinergic system with the dilating transmitters acetylcholine and vasoactive intestinal polypeptide may prevent ischaemia. Besides the intracerebral noradrenergic and serotonergic perivascular innervation with an unclear function, a trigeminal innervation has been described. Its dilating transmitters substance P, calcitonin gene-related peptide, and neurokinin A can be released in an axon reflex-like manner. This system may be involved in vascular headache or vasospasm. In conclusion, the regulation of cerebral circulation involves the same mechanisms as in peripheral organs.