Editing Haemodynamic response (section)

===Astrocytes===
Astrocytes are unique in that they are intermediaries that lie between blood vessels and neurons. They are able to communicate with other astrocytes via [[gap junctions]] and have [[endfoot processes]] that interact with neuronal [[synapses]]. These processes have the ability to take up various [[neurotransmitters]], such as [[norepinephrine]] (NE) and [[glutamate]], and perform various other functions to maintain chemical and electrical [[homeostasis]] in the neuronal environment.

Constriction has been shown ''in vitro'' to occur when NE is placed in the synapse and is taken up by astrocyte receptors. NE uptake leads to an increase in intracellular astrocyte [[Ca2+|Ca{{sup|2+}}]]. When these calcium ion waves spread down the length of the astrocyte, [[Phospholipase A2|phospholipase A]] (PLA{{sub|2}}) is activated which in turn mobilizes [[arachidonic acid]]. These two compounds are transported to the smooth muscle and there react with [[cytochrome P450]] to make 20-hydroxyeicosatetraenoic acid (20-HETE), which acts through yet to-be-determined mechanisms to induce vasoconstriction. It has also been shown that [[agonists]] of [[metabotropic glutamate receptors]] (mGluR) also increase intracellular Ca{{sup|2+}} to produce constriction.<ref name="jap.physiology">{{cite journal | author = Koehler Raymond C | year = 2006 | title = Role of Astrocytes in Cerebrovascular Regulation | journal = Journal of Applied Physiology | volume = 100 | issue = 1| pages = 307–17 | doi=10.1152/japplphysiol.00938.2005| pmid = 16357084 | pmc = 1819408 }}</ref>