Hemodynamic response

Overview
Haemodynamics is a medical term for the dynamic regulation of the blood flow in the brain. It is the principle on which functional magnetic resonance imaging is based.

Neurons, like all other cells, require energy to function. This energy is supplied in the form of glucose and oxygen (the oxygen being carried in haemoglobin). The blood supply of the brain is dynamically regulated to give active neural assemblies more energy whilst inactive assemblies receive less energy.

The molecular biology of haemodynamic regulation
The precise mechanism behind neurovascular coupling is currently under active research and furious debate, but roughly the following model has gained consensus:
 * 1) Active excitatory neurons release the neurotransmitter glutamate
 * 2) Glutamate opens NMDA receptors on other neurons which allows calcium ions into the neuron
 * 3) Calcium activates nitric oxide synthase which produces nitric oxide
 * 4) Nitric oxide diffuses out and dilates smooth muscle surrounding local arterioles
 * 5) This dilation allows more blood into the local capillaries
 * 6) Hence more oxygen and glucose reaches the neurons
 * 7) Glutamate also binds to the metabotropic glutamate receptor on astrocytes
 * 8) This allow calcium into the astrocyte
 * 9) Which activates phospholipase A2 (PLA2)
 * 10) Which produces arachidonic acid (AA)
 * 11) Which is converted by cyclo-oxygenase
 * 12) Into PGE2 and released through the astrocytic endfeet
 * 13) Which dilates local smooth muscle and allows greater blood flow