How does a busy patch of cortex get more blood within seconds? · Grey Matter

Active neurons and astrocytes release signals that relax the smooth muscle of nearby arterioles and the pericytes on capillaries, widening them within a second or two.

How does a busy patch of cortex get more blood within seconds?

Active neurons and astrocytes release signals that relax the smooth muscle of nearby arterioles and the pericytes on capillaries, widening them within a second or two. Nitric oxide from neurons is one of these signals, along with prostaglandins and other messengers triggered by glutamate and by calcium rises in astrocytes. The response is fed forward from signalling itself, often arriving before any shortage of energy, and it delivers more oxygen than the tissue uses, which is why active areas show more oxygenated blood. That surplus of oxygenated blood is what functional MRI detects as the BOLD signal.