← Reference library

REFERENCE LIBRARY

Brain Energy Supply & Neurovascular Coupling

The brain stores almost no fuel; blood delivers it in real time. Wherever tissue gets busy, local vessels widen within seconds — and they deliver more oxygen than gets used. That surplus is the whole reason fMRI can show you a "region lighting up".

No warehouse: ten seconds and the lights go out

Muscle stores glycogen; fat is an entire oil depot. The brain stores essentially nothing (astrocytes hold a small glycogen reserve that does not last long). Its glucose and oxygen arrive just in time — which is why an organ that is about 2% of body weight takes roughly 15% of the heart's output.

The price is that there is no buffer: stop the blood flow and consciousness goes within about 10 seconds, with irreversible damage in minutes. That is why stroke is an emergency and why "time is brain".

The neurovascular unit: who opens the valve

How does busy tissue tell the vessels to send more? Through a small team of cell types known collectively as the neurovascular unit:

Astrocytes

One end wraps synapses, the other grips a vessel with an "endfoot" — sitting exactly between demand and supply, they are the main messenger.

Pericytes & smooth muscle

Contractile cells hugging capillaries and arterioles: the hand that actually widens or narrows the pipe.

Functional hyperemia

Blood flow to a busy patch rises within seconds. The response is local — which is the only reason "which region lit up" is a meaningful question.

Signalling molecules

Nitric oxide, prostaglandins, and the potassium that leaks out during activity all relax vascular smooth muscle.

Flow overshoots oxygen use — and that gave us fMRI

Intuition says flow should rise just enough to cover the extra oxygen burned. What Fox and Raichle measured in 1986 was different: stimulating a finger raised local blood flow by about 29% while oxygen consumption rose only about 5%. The surplus oxygen stays in the venous blood — and oxygenated blood is magnetically different from deoxygenated blood. The scanner reads that difference: the BOLD signal (blood-oxygen-level-dependent).

So three things are worth keeping in mind permanently. fMRI measures blood, not spikes. It is slow (the vascular response takes seconds; spikes take milliseconds). And it cannot separate excitation from inhibition — inhibiting a circuit also costs energy and also raises blood flow. "This region lit up" is several inferential steps away from "this region does the job".

Fuel: glucose first, with a couple of spares

The brain runs almost entirely on glucose, roughly 120 g a day, ferried across the blood–brain barrier by dedicated transporters (CSF & brain barriers). During prolonged fasting or a ketogenic diet, liver-made ketone bodies can cover a substantial share — one of the brain's few backup fuels. Whether astrocytes also hand lactate to neurons as fuel (the "lactate shuttle" hypothesis) has been argued over for more than two decades and is still unsettled.

Read backwards, this explains why hypoglycemia hits so hard: no storage, a single main fuel, and no option to pause. When blood sugar drops, judgement and attention go first.