Brain energy budget · Grey Matter

The brain energy budget is the account of how much energy the brain consumes and what it spends it on: about 20 W in an adult at rest, a fifth of the body's resting use from 2 % of its mass, spent mostly on restoring the ion gradients that signalling runs down.

Brain energy budget. The brain energy budget is the account of how much energy the brain consumes and what it spends it on: about 20 W in an adult at rest, a fifth of the body's resting use from 2 % of its mass, spent mostly on restoring the ion gradients that signalling runs down.

Almost all of that energy comes from glucose burnt with oxygen, so the brain needs a constant supply of both and stores almost none of either. Budgets built for grey matter by Attwell and Laughlin (2001, for rodent cortex, extended to humans since) divide the cost by task. Most goes to signalling: pumping back the sodium that enters through glutamate receptors at synapses and through sodium channels during action potentials, with smaller shares for the resting potential, for recycling transmitter and for calcium. The sodium-potassium pump is the main consumer.

The cost of a spike sets how much the brain can fire. With this budget, only a small fraction of cortical neurons can be highly active at any time; average cortical firing rates are low (around a few spikes per second or less), which favours sparse codes.

Activity changes the total little. Local blood flow and glucose use rise in active areas, but the whole brain's consumption changes by only a few per cent between rest and demanding tasks; most of the budget is spent on ongoing activity.

Supply is tight. The energy comes through glycolysis and oxidative phosphorylation, and with blood flow stopped the brain loses consciousness in about ten seconds.

The brain is paid for by the charge it lets through.

Every synaptic current and every spike is a debt the pump repays in ATP, so energy, more than wiring, limits how much of the brain can be busy at once.

Questions: 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. Where do the brain's 20 watts go? Mostly into pumping ions back after signals. In the budget Attwell and Laughlin built for rodent grey matter, the largest items are the currents through glutamate receptors at synapses and the sodium entry of action potentials, both repaid by the sodium-potassium pump, with smaller shares for keeping the resting potential, recycling transmitter and clearing calcium. Housekeeping that any cell needs (making proteins and lipids, maintaining the cytoskeleton) takes a smaller part. Because demanding tasks change the whole brain's consumption by only a few per cent, most of the 20 W pays for ongoing activity, whatever the person is doing. If thick axons are faster, why are most axons in the brain thin and slow? Speed grows slowly with width while cost grows fast. In a bare axon conduction velocity rises roughly with the square root of the diameter, but the volume the axon takes up rises with the square of it, so doubling the speed means quadrupling the diameter and making the cable sixteen times bulkier. Wider axons also have more membrane to recharge, which costs pump energy with every spike. The brain therefore keeps most local wiring thin and slow (a millimetre takes about a millisecond even at 1 m/s) and spends myelin and width on the long paths where delay matters.