Why does the brain use about a fifth of the body's oxygen at about 2 % of its mass? · Grey Matter

Signalling is paid for by ion gradients, and the gradients are restored by the sodium-potassium pump, which burns ATP made almost entirely by oxidative metabolism.

Why does the brain use about a fifth of the body's oxygen at about 2 % of its mass?

Signalling is paid for by ion gradients, and the gradients are restored by the sodium-potassium pump, which burns ATP made almost entirely by oxidative metabolism. In an adult at rest the brain accounts for about 20 % of the body's oxygen use, around 20 W of a resting total near 100 W. Budgets for rodent cortex attribute most of the signalling cost to pumping ions back after action potentials and after the currents opened by glutamate at synapses, with the resting potential and transmitter recycling costing much less. That is why the brain has almost no energy reserve and fails within seconds to minutes when its blood supply stops.