When they get active, they need increased blood flow to supply that activity, and lucky for us, blood flow control to the brain is local, so if a bunch of neurons, say, right there get active and start firing, then blood flow increases just right there.
活発な時は その活動の為に 送り込む血量を増やします 幸いに脳の血流制御は 局部的なので 例えばここで 多くのニューロンが 活発に発火し始めたら そこの血流は増します