Increases in brain activity are matched by increases in blood flow. Neurons require a huge amount of energy, but can’t store it themselves, so must rely on blood to deliver the nutrients they need.
Two new studies help explain how blood flow is controlled.
The first study found blood appears to be stored in the blood vessels in the space between the brain and skull.
When the heart pumps blood into cranium, only a fraction of it flows into the capillaries that infuse the brain. The arteries in the cranium expand to store the excess blood. This expansion pushes out cerebrospinal fluid into the spinal column. When the heart relaxes, the drop in the pressure pushing blood through the arteries causes them to contract and the blood is pushed into the brain's capillaries. This in turn forces used blood out of the brain into the veins between it and the skull. These cerebral veins expand to store this blood as it leaves the brain.
Crucially, the study shows that the flow of blood in the veins leading out of the cranium is closely linked to the flow of cerebrospinal fluid in and out of the brain's ventricles.
The second study looked at what happens further down the track.
It had been thought that capillaries were passive tubes and the arterioles were the source of action — but the area covered by capillaries vastly surpasses the area covered by arterioles. So new findings make sense: that capillaries actively control blood flow by acting like a series of wires, transmitting electrical signals to direct blood to the areas that need it most.
To do this, capillaries rely on a protein (an ion channel) that detects increases in potassium during neuronal activity. Increased activity of this channel facilitates the flow of ions across the capillary membrane, thereby creating a small electrical current that communicates the need for additional blood flow to the arterioles, resulting in increased blood flow to the capillaries.
If the potassium level is too high, however, this mechanism can be disabled. This may be involved in a broad range of brain disorders.
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(2017). Capillary K+-sensing initiates retrograde hyperpolarization to increase local cerebral blood flow.
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