Blood Flow Feeds the Brain

Continuing to examine Nature v. 431, 9. Sept. 2004...
p. 137, "Feeding the brain" by Claire Peppliatt and David Attwell is a review of studies of the systems that support the brain cells that do our thinking on the physical level. They also show another aspect of the marvelously designed complexity of our bodies.

Our brains need energy to think just as computers need electricity. The simple sugar glucose is the fuel, and oxygen is required to metabolize it, and these are provided through blood flowing through arteries, arterioles, and capillaries. When we're not thinking so hard, our brains don't need as much energy, and too much fuel would be wasteful, if not harmful. So the blood flow is regulated according to need, "but exactly how the flow is increased is uncertain." A study on p. 195 casts some light on part of the system.

Actually, it somewhat muddies the waters (to switch metaphors in mid-stream), as "the new data contradict a previous suggestion" for how the system works. We do know that in addition to the "computationally active" neurons, there are support cells such as astrocytes. Some of the neurons also form "Dedicated neuronal networks" that "signal to the smooth muscle to constrict or dilate arterioles and thus decrease or increase blood flow."

Apparently, just "the neuronal activity associated with information processing increases local blood flow." This alone is not a simple matter, as it involves a series of steps, starting with "the transmitter glutamate" a chemical which "raises the intracellular concentration" of calcium ions (calcium atoms with, in this case, two electrons removed) in other neurons, setting off an enzyme that produces nitric oxide, which (finally) causes the arterioles to dilate (open up more) and allow more blood to flow.

That's just one regulatory pathway, and the simple one at that. An earlier study indicated that the glutamate transmitter also triggered astrocytes to cause the dilation. As noted, astrocytes support the neurons, wrapping around them, but "they also send out an extension, called an endfoot, close to blood vessels: thus, astrocyte anatomy is ideal for regulating blood flow..." However, the new data indicate the situation isn't so simple as the earlier data indicated. In this study, increasing calcium ions inside the astrocytes "produces a constriction of nearby arterioles."

The investigation showed that, in astrocytes, the ions didn't activate a nitric oxide release, and instead different enzymes go into action. It's suggested in the review that this part of the system can produce different effects at different times.

Until Next Time,

David Bump
Philippians 3: 13 Brethren, I
count not myself to have
apprehended: but [this] one thing
[I do], forgetting those things
which are behind, and reaching
forth unto those things which are
before, 14 I press toward the
mark for the prize of the high
calling of God in Christ Jesus.

http://home.att.net/~david.bump