Blood flow in the brain is linked to neuronal activity. Therefore, various ‘brain scanning’ techniques can be used to observe neuronal activity in the brain. This has led to an astonishing revolution in knowledge of how the brain works. Of course, you knew that already.
Also astonishing is that the reason for changes in blood flow in relation to what neurons are doing is unknown! We know this system works, but we don’t know why!
It turns out that it is the glia cells. There a different kinds of glia, and they are very important in brain function. Glia do a lot of different kinds of work in the brain. If the brain’s neurons are the faculty, the glia are the department secretary, administrators, the janitorial staff, security … everybody else. (We’re not sure where graduate students fit into this … perhaps further research will elucidate this mystery).
In particular, it turns out that a particular kind of glia … Astrocytes … bridge the gap between neuronal activity and blood flow. Astrocytes are actually involved in neuronal activity. They do not have very much electonic activity, and therefore, have gone largely unnoticed. You see, much early research on neurons involved observing their activitiy by sticking tiny electrones into nerual tissue. Astrocytes do not show up on that particular ‘radar screen.’ However, other observational techniques indicate that this particular kind of glial cell is involved in modulating neural activity.
According to one of the study’s authors, James Schummers:
Electrically, astrocytes are pretty silent … A lot of what we know about neurons is from sticking electrodes in them. We couldn’t record from astrocytes, so we ignored them.
[When astrocytes were imaged with two-photon microscopy] the first thing we noticed was that the astrocytes were responding to visual stimuli. That took us completely by surprise … We didn’t expect them to do anything at all. Yet there they were, blinking just like neurons were blinking. We didn’t know if the rest of the world would think we were crazy.
According to Mriganka Sur, another co-author:
This work shows that astrocytes–which make up 50 percent of the cells in the cortex but whose function was unknown–respond exquisitely to sensory drive, regulate local blood flow in the cortex and even influence neuronal responses. … What’s more, astrocytes are arranged in orderly feature maps, exquisitely mapped across the cortical surface in sync with neuronal maps.
So how did they figure this out? The researchers used a two-photon imaging of calcium signals in the visual cortex of a ferret (in vivo). Calcium would be active during cell activity because of its role in basic cell metabolism.
This kind of imaging involves looking at tissue with a fancy microscope that is able to focus on things happening at depth within that tissue. (By “depth” we mean about one millimeter.) The reason this works is rather spooky. Under certain conditions, two photons act in a quantum mechanical way to cause a fluorescent event which can be detected by the two-photon microscope.
Schummers, J., Yu, H., Sur, M. (2008). Tuned Responses of Astrocytes and Their Influence on Hemodynamic Signals in the Visual Cortex. Science, 320(5883), 1638-1643. DOI: 10.1126/science.1156120