Brownian thought space

Cognitive science, mostly, but more a sometimes structured random walk about things.

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Tuesday, February 06, 2007

Blood in the brain

Over the last few days we had a remarkable series of talk on funtional neuroimaging of the human brain by Hellmuth Obrig; a most likeable and clever person. The main lessons learnt from these talks (for me), was what exactly is being measured by functional neuroimaging methods. I should 'fess up - my ideas were pretty crap on the topic.

Anyhow. What's clear is why it is called a Blood-Oxygen-Level-Dependent measure. What's far from clear is what causes the change in blood oxygenation in local pieces of the brain tissue. There is some serious energetic stock-taking to be done in brain tissue, and it's time to look at the literature.

What I Learnt: Here's a model. The brain cells are idling, sending the occasional spike and so on, breaking down glucose through the TCA cycle and making ATP through oxidative phosphorylation. Along comes a stimulus, and the cell tries to step up ox. pho.; and in the meantime uses some anaerobic means to break down glucose, which is energetically demanding, and produces lactic acid. [although, neurons also seem to use lactate, presumably produced by astroctytes, as an energy source; ref1, but see ref2]. The cells somehow signal the need for more oxygenated blood, and so the blood flow rate goes up. This brings about an increase in the concentration of oxygenated hemoglobin (Oxy-Hb), presumably because the concentration is something like an integral over time in a certain volume, so faster rate = higher concentration.

Clearly, the higher flow rate is to supply more O2, so more Oxy-Hb should be getting converted to Deoxy-Hb. But, since overall Oxy-Hb seems to increase, it looks like the extent of Oxy-Hb increase is greater than its conversion to Deoxy-Hb. Consequently, the increase in [Deoxy-Hb] is smaller than the increase in [Oxy-Hb] (square brackets - concentration), and as a result, because conversion to Deoxy-Hb cannot keep up with the increased flow-rate, the [Deoxy-Hb] goes down a little, explaining the observation from Optical Topography as in this figure (courtesy of Herr Obrig; thanks!): the red dots are [Oxy-Hb], blue dots are [Deoxy-Hb]; the x-scale is seconds of visual stimulation, the data is recorded from a near-infrared emitter-detector pair over the visual cortex (back of the head).
Presumably, once the stimulus is switched off, the balance between energetic need and enhanced blood flow is re-established, with some overshoot, and the neurons are back to idling.


Another cool thing was about the difference between volume changes and flow-rate changes. You could imagine that either the flow-rate speeds up, somehow, or that the local volume diminishes: both lead to faster blood flow. But how do you get local changes in blood velocity?

Two things: first, several blood vessels in the brain, specially the larger ones, have smoothmuscle control. Even cooler, the very smallest capillaries, which don't have muscles, have pericytes, which grasp the capillaries and squeeze them when required (ref)!

1 Comments:

Blogger amruta said...

'koffee with karan' had shah rukh and rani and kajol. you would have enjoyed it.

February 11, 2007 7:46 PM  

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