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May I know causal dependence of the blood flow in the brain?

How it is determined by the brain that particular brain region requires additional blood?

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  • $\begingroup$ Do you wish to know whether there is a causal relationship yes or no, or are you after the physiological mechanism? $\endgroup$ – AliceD Feb 7 '15 at 12:15
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    $\begingroup$ There must be a causal relationship, so I want to know how? Yes, it can be in terms of physiological mechanism. $\endgroup$ – Karthi prime Feb 7 '15 at 12:18
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    $\begingroup$ I could chase the answer down, but it will take me a while. I know that very small changes in neural activity already causes changes in blood flow (fMRI-BOLD signal). So my guess is neuronal activity and/or glial cell activity is the key. It is certainly not the case that the brain has a specific region dedicated to regulating the brain's local blood supply. It must be something metabolical or physiological. $\endgroup$ – AliceD Feb 7 '15 at 12:24
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    $\begingroup$ See a perhaps informative question: What does fMRI measure exactly? - cogsci.stackexchange.com/questions/8650/… $\endgroup$ – AliceD Feb 7 '15 at 12:26
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The ion gradient that neurons use to transmit electrical signals is not free and costs energy to maintain, therefore needs oxygen and blood.

A brain region is considered "Active" if the neurons in said region are firing (or at least some subset of them). If a neuron fires then it releases chemicals known as neurotransmitters that stimulate (or suppress) activity in the down stream neuron it is connected to.

The cool thing is that the entire process of creating and sending neurotransmitters produces Nitric Oxide (NO) and in fact released as co-transmitters along with the main neurotransmitter.

Nitric Oxide is a small gas and as such diffuses through the brain relatively fast and causes the nearby blood vessels to relax( know as vasodilatation). This causes the blood cells to open and increase blood flow to the brain region.

However their are a few caveats as the relationship is not perfect one to one. What I mean by this is a group of neurons could be releasing a inhibitory neurotransmitter (and thus Nitric Oxide) onto a brain region and thus producing less activation, but still increases blood flow.

Here is a source on the internet if your interested in more. Sections 14.8 and 14.13 are of particular interest for NO and vasodilation in general.

Hope this helps. :)

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    $\begingroup$ Welcome to cogsci.se! This is a nice answer. It would be great if you could provide a reference or two in your answer, both for the sake of verification and for people who were interested in exploring the topic further. $\endgroup$ – Josh de Leeuw Feb 7 '15 at 21:32
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The haemodynamic response, through which neurons are supplied with oxygen, occurs as a result of incoming input and local processing in a brain region (Logothetis et al., 2001). Thus, the causal link is that (incoming or originating, but not outgoing) acticity in an area increases the oxygen requirements for that area.

In other words: local changes in oxygen consumption drive blood supply.

Reference

Logothetis, N. K., Pauls, J., Augath, M., Trinath, T., & Oeltermann, A. (2001). Neurophysiological investigation of the basis of the fMRI signal. Nature, 412, 150-157. doi: 10.1038/35084005 http://www.nature.com/nature/journal/v412/n6843/abs/412150a0.html

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