Myles Jones - Lecture 3 - Measurements of Blood Oxygenation and Oxygen consumption

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What is Myles Jones lecture 3 about?
Measurements of Blood Oxygenation and Oxygen consumption
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What is learning objective 1?
To understand how the blood ('hemodynamic') response can be measured
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What is learning objective 2?
To understand some 'ideas' about why blood flow goes 'up' (increases) more than oxygen use
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How was it done?
The need for an animal model
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fMRI is... - but doesn't measure .... changes .... or '.......'
based on; blood; directly; independently
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Optical techniques can...
Accurately measure hemoglobin concentration and blood flow at very high temporal and spatial resolution
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Optical techniques are....so......
Invasive so we need an animal model
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Need to be able to activate a .....
Specific area of cortex
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Rat whisker barrel cortex contains ....
Somatosensory 'representations' of each whisker on the contralateral (opposite side) face
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Stages
Stimulus (Whiskers) -> Brain activity ('Whisker' cortex) -> Hemodynamics - Blood changes (Optical Imaging, spectroscopy) -> BOLD fMRI
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Oxyhemoglobin (HbO2) =
Light source (different colours/wavelengths) -> -> O2 -> -> Reflected light captured by camera
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Deoxyhemoglobin (Hbr)
Light source (different colours/wavelengths) -> -> * -> -> Reflected light captured by camera
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What does LDF stand for?
Laser Doppler Flowmetrey
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LDF to measure...
Cerebral Blood Flow (CBF)
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What is the easiest explanation of why does flow increase more than oxygen consumption?
Oxygen consumption doesn't go up after all
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What did Raichle and colleagues also do with cerebral glucose consumption?
Measured 'glucose-use' by injecting a special type of glucose which doesn't get consumed ('broken-down') by the brain. Cerebral Glucose Consumption - They found large focal increase in glucose consumption.
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What did Raichle and colleagues also do with cerebral oxygen consumption?
Measured oxygen use getting the subjects to breathe radioactive oxygen. They found little increase in oxygen consumption.
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Why could Marcus Raichle have been wrong? Hint: remember the limitations of the PET technique?
Oxygen consumption is fast and nimble - PET is slow and 'blocky' - it misses it
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Are there any other ways we can investigate oxygen consumption?
Hypercapnia - By increasing CO2 in the area you breathe in you can increase CO2 in the blood
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What does hypercapnia do?
Increases flow (CBF) without altering neural activity. Therefore it provides a useful 'control' condition.
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CO2 (Hypercapnia) and neural activation...
Match flow changes
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When comparing oxygenation changes....
For the same flow increase the oxygenation increase is less for neural activity than for hypercapnia
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Why for the same flow increase does the oxygenation increase is less for neural activity than for hypercapnia?
Because increased neural activity has used up oxygen!
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So if oxygen consumption does go up...Why does blood flow go 'up' to such an extent that blood oxygenation goes 'up'? Why does flow exceed the need for oxygen consumption during activation?
The 'Diffusion limited' theory. Buxton & Frank.
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What is the 'Diffusion limited' theory by Buxton & Frank?
Need to increase oxygen gradient to get more oxygen from capillary to brain; assume zero oxygen in brain tissue;can't increase gradient by lowering brain tissue oxygen-its already zero!;get more oxygen across from blood is to increase flow
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Way to increase oxygen gradient?
Raise blood
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How can you increase oxygen in the blood?
The only way is to increase flow
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But when flow increases, it....
Goes faster so it's more difficult to get oxygen out
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But all we can do is increase flow, so increase it some more! Which leads to....
'End up' with disproportionately large increase in flow to support an increase in oxygen consumption
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Most studies find...
Blood flow increases are twice that of oxygen consumption increases
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Most studies find...
Blood flow increases are twice that of oxygen consumption increases
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Other cards in this set

Card 2

Front

What is learning objective 1?

Back

To understand how the blood ('hemodynamic') response can be measured

Card 3

Front

What is learning objective 2?

Back

Preview of the front of card 3

Card 4

Front

How was it done?

Back

Preview of the front of card 4

Card 5

Front

fMRI is... - but doesn't measure .... changes .... or '.......'

Back

Preview of the front of card 5
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