Reasons for increase in CBF

Question 1

What were the findings of Raichle’s PET scan studies

Answer 1

Presented a visual stimulus to human subjects, found that in the visual cortex cerebral blood flow increased (50%), cerebral glucose consumption increased (50%) but that oxygen consumption hardly increased at all

Question 2

Why is it difficult to measure oxygen consumption during brain activation?

Answer 2

Due to the large cerebral blood flow response washing away the deoxygenated blood.

Question 3

What is hypercapnia?

Answer 3

Elevated carbon dioxide in the blood.

Question 4

What is hypercapnia useful for?

Answer 4

Increases flow (CBF) without altering neural activity therefore it provides a useful ‘control’ condition.

Question 5

How does hypercapnia support the idea that neural activity causes increased cerebral oxygen consumption?

Answer 5

For a CBF increase produced by hypercapnia there will be an oxygenation increase
However for the same CBF increase produced by neural activity (a stimulus) there will be less of an oxygenation increase

Question 6

What are two potential reasons that Raichle’s findings could have been wrong

Answer 6

PET has poor spatial and temporal resolution.

Calculations for inhaled substances are more difficult than with injected substances.

Question 7

Why does CBF increase so much during neural activity?

Answer 7

Buxton and Frank diffusion limited theory of oxygen delivery

To get more oxygen from the blood into the brain you need a gradient

Oxygen tension in the brain is very low

Increase blood flow to increase amount of oxygen in the capillaries

Increased blood flow means less time to get oxygen as it flows through the brain faster so need for more oxygen

So increased blood flow etc on a loop

Question 8

How can PET scans be used to measure blood oxygenation and glucose level?

Answer 8

Ingestion of radioactive substances

Glucose - version that doesn’t get broken down is injection

Oxygen- radioactive oxygen inhaled