Session 11 Cerebral Blood Flow pptx

8/7/2019 Session 11 Cerebral Blood Flow pptx

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Schedule for the day

1. Importance of studying cerebral blood flow

2. Anatomy of cerebral vasculature

3. Autoregulation of cerebral blood-flow:a. Munroe-Kelly Principle

b. Role of Medullary Vasomotor Centresc. Cerebral Microvasculature

d. Windkessel design

e. Neurovascular Network and astrocytes

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fMRI EEG

spectrum

CerebralBloodFlow

ElectricalA

ctivity

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Interesting facts

THE BRAIN

1. ENTIRELY depends on OXYGEN for itsmetabolism

BUT, the reserve is LIMITED&

2. ENTIRELY depends on GLUCOSE for itsmetabolism

BUT, the reserve is LIMITED

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YET.

There is LIMITED/ FIXED/ RATIONED

BLOOD SUPPLY of 700 ml/minute

because ofRestricted skull volume and distribution of

volume between brain, CSF and blood

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The outcome therefore.!!

A) 1 SECOND INTERRUPTION of blood

all dissolved oxygen in brain is used up!

B) 6 SECOND INTERRUPTION of bloodUNCONSCIOUSNESS!!

C) 6 MINUTE INTERRUPTION of blood

IRREVERSIBLE BRAIN DAMAGE!!!

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Stable blood flow to the brain despite wide

range of peripheral mean arterial pressure

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Hippocampus vs. Ear-lobe blood flow (mouse)

Different vascular regions

in hippocampus identified

Blood flow at single

capillary resolution indifferent regions of

hippocampus and

ear-lobe studied

Blood flow in most

hippocampus remainedconstant across wide

mean arterial pressures

(Kuga et al J. Physiol 2008)

Hippocampus

Ear-lobe

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In spite of constant blood flow

Blood supplyPROMPTLY & DELICATELY

alters to match the levels of

REGIONAL NEURONAL ACTIVITYHow?

Autoregulation of Cerebral Blood Flow

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2a. Anatomy of Cerebral Vasculature: The Circle of Willis

Basilar artery

Internal Carotid

artery

Basilar artery

Internal Carotid

artery

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2a. Anatomy of Cerebral Vasculature: The Circle of Willis

Circle of Willis: The most important communicating

system between the major arteries to the brain

Prog Cardiovasc Dis (2007)

Anatomical Details of Circle of Willis will not be tested

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2b. Anatomy of Cerebral Vasculature:

Distribution of the major arteries

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3. Autoregulation:

a. Munroe-Kelly Principle

Volume ofBrain tissue

Volume ofCerebral

Blood flow

Volume of Cerebrospinal fluid

Brain, Blood and CSF share the fixed limited volume of

the rigid skull. This keeps the blood flow constant.

Fixed

skull

volume

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3. Autoregulation:

b. Role of Medullary Vasomotor Centres

Autonomic Nervous System

Cerebral Cortex

Bloodflow

Medulla

Oblongata

- +

Medullary vasomotor

centres detect

changes in blood

pressure in incomingblood vessels

They reflexly regulate

the blood flow to thebrainvia AutonomicNervous System

+-

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But, this is far from the fine-tuningof blood-flow to specific corticalareas!

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3. Autoregulation:

c. Cerebral Microvasculature

Note the

branching

pattern ofblood vessels

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3. Autoregulation:

d. Windkessel design

Smooth muscle valves

work like throttles/

Windkessel

Regulate the rate ofblood flow

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The problem of fine-tuningfor demand-supplyremains!.......

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Area of

increaseddemand

Adjacentareas

Remoteareas

Blood vessels

3. Autoregulation:

e. Neurovascular network

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Area of

increaseddemand

Adjacentareas

Remoteareas

Strategy-1: Steal blood from the neighbouring areaBlood vessels

Steal

from theneighbor

3. Autoregulation:

e. Neurovascular network

NOT A GOOD

STRATEGY!22


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Area of

increaseddemand

Adjacentareas

Remoteareas

Blood vessels

Distributed Stealing

GOOD, BUT HOW?

Strategy-2: Steal blood in a distributed fashion

3. Autoregulation:

e. Neurovascular network (contd)

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Area of

increaseddemand

Adjacentareas

Remoteareas

Blood vessels

Distributed Stealing with the help of

neurovascular network

EFFECTIVE AND FAST!

Neurovascular

network

Steal blood in a distributed manner with quicker

response due to Neurovascular Network

3. Autoregulation:

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Neurovascular network regulates regional

cerebral blood flow

1. Network of nerve fibresconnect blood vessels withone another

2. The network is in turn

orchestrated by major areassuch as basal forebrain

3. O2VasoconstrictionCO2 VasodilatationGlutamate VasodilatationGABA Vasoconstriction

4. Astrocytes are majorcomponents of the nerve net!

Progress in Cardiovascular Diseases (2007)

(Details in the diagram

will not be tested)

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CEREBRAL BLOOD FLOW

Summary

1. Oxygen and Glucose are the sole raw materials forbrain functions. Yet they are in limited supply.

2. Circle of Willis forms an important communicationbetween major arteries to the cortex thus providing

alternative source of blood in the event of an

obstruction. But, blockade ofterminal blood vesselscannot be compensated. This leads to stroke.

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CEREBRAL BLOOD FLOW

Summary

3. Munroe Kelly principle and medullary vasomotormechanisms provide a constant blood flow to braindespite wide fluctuations in the peripheral meanarterial pressures.

4. Branched pattern and Windkessel design of bloodvessels along with theirneurovascular network makeinstant, micro-adjustments of blood flow depending onlocal demands.

5. Astrocytes are implicated in swift vascular changesmatching neuronal demands.

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Documents

Session 11 Cerebral Blood Flow pptx