cerebral blood flow

Question 1

what happens when blood flow to brain is reduced by >50%

Answer 1

insufficient oxygen delivery, function becomes significantly impaired

Question 2

what happens if total cerebral blood flow is interrupted for a) as little as 4 seconds, and b) after a few minutes

Answer 2

a) unconsciousness, b) irreversible damage occurs to brain

Question 3

define syncope

Answer 3

fainting: common manifestation of reduced blood supply to brain

Question 4

causes of syncope

Answer 4

hypotension, postural changes, vaso-vagal attack, sudden pain, emotional shock etc.

Question 5

result of syncope

Answer 5

temporary interruption or reduction of blood flow to brain

Question 6

what is the principal energy source of brain

Answer 6

glucose via blood

Question 7

what % of the body’s glucose does the brain do

Answer 7

up to 50-60%

Question 8

why does the brain use glucose

Answer 8

can’t store, synthesise or utilise any other source of energy; except in starvation where it metabolises ketones to a limited extent (CNS can adapt in chronic undernutrition)

Question 9

common condition causing hypoglycaemia, and effects of hypoglycaemia

Answer 9

insulin-dependent diabetes (type 1); appear disorientated, slurred speech, impaired motor function

Question 10

effect of [glucose] below 2nM

Answer 10

unconsciousness, coma, ultimately death

Question 11

what mechanisms regulate cerebral blood flow

Answer 11

mechanisms affecting total cerebral blood flow, mechanisms relating activity to requirement in specific brain regions by altered localised blood flow

Question 12

at what mean arterial blood pressure is total cerebral blood flow autoregulated

Answer 12

between approx. 60 and 160 mmHg

Question 13

how is blood flow maintained

Answer 13

arteries and arterioles dilate or contract: stretch-sensitive cerebral vascular smooth muscle contracts at high blood pressure and relaxes at lower blood pressure

Question 14

what happens below autoregulatory range

Answer 14

insufficient supply leads to compromised brain function

Question 15

what happens above autoregulatory range

Answer 15

increased flow can lead to swelling of brain tissue which is not accomodated by “closed” cranium, increasing intracranial pressure

Question 16

what determines O2 and glucose demands, and impact on autoregulation of blood supply

Answer 16

local brain activity, so must be local autoregulation

Question 17

2 controls of local regulation of cerebral blood flow

Answer 17

neural, chemical

Question 18

what do branches of surface pial vessles do

Answer 18

penetrate brain parenchyma and enter the CNS tissue (penetrating artioles and penetrating venules)

Question 19

what do these branches of surface pial vessles branch to form, and function

Answer 19

capillaries which drain into venules and veins, then draining into surface pial veins

Question 20

features of CNS vascularisation

Answer 20

densely vascularised (no neurone more than 100um from a capillary)

Question 21

4 neural factors which regulate cerebral blood flow

Answer 21

sympathetic nerve stimulation, parasympathetic (facial nerve) stimulation, central cortical neurones, dopaminergic neurones

Question 22

sympathetic nerve stimulation: location and effect

Answer 22

to main cerebral arteries, producing vasoconstriction (probably only when high arterial blood pressure)

Question 23

parasympathetic (facial nerve) stimulation: effect

Answer 23

slight vasodilation

Question 24

what do central cortical neurones release

Answer 24

vasoconstrictor neurotransmitters e.g. catecholamines (e.g. adrenaline, noradrenaline)

Question 25

dopaminergic neurones: effect

Answer 25

vasoconstriction (localised effect relating to increased brain activity)

Question 26

what do dopaminergic neurones (local effect) innervate

Answer 26

penetrating arterioles and pericytes around capillaries, causing contraction/relaxation and subsequent diversion

Question 27

what are pericytes and what are their function

Answer 27

cells that wrap around capillaries; have diverse activities (e.g. immune function, transport properties, contractile)

Question 28

function of dopaminergic neurones

Answer 28

local effect: diversion of cerebral blood to areas of high activity

Question 29

effect of dopamine secreted by dopaminergic neurones

Answer 29

contraction of pericytes (in small vessels) via aminergic and serotoninergic receptors

Question 30

examples of localised chemical factors that regulate cerebral blood flow by causing vasodilation

Answer 30

most clinically relevant: CO2 (indirect), pH; also: NO, K+. adenosine, anoxia, kinins, prostaglandins, histamine, endothelins

Question 31

shape of pCO2 vs cerebral blood flow graph

Answer 31

sigmoid

Question 32

how does a high pCO2 cause vasodilation of cerebral arteries, increasing blood flow

Answer 32

CO2 is derived from neural metabolic activity and combines with water to form HCO3- and H+ (via carbonic anhydrase) in cells, then enters vascular smooth muscle cells/pericytes; CO2 in blood also diffuses into vascular smooth muscle cells/pericytes and combines with water to form H+ (via carbonic anhydrase); finally H+ in blood cannot cross the BBB; all of this causes a decrease in pH, causing relaxation of contractile smooth muscle cells, increasing blood flow

Question 33

imaging techniques because of local changes to cerebral blood flow

Answer 33

PET, functional MRI; increased neuronal activity = increased CO2 and subsequent H+ production = increased blood flow

Question 34

what produces CSF in brain and where

Answer 34

regions of choroid plexus in cerebral ventricles; protects brain

Question 35

CSF production: location and histology of cells

Answer 35

ependymal cells (epithelial-like glial cells, often ciliated) lining ventricles, aqueducts and canals, which in some regions of ventricles is modified to form branched villus structures (choroid plexus)

Question 36

formation of CSF in choroid plexus: characteristics of capillaries and local ependymal cells

Answer 36

leaky capillaries (enters plasma via arachnoid granulations), but local ependymal cells have extensive tight junctions; secrete CSF into ventricles which then circulates

Question 37

ventricles supplied by CSF via interventricular foramina

Answer 37

lateral and 3rd

Question 38

ventricles supplied by CSF via cerebral aquaduct from 3rd

Answer 38

4th

Question 39

pathway of CSF from 4th to circulation

Answer 39

into subarachnoid space via medial and lateral apetures

Question 40

CSF volume

Answer 40

80-150ml

Question 41

CSF functions

Answer 41

protection (physical and chemical), nutrition of neurones, transport of molecules

Question 42

why is it clinically important that CSF has little protein

Answer 42

if high could indicate bacterial infection or damage to CSF vessels