Neuro 5 - Regulation of Blood flow Flashcards
Approximately how much of the CO does the brain take? (only 2% of body mass)
15%
What is syncope?
(=fainting)
Due to reduced blood supply to the brain
What can cause syncope?
- Low BP
- Postural changes
- Vaso-vagal attack
- Sudden pain
- Emotional shock, etc
What is the principal energy source of the brain?
Glucose
What are the normal fasting glucose levels?
4-6 mM
Below 2mM can cause unconsciousness, coma and death
Total cerebral blood flow is auto regulated between which mean arterial blood pressures?
60 - 160 mmHg
How is blood flow maintained over the wide range of MABPs?
Stretch-sensitive cerebral vascular SM contracts at high BP and dilates at low BP
What happens above the autoregulated pressure range of 60-160 mmHg
Increased blood flow leads to swelling of brain tissue - ICP increases which is dangerous
How can local regulation of cerebral blood flow be controlled?
- Neural control
2. Chemical control
Describe the pattern of vascularisation of CNS tissues
Surface Pial vessels —> branches penetrate into brain parenchyma —> form capillaries, which form veins which drain into surface pial veins
What are the 4 neural factors that regulate cerebral blood flow
- SNS nerve stimulation - causes main cerebral arteries to vasoconstriction - probably when there is high MABP
- PNS stimulation (facial nerve) - produces slight vasodilation
- Central cortical neurones - release many vasoconstrictor neurotransmitters (e.g. catecholamines or adrenaline)
- Dopaminergic neurones produce vasoconstriction - effect can be localised according to increased brain activity
How do dopaminergic neurones exert a local effect?
They innervate the penetrating arterioles and pericytes around capillaries
They may participate in diverting cerebral blood to regions of high activity
Dopamine may cause pericytic contraction vie aminergic and serotoninergic receptors
What are pericytes?
Cells that wrap around capillaries, and have a variety of functions
What chemical factors can regulate local cerebral blood flow?
- CO2
- pH
- NO
- K+
- Adenosine
- Anoxia
(ALL VASODILATOR)
What produces CSF?
Regions of choroid plexus in cerebral ventricles
What cells line the ventricles, aqueducts and canals of the brain?
Ependymal cells (they are epithelial-like glial cells that are often ciliated)
What are the choroid plexus?
Ependymal lining that has been modified to form branched villus structures
Describe the capillary and ependymal cell structure of choroid plexus cells
Capillaries are quite leaky
But the local ependymal cells have got extensive tight junctions
CSF is secreted by choroid plexus into ventricles. via what does CSF enter the 3rd ventricle?
Interventricular foramina
How does CSF enter the subarachnoid space?
Via medial and lateral apertures
What are the functions of CSF?
Protection, nutrition of neurones and transport of molecules
Roughly what is the volume of CSF?
80-150ml
CSF has little protein in it, how can this be used clinically?
If there is protein found in CSF, could indicate a problem
At what level is the blood brain barrier present?
At the level of the CNS capillaries
What are the 3 types of capillary?
- Continuous
- Fenestrated
- Sinusoid
BBB capillaries have extensive tight junctions, more as you go down from pial arterioles to capillary
T
Name 2 differences between BBB capillaries and peripheral capillaries
Peripheral vessels = spare pericyte coverage, BBB vessels = dense pericyte coverage
BBB capillaries have “end-feet” astrocyte coverage - important for maintaining BBB properties
How are pericytes important for BBB vessels?
Pericytes help maintain capillary integrity and function
Hydrophilic solutes cannot cross the BBB. What does this mean?
Membrane transporters are present on BBB to control substance access into and out of CNS (influx / efflux transporters)
Are the meninges’ vessels BBB?
No
How does access to / removal of lipophilic substances occur in the CNS?
Diffusion down concentration gradients
Name 4 hydrophilic substances that access the brain and how
- Water, via AQP1 and AQP4 channels
- Glucose via GLUT1 transporter proteins
- AA, via 3 different transporters
- Electrolytes via specific transporter systems
What are CVOs?
Circumventricular organs. Regions of the brain (near ventricles) whose capillaries lack BBB.
Describe the capillary structure of CVOs
Fenestrated capillaries, ventricular ependymal lining is much tighter than in other areas
What are CVOs used for?
Secreting into the circulation of to sample the plasma
Give 3 examples of CVOs
- PPG
- Median eminence
- Area postrema - samples plasma for toxins and induces vomiting
