bbb and csf Flashcards
what are the 3 layers of the cerebral meninges?
dura
arachnoid
pia
where is the dura?
adjacent to the skull and attached to the skull in places
what are the layers of the dura?
periosteal and meningeal
what is the dura made of?
collagen - makes it thick and strong
where is the arachnoid?
presses against inner surface of dura
what are the layers of the arachnoid?
2 layers – outer compact layer of “barrier” cells and inner “trabecular” meshwork
where is the pia?
thin membrane tightly attached to basement lamina of the brain
where are veins found in the brain?
in the inner layer of the dura which cross between dura and arachnoid
where are blood vessels found in the brain?
in the dura and subarachnoid space
what are the 3 types of cerebral haemorrhage?
epidural
subdural
subarachnoid
what is an epidural haemorrhage?
bleeding outside the dura, usually arterial
how does an epidural haemorrhage present and why?
o May be no symptoms at first
o Minutes/hours later there’s a headache bc the hematoma compresses the brain
what is a subdural haemorrhage?
bleeding between dura and arachnoid, usually venous.
which vessels does a subdural haemorrhage involve?
normally involves the bridging veins
how do subdural haemorrhages present and why?
o Bc it’s venous the onset of symptoms is slow (24 hours)
what is a subarachnoid haemorrhage?
bleeding into the arachnoid space (between the pia and the arachnoid), usually arterial
what causes a subarachnoid haemorrhage?
ruptured arterial aneurysm (a type of stroke)
what are the symptoms of subarachnoid haemorrhage?
sudden severe headache – “thunderclap headache”
what is a subarachnoid haemorrhage similar to?
intracranial haemorrhage
how does an epidural bleed present on an MRI?
has a lens shape on an MRI
rapidly increasing in size bc its arterial
what can cause an epidural bleed?
acute skull trauma
what symptom helps in the diagnosis of an epidural bleed?
lucid interval
what is a lucid interval?
when patients regain consciousness. After this interval they descend suddenly and rapidly into unconsciousness
how does a subdural bleed present on an MRI?
• Similar MRI appearance to epidural bleed but more spread out
how is an epidural bleed treated? what is the consequence of slow treatment?
surgical intervention
death
what can cause a subdural bleed?
• Caused by rapid acceleration/deceleration of the head which moved brain relative to the skull – e.g. in road traffic accident
what are symptoms of a subdural bleed?
confusion, headache, vomiting
what is the falx?
a sheet formed by the dura extending down between the hemispheres
Runs antero-posteriorly, dividing the 2 hemispheres
how does the falx make a t-juction?
Falx makes a t-junction at the occipital pole with the tentorium cerebelli
what is the tentorium cerebelli?
pair of approximately transverse sheets of dura that extend laterally below the base of the occipital cerebrum – divide the cerebrum from the cerebellum below
what passes between the gap between the tentorium?
brainstem
how many cerebral ventricles are there?
4
what are the cerebral ventricles?
spaces in the brain containing CSF
what are the cerebral ventricles?
Two lateral ventricles; a third ventricle (between the thalami) and a fourth ventricle (between the brainstem and cerebellum)
what is the choroid plexus?
a meshwork of capillaries covered by ependymal cells protruding into the ventricles of the brain
what does the choroid plexus make?
CSF
where is the choroid plexus found?
A choroid plexus is found in all cerebral ventricles, but majority is in the lateral ventricles
how does the choroid plexus make CSF?
• Choroid capillaries are fenestrated – ultrafiltrate of blood passes through the capillary fenestration into the subependymal layer
what is the flow of CSF in the brain?
500ml/day
describe the flow of CSF and why it’s like this?
pulsatile with heart rate
what % of CSF is plasma proteins?
0.3 - 1%
how is the concentration of plasma proteins in the CSF measured?
lumbar puncture
what is the consequence of the low % of plasma proteins in the CSF?
pH isn’t buffered without proteins –> small changes in PaCO2 in the blood causes large changes in the CSF pH
how much CSF does the brain hold?
135 - 150ml
how does CSF drain in the brain?
drains into the veins through arachnoid granulations in the superior saggital sinus –> confluence of sinuses at the rear of the skull
how much glucose does the CSF contain?
about 60% of the glucose conc of plasma
what is the pressure range of CSF? why does it vary?
4.4 - 7.3 mmHg (0.6-0.9 kPa)
Variation due to coughing or internal compression of jugular veins in the neck.
how is the immune system different in the CSF?
controlled by microglia
describe the CSF pathway in the brain?
o Lateral ventricles –> 3rd ventricles –> 4th ventricles
o Then flows out of the median aperture into the subarachnoid space
o Flows upwards over the surface of the brain –> superior saggital sinus
o Absorbed into the arachnoid granulations and joins the venous blood
where are cisterns of CSF found?
found where the surface of the brain is convoluted and the arachnoid is some distance from the pia
E.g. cisterna magna, superior cistern and the interpenduncular cisterns
what is hydrocephalus?
Accumulation of CSF in the ventricular system
what causes hydrocephalus?
obstruction of the normal CSF circulation, causing ventricles to therefore dilate – usually due to the blockage of the cerebral aqueduct
o Exception: overproduction of CSF by a rare papilloma of the choroid plexus
how is hydrocephalus treated?
shunt (tube inserted in third ventricle leading to subarachnoid space)
what is the prognosis for hydrocephalus?
good if treated early
how is hydrocephalus detected?
by translucent skull (no brain matter to block light)
what are the majority of cells in the CNS?
glial cells
what is the main type of tumour found in the brain and why?
gliomas
Bc neurones can’t undergo mitosis – cellular apparatus usually used for mitosis are used to produce axons and dendrites in nerve cells
what causes epilepsy?
caused by malfunction of the glial cells in a region where infarct or other neuronal ‘insult’ (contusion, impact, infection) has occurred
what is the function of oligodendrocytes?
produce myelin for intracerebral axons
One oligodendrocyte can provide myelin for several adjacent axons
why are oligodendrocytes subject to autoimmune attack in central demyelinating diseases?
Surface antigens on oligodendrocytes are different to those on Schwann cells
what is the function of ependymal cells?
line the walls of the ventricles and are involved in the transport of materials in and out of the CSF
what is the function of microglia cells?
phagocytose and remove cellular debris –> immune role in the brain
what are the functions of astrocytes?
- Maintain local pH and glucose by removing excess neurotransmitters, metabolites etc
- Secrete growth factors vital to support of some neurons
- form an astrocytic scar during injury to the cns
- astrocytes can proliferate and divide
how do astrocytes form an astrocytic scar?
astrocytes fill the space created by the breakdown of debris through proliferation and/or hypertrophy of astrocytes
what are end feet?
processes of astrocytes
what do astrocyte end feet cover?
• Most of the free surface of neuronal dendrites and cell bodies, as well as some axonal surfaces
- Every blood vessel in the CNS is covered by a layer of astrocyte end feet that separate it from the neural tissue
what are glia limitans
a coating of joined end feet where the outer surface of the brain meets the inner surface of the pia
what is the tight junction system?
Barrier between the blood and extracellular space of the brain
what is the function of tight junctions?
o Prevent proteins from leaving the blood
o Prevent macromolecules from entering/leaving the CNS
what is the blood brain barrier?
the combination of astrocyte end feet and endothelial cell tight junctions
how can some molecules pass from the blood into the brain extracellular space?
special transport mechanism; For example, amino acids have three specific transport systems designed for acidic, neutral, and basic amino acids. D-Glucose also has a specific transporter
which molecules can pass through the blood brain barrier unaided?
lipid-soluble
why do opiates have a high speed of action?
• Opiates such as diacetylmorpine (heroin) are lipid soluble and so can cross the bbb rapidly after I.v. injection –> increases their effect on the brain
why are opiates so addictive?
have a high speed of action
why cant antibiotics reach the brain?
polar molecules - soluble in water –> prevents them from reaching the brain
what happens to the BBB in meningitis?
breaks down due to inflammation
allows penicillin to penetrate the brain tissue
why can neoplasms be differentiated from healthy brain tissue by an MRI?
• Endothelial cells within rapidly growing neoplasms lack the tight junctions found in normal brain capillaries. The loss of this barrier affects the relative hydration of the neoplasm, which allows it to be differentiated from healthy brain tissue by MRI
