Lectures 6 + 7 - Brain and Environs Flashcards
What are foramina?
Holes in the skull, through which cranial nerves, blood vessels and the spinal cord enter and leave the cranial vault.
What is the largest foramen?
Foramen Magnum.
This is where the brain stem and spinal cord sits.
It is at the base of skull.
This is also where the medulla and spinal cord meet - cervicomedullary junction
Which fossa contains the frontal lobes?
Each side of the anterior fossa
Which fossa contain the temporal lobes?
Middle fossa
Which fossa contains the brainstem and cerebellum
Posterior fossa
What is the sphenoid bone?
It divides the anterior fossa and the middle fossa (just the lesser wing)
What divides the middle and posterior fossa?
ridge of temporal bone and sheet of meninges.
What are the three layers of the meninges.
from outside the inside
Dura
Arachnoid
Pia
What is the dura?
It is the outermost layer of the meninges.
It is composed of two layers which are fused together, except for in two cases where they seperate.
Where are the two areas where the two dura layers are un-fused?
- Falx cerebri - sheet of dura that seperates the right and left hemispheres in the interhemispheric fissure.
- Tentorium cerebelli - tent-like sheet of dura that covers the upper surface of the cerebellum
What is the falx cerebri?
sheet of dura that seperates the right and left hemispheres in the interhemispheric fissure.
This is one of two areas where the two dura layers seperate.
What is the tentorium cerebelli
ent-like sheet of dura that covers the upper surface of the cerebellum
This is one of two areas where the two dura layers seperate.
What are the two layers of the dura called?
- Periosteal layer (adhered to inner surface of skull) & Meningeal Layer
What are cerebral sinuses?
The dural venous sinuses lie between the periosteal and meningeal layers of the dura mater. They are best thought of as collecting pools of blood, which drain the central nervous system, the face, and the scalp.
All the dural venous sinuses ultimately drain into the internal jugular vein. Unlike most veins of the body, the dural venous sinuses do not have valves.
main one would be the superior sagittal sinus.
Describe the arachnoid
spidery meningeal layer that adheres to the inner surface of dura
CSF trickles through this
What is the pia
it is the innermost layer of the meninges.
It is a thin layer of cells that adheres very closely to the surface of the brain, follows along the gyri and deep into the sulci.
What supplies the dura mater blood?
middle meningeal artery.
runs through the epidural space.
Describe the subdural space
The subdural space is between the dura and arachnoid.
- Bridging veins go through this subdural space.
- These veins drain cerebral hemispheres and pass through subdural space to several dural venous sinuses.
What is the superior sagittal sinus?
It is a dural venous sinus that lies between the two layers of dura, and drains deoxygenated blood mainly via the SIGMOID sinuses to reach the INTERNAL JUGULAR VEINS.
What is the subarachnoid space
This is the space between the arachnoid and pia.
It is filled with CSF.
Also has major arteries travelling through it, and send smaller penetrating branches through the pia.
Where can you find the three meningeal layers?
It covers the entire CNS.
This includes the brain, and the entire spinal cord.
What are the inner walls of ventricles lined by?
ependymal cells
these keep the CSF from leaking out onto the surface of the brain.
Where is CSF produced?
by the choroid plexus, which is inside the ventricles.
Describe what the ventricles consist of.
- 2 lateral ventricles
- 3rd ventricle which is in the diencephalon
- 4th ventricle which is surrounded by the medulla, pons and cerebellum.
Where is the atrium in the ventricles?
also known as trigone
This is the triangular area, at the junction of the body, temporal and occipital horn of the lateral ventricle.
Name some c shaped structures in the brain
lateral ventricles corpus callosum fornix caudate nucleus stria terminalis
What are the walls of the third ventricle made of
the thalamus and hypothalamus
What are the walls of the fourth ventricle made of
top is made of the cerebellum, the bottom is made of the pons.
What connects the lateral ventricles to the 3rd ventricle
intraventricular foramen
OR
formen of monro
What connects 3rd and 4th ventrices
cerebral aqueduct
or
aqueduct of sylvius.
Where does CSF leave the ventricular system?
via various foramen in the fourth ventricle.
These are the foramen of lushka and midline foramen of magendie.
Where does CSF go after it leaves the ventricular system?
It percolates the subarachnoid space of the spinal chord and the brain., until it is ultimately absorbed by the arachnoid granulations, into the superior saggital sinus, then back into the bloodstreams.
where is most of our CSF
cranial subarachnoid space
What are cisterns
This is where the subarachnoid space opens wider to form larger pools of CSF.
Name the two main cisterns
cisterna magna - located beneath the cerebellum near the foramen magnum
Quadrigeminal cistern - posterior to midbrain, and beneat the posterior park of corpus callosum.
What creates the blood brain barrier?
The endothelial cells that form the capillary walls in the brain have tight junctions, which restricts the metabolites which can get through to the brain.
Cannot pass things through from blood supply to brain.
Describe the Blood-CSF Barrier
this is in the choroid plexus.
The epithelial cells form a barrier between capillaries and the CSF
What can cross the blood brain barrier
lipid solubles, such as oxygen, CO2
What are circumventricular organs?
These are organs or areas whereby the blood brain barrier is interrupted.
These are mostly hormonal brain regions
They need to respond to changes to the chemicals in the rest of the model in order to secrete the appropriate modulatory neuropeptides into the bloodstream.
eg. Median eminence -
Where ist he blood brain barrier interrupted?
Circumventricular organs
eg. median eminence.
neurohypophysis of pituitary
area postrema - for detecting toxins that cause vomiting.
What is an intracranial mass lesion
anything that occupies volume within the cranial vault.
What is a mass lesion?
this causes local tissue damage and remove effects due to mechanical distortion
mass effect
What happens when there is a space occupying or mass lesion within the skull?
- Something must leave to accomodate for the extra volume.
- small lesions can be accomodated for by decreasing CSF and blood
- larger lesions will cause raised intercranial pressure, ultimately leading to herniation and death.
Describe what herniation is.
This occurs when mass effect is severe enough to push brain structures from one compartment into another.
3 most common herniation syndromes are:
- herniation through tentorial notch
- central herniation
- herniation down through the falx cerebri (subfacine herniation)
What are signs of raised intercranial pressure?
Headache Altered mental status, especially irritability & depressed level of arousal & attention Nausea and vomiting Papilloedema Visual loss Diplopia (double vision) Cushing’s triad: hypertension, bradycardia, and irregular respirations
- optic cup becomes convex.
What is cushing’s triad?
It is a sign of ICP
hypertension, bradycardia, and irregular respirations
What is normal intracranial pressure?
less than 20cm h20
or less than 15 mm hg.
usually measuring during lumbar puncture.
do not do lumbar puncture if severely elevated ICP is suspected.
Name the three most common herniation syndromes.
central herniation - the brainstem is displaced downwardly.
transtenorial herniation - herniation of the temporal lobe, especially the uncus. This causes a clinical triad of blown pupil (due to CN III being comperssed, ipsilateral), hemiplegia (usually contralateral, except for when big herniation and it compresses the other side of the corticospinal tract) and coma (due to reticular formation compression).
subfalcine herniation - herniation under the falx cerebri, going from one side of the cranium to another.
no clear clinical signs are attributed to this.
Describe central herniation
downward shift of brainstem.
usually occurs through the foramen magnum
tonsillar herniation is associated with the medulla being compressed leads to respiratory problems, BP instability and eath.
Describe subfalcine herniation
Herniation under then falx cerebri.
This is caused by unilateral mass lesions and can mostly cased the cingulate gyrus to herniate from one side to other side.
What are some reason hydrocephalus can occur?
- excess csf production
- obstruction of flow - most common
- decrease in reabsorption via subarachnoid granulations.
excess csf production is very rare and only occurs in certain tumurs.
What are the types of hydrocephalus
- communicating hydrocephalus, which is caused by impaired CSF reabsorption, obstruction of flow in the subarachnoid space or excess CSF prduction
- noncommunicating hydrocephalus - caused by obstruction of flow WITHIN THE VENTRICULAR SYSTEM.
- normal-pressure hydrocephalus - chronically dilated ventricles, usually in elderly. usually have gait difficulties, urinary incontinence and mental decline. FORM OF COMMUNICATING hydrocephalus.
- hydrocephalus ex vacuo .- excess csf in a region where brain tissue was lost as a result of stroke, surgery, atrophy, trauma etc.
What are symptoms of hydrocephalus?
- depends on acuteness or chronicity.
- headache, vomitting, cog impairment, decreased cnsciousness, papilloedema, decreased vision and nerve palsies.
- ventricular dilation could compress descending white matter pathways in frontal lobes, leading to unsteady magnetic gait and incontinence.
Where does the lumbar puncture occur?
Into the subarachnoid space of the lumbar cistern.
This is at L4.
What is CSF?
clear, colourless fluid.
- chemical buffer to maintain constant ionic enviro
- modular pressure change
- protects cranial contents
- around 150 ml in adult
- 400-500ml produced daily
- Cells are believed to actively secrete Na+ into the ventricular system in exchange for K+. Sodium ions electrically attract Cl- and osmotically draw water from the blood vascular system to constitute the CSF.
describe the circulation of CSF
- Due to pulsation of blood in choroid plexus
- Due to pulsation of ependymal cells
- Lateral ventricles > interventricular foramen of Monroe >third ventricle >mesencephalic aqueduct (aqueduct of
Sylvius) >fourth ventricle >spinal cord central canal; also, out the lateral apertures to the subarachnoid space to the venous system
What can cause blockage of CSF absorption?
- Subarachnoid hemorrhage.
- brain tumour
- debris from hemorrhage
- infection, inflammation of meninges.
Describe bacterial infections in the CNS
- cocci, bacilli –> bacterial meningitis, brain abscess and epidural abscess.
- occurs through blood stream, usually through having infections in order areas of the body.
- oral nasal passages or surgery, trauma.
Describe infectious diseases in the CNS
- infectious meningitis is an infection of the CSF in subarachnoid space
- caused by bacteria, viruses, fungus or parasite.
usually symptoms are signs of meningeal irritation
What are the signs of meningeal irritation
- headache
- lethargy
- sensitivity to light
- sensitivity to noise
- fever
- nuchal rigidity.
What is nuchal rigidity
the neck muscles contract involuntarily resulting in resistance to active or passive neck flexion and neck pain
