Week 2 - coverings, ventricles and barriers of the nervous system Flashcards
Why are there barriers between the nervous system and the rest of the the body
to enable efficient performance of it’s functions, for protection
name three types of barriers
- bony structures that physically protect
- meanings: three membranes that surround and support
- barriers between the internal and external chemical environment
name 2 examples of barriers between the internal and external chemical environments
- blood - CSF barriers: at its site of production and return to venous blood
- blood - brain barrier
Where is the brain situated?
in the cranial cavity
what is the cranial cavity divided into?
three sections: anterior, middle and posterior cranial fossae
What bones are included in the anterior fossae?
- frontal
- ethmoid
- sphenoid
What bones are included in the middle fossa?
sphenoid and temporal
What bones are included in the posterior fossa?
sphenoid, temporal, parietal, occipital
What other contents are included in the anterior fossa?
frontal lobes and olfactory nerves
What other contents are included in the middle fossa?
temporal lobes and pituitary glands
What other contents are included in the posterior fossa?
brainstem and cerebellum
What are meninges?
three membranes, surround and support the brain in the crania cavity and extend down around the brainstem and spinal cord
Name the three meninges
dura mater
arachnoid mater
pia mater
describe the dura mater
the outermost, tough fibrous membrane loosely surrounding the brain and attached to the skull in places
- has blood and nerve supply
describe the arachnoid mater
soft sac-like membrane that also envelops the brain
- fibrous processes extend to the inner Pia
- subarachnoid spaces contains CSF and the CNS arteries and veins
describe the Pia mater
follows contours of the brain and adheres to the basement membrane of the brain’s glia surface lining (glia limitans)
Name the layers form skull to CNS
- skull
- dura mater include. periosteal arteries and veins
- arachnoid
- subarachnoid space include. CSF, CNS arteries and veins, arachnoid tabreculae
- Pia mater
astrocyte end-feet - CNS
What are dural reflections?
two large reflections (folds) of the dura which extent into the cranial cavity at the longitudinal and transverse fissures
name the two dural reflections
fall cerebri
tentorium cerebelli
What is the fall cerebri?
extends from the cranial roof between the cerebral hemispheres
What is the tentorium cerebelli ?
horizontal shelf of dura between the cerebrum and cerebellum
Where are the fall cerebri and tentorium cerebella continuous?
posteriorly
Where does the brainstem pass through?
the gap in the shelf called tensorial incisure
What two layers is the dura mater comprised of?
periostea and meningeal
Do the periosteal and meningeal layers ever separate? If so why?
yes - to enclose dural sinuses
What dod Ural sinuses contain?
venous blood leaving the brain
What is the site of CSF production
choroid plexus
What forms the choroid plexus?
an invagination fo vascular pa mater into the ventricle forming a highly convoluted tuft of capillaries
Name the three cellular componenets of the chord plexus
cellular constituents of the vascular core are the blood vessel endothelium (1) and Pia cells (2), on its ventricular surface, choroidal epithelium (3)
What secretes the CSF into the ventricle?
specialised ependymal cells with tight intercellular junctions between them
Describe CSF
- clear and colourless, acellular fluid
- low in protein, similar but not identical to serum in its ionic composition, contains approx 60% of the glucose of serum
Where is the choroid plexus found?
in each ventricle, filled with CSF which flows form one to the next
Into what do the ventricles empty their CSF ?
subarachnoid space
What are arachnoid villi?
act as one-way valves that allow CSF to leave form the subarachnoid space into venous blood of the dural sinuses
Into what structure does the CSF through the arachnoid villi?
form the subarachnoid space into venous blood of the dural sinuses
Where os the CSF in the subarachnoid space collected?
into pockets formed where there are irregulatites on the surface of the CNS - cisterns
How many ventricles are there?
4
Describe the lateral ventricles (1 and 2)
- c-shaped structures in the deep cerebrum
- communicate via interventriuclar foramen of Monro w 3rd
How do 1&2 communicate with the 3rd ventricle?
through the interventriuclar foramen of monro
Describe the 3rd ventricle
a midline slit within the diencephalon, communicates via cerebral aqueduct of the midbrain with the 4th ventricle
name the three openings of the 4th ventricle
2 lateral foramina (of Luschka)
1 midline foramen (of Magendle)
describe the 4th ventricle
- tent-like roof pokes up into cerebellum and widens laterally
- caudal it narrows to form vestal central canal of the casual medulla and spinal cord
- 3 openings aria CSF to subarachnoid space
What drives the different parts of the ventricular system?
the neural canal
What parts of the adult brain derives from the telencephalon (forebrain)
cerebral cortex, basal ganglia, hippocampus, olfactory bulb, basal forebrain
What ventricular space is associated to the cerebral cortex, basal ganglia, hippocampus, olfactory bulb, basal forebrain?
lateral ventricles
What brain parts of the adult brain are derived from the diencephalon?
the dorsal thalamus and hypothalamus
What ventricular space is associated to the dorsal thalamus and hypothalamus?
third ventricle
What adult brain parts derive form the mesencephalon ?
midbrain (superior and inferior colliculi)
What ventricular space is associated to the midbrain (superior and inferior colliculi)?
cerebral aqueduct
What part of the adult brain is derived from the metencephalon?
cerebellum, pons
What ventricular space is associated to the cerebellum/ pons?
fourth ventricle
What part of the adult Brian is derived from the myelencephalon?
medulla
What ventricular space is associated to the medulla?
fourth ventricle
What ventricular space is associated to the spinal cord?
central canal
Summaries the circulation of CSF
- flows form lateral ventricles to 3rd
- to 4th
- to subarachnoid space
- CSF passes through arachnoid villi (granulations)
- joins venous blood mainly in sinuses
Is the flow from subarachnoid space to the venous sinus unidirectional or both ways?
only in one direction (space ti sinus)
Name the four main functions of CSF
- physical support and protection
- spatial buffer
- elimination of waste
- chemical communication
Describe CSFs role in physical support and protection
- CNS partially floats in subarachnoid CSF, makes it easier for meninges to stabilise the CNS in the head and spinal cord
- cushions Brian, eliminates effects of movement and reduces effects of trauma
Describe CSFs role in being a spatial buffer
- some CSF can be displaced as necessary ie. to limit rising intracranial pressure if for example there is damage to a blood vessel
-> prevents ischaemia due to reduced blood perfusion
Describe CSFs role in elimination of waste
- regulates chemical environment of the CNS by removing water-soluble metabolites
- these diffuse form Brian into CSF for excretion into venous blood, facilitated by lymphatic system
Describe CSFs role in chemical communication
provides a medium for changes in ionic conc., hormones and other neurochemicals to reach neurones
What does the subarachnoid space contain beside CSF ?
blood vessels of CNS
Do CNS blood vessels ever penetrate the brain?
yes, for a short time tithe with the Pia mater
What are the areas around the vessels of the CNS called?
perivascular spaces
What do perivascular spaces do?
they provide a path for CSF to flow from the subarachnoid space into the interstitial spaces of the brain and spinal chord
list the different levels from white matter to dura mater
white matter
cerebral cortex
perivascular space
artery
pia mater
arachnoid trabeculae
subarachnoid space
arachnoid mater
dura mater
CSF flow though the brain is pulsatile in phase with what?
the heart rate
What are astrocyte processes called?
end feet
What surfaces possess astrocyte end feet?
most neuronal surfaces, of dendrites and cell bodies, some axonal surfaces
Where in the brain do astrocyte end feet exist?
where the outer surface of the brain meets the inner surface of the Pia
What is the lining of astrocyte end feet also called?
glia limitans
What do end feets do?
facilitate CSF - interstitial space interchange of fluid
What does every blood vessel in the CNS have?
a coat of end feet that separates them from natural tissue
What is the glymphatic system?
- analogous tot he body’s lymphatic system that moves excess fluids and wastes into the blood
- comprises glial cells (astrocytes) and the perivascular space
What is CSF’s relation to arteries?
- CSF influx is organised around arteries
- Arterial pulsation derives CSF movement
What Is special about astrocytes expressing aquaporin 4?
they are essential for perivascular CSF - interstitial fluid exchange
Where do interstitial fluid exists?
along veins
What happens during slow wave sleep?
- increase of 60% in the interstitial space, which dramatically increases CSF interstitial fluid exchange and metabolite clearance
What makes up the blood-brain barrier
- astrocyte end feet and 2 endothelial cell tight junctions
What does the blood brain barrier do ?
- prohibits the free entry of many substances form the blood into the brain
- lipid soluble molecules can cross this barrier unaided
What allows selective transport of material required for brain function?
specific transported int he endothelial cell membranes
What percentage makes the brain up of the entire body weight?
around 2 percent
How much percent of oxygen and cardiac output does the brain use?
20% of O2 and 15% of cardiac output
what can lead to unconsciousness?
20 seconds of anoxia (lack fo oxygen)
when can anoxia have permanent effects?
it it lasts for more than 5 min
Describe arteries
thin walled, easily blocked and ruptured
Describe veins
no valves, thin walled, no muscular or elasticity
The brain uses autoregulation which occurs in two ways, describe both
- brain blood flow remains constant despite changes in systemic blood pressure, via dilation or constriction of arterioles
- local responses, mediated largely by astrocytes, provide increased blood flow to activate Brian areas
What vessel supplies anterior circulation?
carotid artery
What vessel supplies posterior circulation?
vertebral artery
From what artery does the vertebral artery defend from?
the subclavian artery
How many branches does the aorta have?
3 major branches
name the three major branches of the aorta
- brachiocephalic artery
2 left common carotid artery - left subclavian artery
what does the brachiocephalic artery branch into?
the right common carotid and right subclavian arteries
What does the common carotid artery supply?
the external and internal carotid arteries
What does the subclavian artery supply?
the vertebral artery
How high is the percentage for cerebral blood flow of anterior (carotid) circulation?
- 75%
What branches are involved in anterior circulation?
- fed by internal carotid arteries
- main branches include anterior and middle cerebral arteries
how much percentage does the posterior circulation make up of the cerebral blood flow?
around 25 %
what artery feeds the posterior (vertebral) circulation?
the subclavian arteries
What arises from subclavian arteries?
2 vertebral artery
how do the two arteries continue after depending from the subclavian arteries?
join to form the single basiler artery on the ventral surface of the pons
What are the main branches participating in posterior circulation?
poster cerebral artery
what is the sole blood supply for the brainstem and the cerebellum?
the poster circulation
What contributes to the blood supply of the spinal cord?
the posterior circulation through vertebral artery branches (anterior and posterior spinal arteries)
What does the anterior circulation consist of?
the internal carotid artery and its branches
name the branches of the internal carotid artery
- ophthalmic
- anterior choroidal
- middle cerebral
- anterior cerebral
- posterior communicating
What area is supplied by the ophthalmic CA ?
orbit and retina
What area is supplied by the anterior choroidal CA ?
- diencephalic and subcortical telencephalon structures such as internal capsule, choroid plexus of lateran and third ventricle
What area is supplied by the middle cerebral CA ?
- basal ganglia and internal cause - via striate arteries
- lateral 2/3rds of cortex
What area is supplied by the anterior cerebral CA ?
- internal capsule
- medial side of frontal and parietal lobes
What area is supplied by the anterior cerebral CA ?
- internal capsule
- medial side of frontal and parietal lobes
What area is supplied by the posterior communicating CA ?
connects carotid and vertebral artery system
what is being supplied by superficial vessels?
cortical regions
What is being supplied by deep or perforating vessels?
subcortical grey and white matter
What is the main tool used to study blood flow?
cerebral angiography
name one superficial vessel
the anterior cerebral artery (ACA)
What does the anterior cerebral artery supply?
the medial aspects of the cerebral hemisphere
What does the middle cerebral artery (MCA) supply?
most of the lateral cortex and insular cortex within the lateral fissure
What does the MCA posses?
deep perforating arteries - medial/lateral striate arteries
What are the main branches of posterior circulation?
- vertebral
- basilar
- poster cerebral artery
What parts does the vertebral artery supply?
- spinal cord and medulla
- dorsal medulla and cerebellum via posterior inferior cerebellar artery (PICA)
What parts does the basilar artery supply?
- pons
- cerebellum (AICA and superior cerebellar artery)
What parts does the posterior cerebral artery supply?
- midbrain
- thalamus and optic radiation
- inferior and medial aspects of temporal and occipital cortex
How is the basil artery formed?
through fusion of two vertebral arteries
Why does the poster circulation system have deep, perforating vessels?
arise to provide blood to brainstem
What does the poster cerebral artery (PCA) supply?
occipital lobe and inferior and medial parts of the temporal lobe
What two regions are being supplied by deep vessels?
the basal ganglia and the thalamus
By what vessels is the basal ganglia being supplied?
via ACA and MCA (both striate arteries)
By what vessels is the thalamus being supplied?
supplied by PCA (anterior and choroidal)
What subdivision of the brain stem supplies medial structures?
medial vessels - vertebral (and anterior spinal) and basal arteries
What subdivision of the brain stem supplies lateral aspects?
longer cerebellar and poster cerebral arteries
When do the anterior and poster circulations connect?
- terminal ends of cerebral arteries anastomoses (arteries interconnect)
- Circle of Willis - on the ventral brain, proximal parts of carotid and vertebral vasculatures anastomose
Name the 6 cortical functional areas
- primary motor cortex
- primary somatosensory cortex
- primary auditory cortex
- primary visual cortex
- Broca’s expressive speech
- Wernicke’s receptive speech
What could cause functional deficits?
a stroke (cell death due to loss of blood supply)
What is a homunculus?
a topographic representation of the body
What does the MCA supply?
lateral cortex
What does the ACA supply?
medial cortex
What happens if the ACA is blocked/ ruptured?
hemiparesis and hemisensory loss affecting Lowe extremity only
What does anastomoses between the ACA and MCA allow for?
partial compensation for loss of blood supply if one is occluded
Describe the circle of willis
- poster communication arteries of anterior circulation join first part of posterior cerebral arteries
- together with the anterior cerebral arteries + anterior communicating artery -> arterial circle
How many people have the classic circle of wills?
only 33 percent, normally channels are closed
Describe the blood supply of the spinal cord
the single anterior and two poster spinal arteries run the length of the spinal cord
where does the anterior spinal artery arise?
form the vertebral arteries at the medulla
where does the two posterior spinal arteries arise from?
vertebral arteira (or PICA)
What vessel additionally supplies the spinal cord at lower levels?
radicular arteries
Where do radicular arteries ascend from?
segmental medullary arteries
what is venous drainage?
blood from the brain and spinal cord drains form capillaries into veins
Explain the venous drainage form the brain
- both superficial and deep cerebral veins first drain into dural venous sinuses
- final sigmoid sinuses return the blood to the internal jugular vein
Explain the venous drainage form the spinal cord
spinal veins are part of systemic vasculature
Describe superficial venous drainage
- superficial cerebral veins cross the subarachnoid space
- these pierce dura (as bridging veins) as they enter intracranial dural venous sinuses
- superior and inferior sagittal sinuses drip blood form lateral and medial cortex, joining at the confluence of sinuses
- blood then travels via transverse sinuses to sigmoid sinuses
Where does blood form lower parts of the cortex, basal ganglia and thalamus drain into?
the cavernous sinus, which then send the blood the petrosal sing tot he sigmoid sinus
What passes through the cavernous sinus?
the internal carotid artery
what does the great cerebral vein of Galen do?
brings blood from deep structures to the straight sinus
key concepts
- the anterior carotid and posterior - vertebral - circulations supply blood to the brain
- the cortical blood supply is derived form the anterior, middle and poster cerebral arteries. Perforating arteries supply deep cerebral structures
- vessels anastomose (connect) to provide alternative blood flow. At the ventral surface of the brain the circle of willis connects the anterior and posterior circulators
- spinal cord is supplied by anterior and posterior spinal arteries rostrally arising form the posterior circulation and supplemented by the radicular arteries at lower levels
- venous return of blood is mediated by deep veins, superficial veins which drain into the dural sinuses. Blood ultimately returns to the systemic circulations via the internal jugular veins
