CSF and Ventricular System Flashcards
Describe the functions of CSF
Serves as a cushion between the central nervous system and the surrounding bones protecting it from mechanical trauma
As the density of the brain is only slightly greater than that of the CSF it provides the brain with mechanical buoyancy and support
Serves as a reservoir and assists in the regulation of the contents of the skull due to the close relationship of the fluid to the nervous tissue and the blood i.e. if blood volume increases CSF volume decreases
Nourishes the central nervous system as it provides an ideal physiologic substrate
Removes metabolites from the central nervous system
Serves as a pathway for pineal secretions to reach the pituitary gland
Identify the lablled structures
Posterior cerebromedullary (cisterna magna)
B/t the cerebellum and dorsal surface of MO
Lateral cerebromedullary
Along each side of the medulla
Chiasmatic
Behind and above optic chiasm
Cistern of lateral cerebral fossa
Along the lateral sulcus (Sylvian fissure)
Interpeduncular
Interpeduncular fossa
Ambient (cisterna ambiens)
Each side of midbrain
Superior cistern/ Quadrigeminal/ cistern of great cerebral vein
Surrounding the great cerebral vein dorsal to the midbrain colliculi (quadrigeminal bodies)
Pontine
Anterior surface pons and MO, continuous posteriorly cerebromedullary cistern
Identify the labelled structures
Posterior cerebromedullary (cisterna magna)
B/t the cerebellum and dorsal surface of MO
Lateral cerebromedullary
Along each side of the medulla
Chiasmatic
Behind and above optic chiasm
Cistern of lateral cerebral fossa
Along the lateral sulcus (Sylvian fissure)
Interpeduncular
Interpeduncular fossa
Ambient (cisterna ambiens)
Each side of midbrain
Superior cistern/ Quadrigeminal/ cistern of great cerebral vein
Surrounding the great cerebral vein dorsal to the midbrain colliculi (quadrigeminal bodies)
Pontine
Anterior surface pons and MO, continuous posteriorly cerebromedullary cistern
Briefly outline the ventricular system of the brain
The ventricles are fluid filled cavities within the brain; there are 4 in total, 2 lateral ventricles, a third ventricle and a fourth one.
The two lateral ventricles communicate through the interventricular foramina (foramina of Munro) with the third ventricle.
The third ventricle is connected to the fourth ventricle by
the cerebral aqueduct (mesencephalic aqueduct/mesencephalic duct/aqueduct of Sylvius).
The fourth ventricle is continuous with the central canal of the spinal cord and the subarachnoid space through three
foramina (2 Lushka and 1 Magendie).
At the inferior end of the spinal cord there is a small swelling known as the terminal ventricle.
The ventricles are lined throughout with ependyma and are filled with cerebrospinal fluid (CSF).
Describe the lateral ventricles and their boundaries
There are two large lateral ventricles one present in each cerebral hemisphere. Each ventricle is C-shaped and can be divided into a number of parts;
The body of the lateral ventricle
occupies the parietal lobe
Extends from the interventricular foramen posteriorly as far as the posterior end of the thalamus, where it becomes
continuous with the posterior and inferior horns
Contains the choroid plexus which projects into it through a slit like gap (the choroidal fissure) between the fornix and the superior surface of the thalamus.
Roof - under surface of the corpus callosum
Floor - body of the caudate nucleus and lateral margin of the thalamus
Medial wall - septum pellucidum
The anterior horn of the lateral ventricle
extends forward from the body into the frontal lobe
Continuous posteriorly with the body of the ventricle at the interventricular foramen
Roof - under surface of the anterior part of the corpus callosum, the genu of the corpus callosum limits the anterior horn anteriorly
Floor - rounded head of the caudate nucleus,
medially, a small portion of the floor is
formed by the superior surface of the
rostrum of the corpus callosum
Medial wall - septum pellucidum and the anterior column of the fornix
The posterior horn of the lateral ventricle
extends posteriorly into the occipital lobe
Roof and lateral wall - fibres of the tapetum of the corpus callosum, lateral to the tapetum are the fibres of the optic radiation
Medial wall - two elevations
superior swelling caused by the splenial fibres of the corpus callosum (forceps major) called the bulb of the posterior horn
The inferior swelling is caused by the calcarine sulcus and is called the calcar avis
The inferior horn of the lateral ventricle
extends anteriorly into the temporal lobe
- *Roof** - tapetum of the corpus callosum and tail of the caudate nucleus
- *Floor** -laterally by the collateral eminence, produced by the collateral fissure and medially by the hippocampus
Describe the third ventricle
Slit like cleft between the two thalami
Communicates anteriorly with the lateral ventricles
through the interventricular formania and posteriorly with the fourth ventricle through the cerebral aqueduct
Running through the 3rd ventricle is interthalamic adhesion containing thalmic neurones and fibres
Anterior wall - lamina terminalis
Posterior wall - opening into the cerebral aqueduct
- *Lateral wall** - medial surface of the thalamus superiorly and the hypothalamus inferiorly which are separated by the hypothalamic sulcus. The lateral wall is limited superiorly by the
- stria medullaris thalami*
Superior wall/ Roof - two layered fold of pia matter called the tela choroidea of the third ventricle in which lie the internal cerebral veins. The vascular tela choroidea invaginates the roof to form
the choroid plexus of the third ventricle.
Floor - optic chiasma, the tuber cinereum, the
infundibulum and the mammillary bodies.
Describe the fourth ventricle
Tent shaped cavity situated anterior to the cerebellum and posterior to the pons and the superior half of the medulla oblongata
Continuous superiorly with the cerebral aqueduct
and inferiorly with the central canal of the medulla oblongata and the spinal cord
Lateral boundary - caudally i_nferior
cerebellar peduncle_, cranially superior cerebellar
peduncle
Roof - superior part, medial borders of the two superior cerebellar peduncles and the superior medullary velum
inferior part, inferior medullary velum which is pierced in the midline by the median aperture (foramen of Magendie). Lateral recesses extend laterally around the sides of the medulla
oblongata and open anteriorly as foramina of Luschka
These openings permit the CSF to flow from the ventricular system into the subarachnoid space.
Floor - diamond shaped, posterior
surface of the pons and cranial half of the medulla oblongata, divided into two symmetrical halves by the median sulcus
Describe the choroid plexuses
Choroid plexus of the lateral ventricles
projects into the ventricle on its medial aspect and is a
vascular fringe composed of pia matter covered with the ependymal lining, irregular lateral edge is the tela choroidea (a two layered fold of pia matter situated between the fornix superiorly and the upper surface of the thalamus).
At the junction of the body and the inferior horn, the choroid plexus continues into the inferior horn via choroidal fissure.
Choroid plexuses of the third ventricle
vascular tela choroidea projects down on each side of the midline invaginating forming two ridges that hang from the roof of the ventricle
Blood supply of the tela choroidea and choroid plexuses of the lateral and third ventricles is derived from the choroidal branches of the internal carotid and basilar arteries
Choroid plexuses of the fourth ventricle
T shaped, is suspended from the inferior half of the
roof of the ventricle and formed by the tela choroidea
Blood supply to this plexus is from the
posterior inferior cerebellar arteries
Describe the formation of CSF and a common problem that can occur
CSF is formed mainly by the choroid plexuses of the lateral, third and fourth ventricles which actively
secrete CSF creating a small pressure gradient
Choroid plexuses have a highly folded surface and each
fold consists of a core of vascular connective tissue covered with cuboidal epithelium of the ependyma.
At the same time they actively transport nervous system metabolites from the CSF to the blood
Produced at a continuously at a rate of about 0.5ml/min with a total volume of about 150ml corresponding to
a turnover time of about 5 hours
CSF production is not pressure regulated and it will continue to be
produced even if reabsorption mechanisms are obstructed which can lead to hydrocephalus
Hydrocephalus is an abnormal increase in the volume of CSF within the skull,
if accompanied by a raised CSF pressure then it is due to either;
an abnormal increase in the formation of the fluid,
a blockage of the circulation,
or a diminished absorption of the fluid.
Rarely hydrocephalus occurs with normal CSF pressure,
and in these patients there is a compensatory hypoplasia or atrophy of the brain substance.
Describe the subarachnoid space
The subarachnoid space is the space between the arachnoid and pia mater and is present where the
meninges envelop the brain and spinal cord
It is filled with CSF and contains the large blood vessels of the brain
It surrounds the cranial and spinal nerves and follows them to
the point where they leave the skull and vertebral canal. Here
the arachnoid mater and pia mater fuse with the perineurium of
each nerve.
Completely surrounds the brain and extends along the olfactory nerves to the mucoperiosteum of the nose,
extends around the optic nerve where the arachnoid and pia mater fuse with the sclera. The central artery and vein of the retina cross this extension to enter the optic nerve and they may be compressed in patients with raised CSF pressure
Extends along the cerebral blood vessels as they enter and leave the substance of the brain and stops where they become an arteriole or venule
The subarachnoid space also extends around
the arteries and veins of the brain and spinal cord where they penetrate the nervous tissue, but
the pia mater fuses with the outer coat of the blood vessel below the surface of the brain and spinal cord,
closing the subarachnoid space.
Communication occurs between the space and perineural lymph vessels
In certain areas around the base of the brain, the arachnoid does
not closely follow the surface of the brain to form subarachnoid cisterns
(cerebelomedullary cistern, pontine cistern and interpeduncular
cistern)
Inferiorly the subarachnoid space extends beyond the
lower end of the spinal cord and invests the cauda equina, it ends
at the level S2/S3
Describe the circulation of CSF
Circulation begins with its secretion from the choroid plexuses in the ventricles, the fluid passes from the
lateral ventricles into the third through the interventricular foramina
then passes into the fourth ventricle through the cerebral aqueduct,
this circulation is aided by the arterial pulsations of the choroid
plexuses and by the cilia on the ependymal cells lining the ventricles
Fluid passes from 4th ventricle through the median aperture and lateral foramina to enters the subarachnoid space
Then moves through the cerebellomedullary cistern (cisterna magna) and pontine cisterns and flows superiorly through the tentorial notch of the tentorium cerebelli to reach the
inferior surface of the cerebrum
Then moves superiorly over the lateral aspect of each cerebral
hemisphere, assisted by the pulsations of the cerebral arteries
Some of the CSF moves inferiorly around the spinal cord and cauda equina, here the fluid is at a dead end and further
circulation relies on the pulsations of the spinal arteries and movements of the vertebral column,
respiration, coughing and the changing positions of the body.
Describe the subarachnoid cisterns
Arachnoid is adherent to the inner surface of the dura matter, it conforms to the general shape of the brain but does not dip into the sulci or follow the more intricate contours of the surface of
the brain. Therefore there is a subarachnoid space filled with CSF between the arachnoid and pia matter.
This space is very narrow over the surfaces of the gyri, relatively small where the arachnoid bridges over small sulci, and much larger in where it bridges over surface irregularities. These regions
contain a considerable volume of CSF and are called sub arachnoid cisterns
The cerebromedullary cistern (cisterna magna) – largest cistern, space between the inferior surface of the cerebellum and the posterior surface of the medulla oblongata.
The pontine cistern – anterior surface of the pons and the medulla, continuous posteriorly with the cerebromdullary cistern
The interpeduncular cistern – between the cerebral peduncles and contains the posterior part of the Circle of Willis.
The superior cistern (quadrigeminal cistern / cistern of the great cerebral vein) – radiological landmark above the midbrain and is continuous laterally with a thin, curved layer of subarachnoid space
on each side that partially encircles the midbrain before opening into the interpeduncular cistern.
A finger like extension of the subarachnoid space between the fornix and the roof of the 3rd ventricle,
continues anteriorly from the superior cistern.
Describe the absorption of CSF
Main sites for the absorption of CSF are the arachnoid villi that project into the dural venous sinuses, especially the superior sagittal sinus
The villi group together to from arachnoid granulations which
structurally are diverticulums of the subarachnoid space that pierce the dura matter
Arachnoid granulations increase in number and size with age and tend to become calcified with advanced age.
Absorption of CSF from the arachnoid granulations into the venous sinuses occurs when the CSF pressure > venous pressure in the sinus allowing a direct flow from the subarachnoid space into the lumen of the sinuses.
If venous pressure > CSF pressure the
compression of the villi close the tubules and blood is prevented from entering
Some CSF may also be absorbed directly into the veins of the subarachnoid space and some possibly escapes through the perineural lymph vessels of the spinal and cranial nerves.
As the rate of production is constant, it is the absorption via arachnoid granulations that controls the CSF pressure.
Describe the blood-brain barrier
The central nervous system requires a very stable environment in order to function normally
The BBB is a continuous lipid bilayer that encircles the
endothelial cells and isolates the CNS from the blood (reason that
lipophilic molecules can diffuse across readily, whilst hydrophilic
molecules are excluded)
The permeability of the blood-brain barrier (BBB) is inversely related to
the size of the molecules and directly related to their lipid solubility.
Gases and water move readily through the barrier, whilst glucose and electrolytes pass through more slowly (it is almost impermeable to plasma proteins).
The lumen of a blood capillary is separated from the extracellular spaces around the neurons and neuroglia by;
endothelial cells in the capillary wall
continuous basement membrane surrounding the capillary outside the endothelial wall
foot processes of astrocytes that adhere to the outer surface of the
capillary wall
tight junctions around the endothelial cell wall
Research has demonstrated that it is the presence of tight junctions
between the endothelial cells of the blood capillaries that are responsible for the BBB.
The BBB is more permeable to certain substances in the newborn than in the adult and its structure is not identical in all regions of the CNS eg bilirubin which can cross in infants and cause learning development
In areas where the BBB appears to be absent, the capillary epithelium
contains fenestrations across which small proteins and small organic
molecules may pass from the blood to the nervous tissue. Areas in
which this has been suggested to occur are the area postrema of the
floor of the fourth ventricleand thehypothalamus. It is thought that
at these sites neuronal receptors sample the chemical content of the blood plasma directly to monitor and maintain homeostasis.
Under normal conditions the BBB and blood- CSF barrier are two important semipermeable barriers that
protect the CNS from potentially harmful substances while permitting gases and nutrients to enter the
nervous tissue.
Describe the Blood-CSF barrier
It has been suggested that a barrier similar to the BBB exists in the
choroid plexus.
It has been shown that the lumen of a blood vessel is
separated from the ventricle by;
fenestrated thin endothelial cells (these fenestrations are filled by
a thin membrane)
continuous basement membrane surrounding the capillary outside
the endothelial wall
scattered pale cells with flattened processes
continuous basement membrane
choroidal epithelial cells which rest on the basement membrane
It is probable that the tight junctions between the choroidal epithelial
cells serve as the barrier
Under normal conditions the BBB and blood- CSF barrier are two important semipermeable barriers that
protect the CNS from potentially harmful substances while permitting gases and nutrients to enter the
nervous tissue.
Describe the CSF-Brain interface
No comparable barrier between the CSF and extracellular compartment of the CNS, dyes injected into the subarachnoid space or ventricles soon enter the extracellular spaces around the
neurons and glial cells.
However, there are structures that separate the CSF from the nervous tissue;
pia mater covering the surface of the brain and spinal cord.
the perivascular extensions of the subarachnoid space into the nervous tissue
the ependymal surface of the ventricles
The pia mater is a loosely arranged layer of pial cells resting on a basement membrane.
Beneath the basement membrane are the astrocytic foot processes, no intercellular junctions exist between pial cells or
between adjacent astrocytes, therefore the extracellular spaces of the nervous tissue are in almost direct continuity with the subarachnoid space
The ventricular surface is covered with ependymal cells with localized tight junctions. Intercellular channels exist that allow free communication between the ventricular cavity and extracellular neuronal space.
