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Author of Theme Session: Luke Henderson

Title of Theme Session: 6.02 Cerebrospinal fluid and Meninges

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Week 6.02: Cerebrospinal fluid and Meninges

Meninges

The brain is covered by three tissue layers called meninges:

(i) Pia mater (soft mother): closely adheres to the surface of the brain and cannot be removed. It follows the folds and crevices of the brain.

(ii) Arachnoid mater (spiders web): middle layer of meninges - is more loosely fitting. Imagine putting the brain into a plastic bag and pulling it close, but not overly tight, onto the surface; then holding the free

end of the bag firmly around the medulla, with one hand.

(iii) Dura mater (hard mother): outer most meningeal layer composed of thick, inelastic membrane. It adheres to the internal surface of the cranium (periosteum). Consists of two layers (meningeal and

endosteal layers) which for the most part are joined tightly together. In some places they separate and

form the dural venous sinuses i.e. venous drainage of the brain.

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Spaces between meninges

(i) Subdural space: A potential space between the dura and arachnoid mater is called the subdural space

(ii) Subarachnoid space: space between the arachnoid mater and the pia mater. The subarachnoid space

contains the cerebrospinal fluid (CSF). Given that the arachnoid (like the bag) skips from gyrus to gyrus;

around the brainstem, large spaces, full of CSF are left between the pial surface of the brain and the

arachnoid. These large spaces are the cisterns.

The major branches of cerebral blood vessels lie on top of the pia mater, within the subarachnoid space.

As a consequence most cerebral bleed from aneurysms are into the subarachnoid space. Branches of these

vessels press into into the brain surface, taking the pial lining with them. This pial lining forms a glial layer around the vessels (the glia limitans).

The subarachnoid space is extremely delicate and fragile which allows for little tolerance for insult.

Despite this, the subarachnoid space is used often for administration of anaesthesia or other drugs,

together with spinal taps. One should never underestimate sensitivity of subarachnoid space to foreign

body substances or procedural invasion.

(iii) Epidural space: A potential space between the dura and the periosteum. In the cranium, this potential space becomes relevant if filled with blood or any other fluid, and may do so after head trauma.

An epidural hematoma is a collection of blood that accumulates within the epidural space and can be

either arterial (90%) or venous (10%) in origin. Epidural haematoma's are typically associated with blows

to the side of the head that result in a rupture of the middle meningeal artery. This artery is a branch of the

external carotid and enters the skull through the foramen spinosum. Unlike most cerebral vessels that

travel through the subarachnoid space, the middle meningeal artery adheres closely to the inside surface

of the temporal and parietal bones, forming a distinct groove in the middle cranial fossa.

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In the vertebral canal, the epidural space is real, since dura is not fused to the adjacent vertebral

periosteum. The epidural space contains loose areolar connective tissue, semiliquid fat, lymphatics,

arteries, an extensive plexus of veins, and the spinal nerve roots as they exit the dural sac and pass

through the intervertebral foramina. This is the locale where epidural injections are made.

Dural folds

In a few locations, the dura mater forms folds. There are three main dural reflections or folds:

(i) falx cerebri: between cerebral hemispheres

(ii) tentorium cerebelli: between cerebellum and cerebral cortex

(iii) falx cerebelli: between cerebellar hemispheres

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Ventricular system

The brain develops from cells that initially line a hollow tube. The lumen of this tube persists into

adulthood, in a much reduced form, as the ventricular system of the brain, in which cerebrospinal fluid is secreted and circulates.

Below are some of the major brain structures they you need to be able to identify in order to locate the

main parts of the ventricular system.

There are four main ventricles which are connected by small diameter canals:

(i) Two lateral ventricles (ventricles I and II): follow a long C-shaped course through all lobes of

cerebral hemispheres. They each have 5 parts:

anterior horn: in frontal lobe, anterior to interventricular foramen.

body: in frontal and parietal lobes, extending posteriorly to splenium of corpus callosum.

posterior horn: projecting into occipital lobe.

inferior horn: curving down and forward into temporal lobe.

collateral trigone: near splenium of corpus callosum where body, posterior & inferior horns meet.

(ii) Third ventricle: forms a narrow slit between the left and right thalamus and hypothalamus and

occupies most of the midline of the diencephalon. Interthalamic adhesion breaks the continuity,

connecting the two thalami. The posterior end of the third ventricle near the mamillary bodies narrows to

become the mesencephalic (cerebral) aqueduct, which traverses midbrain. Anteriorly, the third ventricle

communicates with the lateral ventricles via the interventricular foramina (Foramina of Munro).

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(iii) Fourth ventricle: is sandwiched between cerebellum and pons and the rostral medulla. Its floor is

called the rhomboid fossa and its roof is formed by the superior and inferior medullary vela of the

cerebellum.

The lateral and third ventricles are closed cavities (and do not house the pineal gland which lies in

subarachnoid space over the midbrain), communicating only with other parts of ventricular system. In

contrast, there are 3 apertures in fourth ventricles through which the ventricular system communicates

freely with subarachnoid space. These are the foramen of Magendie (median aperture) and foramina of

Luschka (x2, lateral apertures).

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Cerebrospinal fluid

Cerebrospinal fluid is a clear fluid that is a blood filtrate. It is low in cells and protein and it differs from

blood plasma in having excess Na and Mg and little K and Ca ions. Further, these concentrations remain

stable during the day, in face of plasma changes. This feature serves to provide a stable environment for

neurones to function properly.

CSF fills the ventricles and the subarachnoid space. It serves at least three vital functions:

(i) provides physical support and buoyancy - makes the brain about 1/30th of its actual weight (less than 50 grams)

(ii) regulates the chemical environment of the CNS (removes wastes) (iii) can provide chemical communication i.e. neurochemicals can be released into the CSF and travel

around the brain and act at regions at a considerable distance

CSF is made primarily in choroid plexus in the ventricles (80%) although some may be formed by

ependyma cells of brain, those that line the ventricular walls. If an overproducing choroid plexus is

removed, CSF is still made - the bulk by ependyma. The choroid plexus is found in all areas of the

ventricular system, except for the anterior and posterior horns of the lateral ventricles. This is important

clinically because if inserting catheters for dye injection (for stereotactic surgery) or measuring pressure,

then areas of the ventricles that do not have choroid plexus would be best.

The choroid plexuses receives a large blood supply - about 10 times greater per gram that of the cerebral

cortex. They produce about 500ml of CSF is made per day and the entire volume of brain CSF is replaced

about 4 times per day. Importantly from a clinical point of view, there is no feedback system limiting its

production. Hence any disruption/blockage to the drainage system may lead to a "ballooning" of the

ventricles and a rise in intracranial pressure.

CSF Cisterns

In a number of places a wide interval exists between the pia and arachnoid mater called the subarachnoid cisterns. There are several:

(i) cerebellomedullary (cisterna magna): space between inferior surface of cerebellum and dorsal

surface of medulla.

(ii) pontine: space around anterior surface of pons: continuous caudally with cerebellomedullary

(iii) interpeduncular: between cerebral peduncles, which contains Circle of Willis

(iv) superior cistern (cistern of great cerebral vein of Galen): a radiological landmark above midbrain.

Occupies interval between splenium of corpus callosum and superior surface of cerebellum. Contains

great cerebral vein of Galen and pineal gland and continues rostrally into transverse fissure.

The term cisterna ambiens is sometimes applied to group of cisterns which completely encircle midbrain.

As this part of subarachnoid space it is clearly visible on a normal CT scan.

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CSF Circulation

Within the system of ventricles, CSF tends to circulate from the lateral ventricles through the Foramen of

Munro into the 3rd ventricle, from where it circulates downwards through the cerebral aqueduct into the

fourth ventricle. From the 4th ventricle most CSF passes into the subarachnoid space, via the Foramen

of Magendie and Luschka, into the cisterns surrounding the cerebellum (cerebellomedullary cistern).

From here CSF circulates

- downwards and around the spinal cord

- anteriorly and upwards into the pontine and interpeduncular cisterns

- upwards and posteriorly, around the cerebellum and into the cistern posterior to the midbrain

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CSF Reabsorption

CSF is resorbed into the circulation by passing into the arachnoid villi (which pierce the dura mater) and

diffusing, under hydrostatic pressure, into the dural venous sinuses. These arachnoid villi clump together

to form the arachnoid granulations which are located extensively within the superior sagittal sinus.

The flow of CSF through the arachnoid granulations is from the subarachnoid space into the dorsal

venous sinuses, never the other way. If so, there is a disruption in the function of the arachnoid

granulations.

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Blood brain barrier

The Blood Brain Barrier (BBB) functions to maintain an optimum environment for neurones to function

properly. Specifically, it serves to:

- maintain electrolyte levels

- control entry of particular substrates (eg, glucose)

- protect from circulating hormones (eg, systemic neurotransmitters)

- remove waste products

- exclude toxins

Unlike capillaries in the body, brain capillaries have no fenestrations and show no pinocytosis. Astrocyte

feet cover 85% of brain capillary surface. The ability to exclude certain substances from brain interstitial

space by the BBB is due to both the anatomy of the capillaries and the selective transcellular transport by

the endothelial cells.

Spina Bifida

Spina Bifida is a disorder that involves an open vertebral column (bifida from Latin, meaning "split in

two"). It is commonly classified into 3 main types:

(i) Spina bifida cystica - myelomeningocele: the spinal cord, spinal roots and meninges protrude

forming a sac or cyst. This defect usually occurs in the lumbosacral region. It is due to failure of the

neural tube to close in the posterior neuropore region. This normally occurs during week 4 of gestation.

Since the neural tube is open in the affected region it will leak cerebrospinal fluid.

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The cause of myelomeningocele is largely unknown but appears to be related to folate in some way.

Maternal folate supplementation prior to closure of the neural tube (weeks 3-4 of gestation) appears to

reduce the risk of the malformation by about 80%.

Many children born with a myelomeningocele will subsequently develop hydrocephalus. The abnormal

part of the spinal cord appears to be tethered to the vertebral column in the lumbosacral region. In late

gestation the vertebral column increases in length at a faster rate than the spinal cord so if there is

tethering the spinal cord is pulled down. This has the effect of pulling the medulla down into the foramen

magnum which compresses the medulla and cerebellum and can close the median and lateral apertures.

The trapped CSF then increases in pressure and can hydrocephalus.

(ii) Spina bifida occulta: this is the mildest form of spina bifida and occurs in ~10% of individuals. Only

the arch of a single vertebra is open usually L5 or S1. The site is often marked externally by a tuft of hair,

a pigmented nevus or a dimple. It is not associated with any clinical features. Embryologically it is

thought to occur some time during weeks 5-12 of gestation and is probably due to failure of the vertebral

precursor cells to migrate around the spinal cord. The spinal cord is normal.

(iii) Spina bifida cystica meningocele: the vertebral defect is similar to that seen in spina bifida occulta only several vertebrae are usually involved. As a result the dura and arachnoid protrude forming a fluid-

filled sac. It is a rare malformation and is thought to occur during weeks 5-12 of gestation and is probably

due to failure of the vertebral precursor cells to migrate around the spinal cord. The spinal cord is normal.

References

Cross section of the spine: http://hcd2.bupa.co.uk/fact_sheets/html/epidural.html

Meninges: www.rci.rutgers.edu/~uzwiak/AnatPhys/Neuroanatomy.html

The meninges: www.csuchico.edu/~pmccaff/syllabi/CMSD%20320/362unit3.html

Nieuwenhuys R, J Voggd, C van Huijzen (1985). The Human Nervous System, Springer 3rd Ed, Berlin.

Ventricles of the brain: http://universe-review.ca/I10-80-ventricles.jpg