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The meninges are a series of membranes that cover the
outside of the CNS.
The meninges act to support and protect the CNS
Attach to the skull and vertebral column.
Attach to the CNS. Fluid in between. Brain
is suspended in fluid. This reduces
the effective weight of the brain (from 1500g to 50 g)
Three layers.
Dura mater
Arachnoid
Pia mater
Dura mater thickest of the three
- Two
layers around the brain
- Periosteal
layer outer layer - forms the periosteum of the skull attached to
the bone
- Meningeal
layer inner layer usually firmly attached to the periosteal layer
- Two
potential spaces usually not real spaces, but can become real
fluid-filled spaces in some pathological conditions
- Epidural
space upon dura between skull and periosteal layer
- Subdural
space below dura between meningeal layer of dura and the arachnoid.
- The
dura surrounds most of the external surface of the brain
- There
are two places where the dura folds down and separates regions of the brain
These are called the dura septa
- Falx
cerebri sickle - extends down into the longitudinal fissure
- Tentorium
cerebelli separates the cerebellum from the posterior cerebrum
- Separates
the skull and brain into supratentorial (cerebrum) and infratentorial
(cerebellum and brainstem) compartments
- There
is a space in the membrane called the tentorial notch, which allows the
brainstem to pass through the membrane
- The
dura mater contains venous sinuses blood from veins in the brain drain
into the sinuses
- The
sinuses are formed by separations between the periosteal and meningeal
layers of the dura- usually at attachment sites of parts of the dura
- Superior
sagittal sinus where the falx cerebri attaches to the rest of the dura
- Straight
sinus attachment of falx cerebri and tentorium cerebelli
- Transverse
sinus where tentorium cerebelli attaches to the dura
- Sigmoid
sinus anterior continuation of transverse sinuses flow into
internal jugular vein
- The
dura has its own blood supply the largest is the middle meningeal artery
supplies lateral surface of dura mater.
- The
dura is pain sensitive only one of the meninges that is
- Brain,
arachnoid, pia not innervated
- Most
of innervation of dura is by trigeminal nerve
Arachnoid layer
- Thin,
avascular layer attached to the dura
- Looks
like a spider web.
- There
are small strands of collagenous material that extend from the arachnoid to
the pia mater these are the arachnoid trabeculae
- The
trabeculae help keep the brain suspended in the skull
- The
arachnoid follows the contours of the dura, not the surface of the brain.
So it does not extend down into the sulci and fissures.
- The
space between the arachnoid and the pia mater is the subarachnoid space.
The space is filled with cerebrospinal fluid.
Blood vessels travel through the subarachnoid space
- The
subarachnoid space varies in thickness:
Thin over the gryi, thicker over other regions.
- Cisterns
are areas where the arachnoid extends over irregular structures in the
surface of the brain. CSF
collects in the cisterns
- The
CSF in the subarachnoid space enters the venous circulation through
arachnoid villi
- Pockets
of arachnoid will pass through the meningeal layer of the dura t enter the
dural sinuses
- Large
arachnoid villi are termed arachnoid granulations
- CSF
flows out through the villi into the venous blood in the sinuses
- Blood
that is in the CSF can irritate and block the arachnoid villi
Pia mater
- Thin
membrane, covers the surface of the CNS tightly
- Cant
be separated from the surface of the nervous system
Up to now, we have been looking at the brain
What happens in the spinal cord?
We have the same three layers of meninges, but there are
some differences
Dura mater
- Only
a single layer the meningeal layer.
There is no periosteal layer of the dura.
- The
vertebrae have a separate periosteum, that is not part of the meninges
- The
is a true epidural space in this case, which is filled with fatty connective
tissue and a vertebral venous plexus
Arachnoid
- Like
in the brain, the arachnoid is attached to the dura mater.
- There
is a subarachnoid space between the arachnoid and the pia mater.
- The
spinal cord ends at the disk between L1 and L2
- The
dural sheath and arachnoid end about the second sacral vertebrae.
This results in a large cistern, the lumbar cistern.
In the cistern is the cauda equina, the nerve roots for the inferior
part of the spinal cord. This
is a site for sampling CSF. Also
for epidural injections.
Pia mater
- Relatively
thick
- Dentate
ligament extension of the pia mater that anchor the spinal cord to the
arachnoid and dura mater
- At
the inferior end of the spinal cord, the pia mater continues as the filum
terminale, which anchors the distal end of the spinal cord to the caudal end
of the dural sheath, which is in turn anchored to the caudal end of the
vertebral canal.
Pathology
- With
damage to blood vessels, both arteries and veins, blood can enter the CSF or
into the potential spaces of the meninges
- Subarachnoid
space
- Subdural
space
- Epidural
space
- In
the figures in the book, look at the lateral ventricles (dark structures)
They are usually pushed to the side away from the hematoma, and are
smaller. Look at the pictures
after surgery for normal.
- Within
the skull, there are brain, blood, and CSF. With increased pressure, something has to go, and it is
usually CSF first.
Epidural Hematoma
- Damage
to an artery that supplies the dura. The
most common site is the middle meningeal artery.
It lies deep to the temporal bone.
A blow to the side of the head can damage the artery.
- Blood
collects in the epidural space, between the skull and the periosteum
- With
the blood coming from an artery, Onset of signs and symptoms occur
relatively quickly. Fig 4-15.
Symptoms at 2 hours
- As
the hematoma expands, it compresses the underlying nervous tissue
- Blood
will collect in a lens-shaped pool
- Less
commonly, a dural venous sinus may be torn. Fig 4-16. 4 days of headaches
Subdural hematoma
- Commonly
due to damage to a vein as it goes through the arachnoid and enters a venous
sinus
- Shearing
motions can tear these veins: Dura and skull are fixed together, the brain
and arachnoid can move laterally somewhat.
- Can
be acute or chronic: Venous
blood is under less pressure than arterial blood, so they tend not to
develop very quickly, can be weeks.
- The
hematomas tend to be crescent shaped
Effects of intracranial pressure
- The
meninges separate the nervous system into compartments.
Pressure in the nervous system can compress parts of the brain
against parts of the meninges, or move parts of the brain from one
compartment to another.
- What
could increase intracranial pressure?
- Fig
4-19, page 95
- With
increased intracranial pressure, the midbrain can be compressed against the
tentorium cerebelli at the level of the notch.
This can compress the cerebral peduncle and some cranial nerves.
- WHAT
Cranial nerves leave at the level of the midbrain?
How could you test the function of the nerves?
Does this give you an idea of a useful method to test for
intracranial pressure?
-
- Lateral
pressure in one of the cerebral hemispheres could force part of the cerebral
cortex under the falx cerebri to the other side.
What gyrus is the most likely to be affected in this way?
- The
hemisphere could also be displaced medially and inferiorly.
This would compress the uncus and adjacent parts of the temporal lobe
against the midbrain. This is
termed an uncal herniation.
- Downward
pressure could force the cerebellum downward into the foramen magnum.
This will usually force the tonsils of the cerebellum into the
brainstem. Tonsillar
herniation. The brainstem
is important in control of the respiratory and circulatory systems.
Compression of these areas of the brainstem can have grave
consequences.
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