Protection of the cns

1. The brain and spinal
cord are covered and
protected by the bones
of the skull, specifically
the cranium, and the
vertebral column.
Protection of the Central Nervous System
1. Axial Skeleton
Cranium
Vertebral
Column
The brain and spinal cord are
surrounded and protected by three
different sets of tissue.

2. The brain has hollow, fluid-
filled cavities called ventricles.
Inside the ventricles is a
structure called the choroid
plexus (see Neuroglia; slide 5;
ependymal cells) that makes a
clear, colorless fluid called
cerebrospinal fluid (CSF).
CSF circulates within and
around the brain and spinal cord
to help cushion it from injury.
2. Ventricles and cerebrospinal fluid
Protection of the Central Nervous System

3. CSF is constantly being
absorbed and replenished. A
“perfect” balance is maintained
between absorption and
replenishment. A disruption or
blockage in the system can
cause a build up of CSF, which
can cause enlargement of the
ventricles, called
hydrocephalus, or cause a
collection of fluid in the spinal
cord, called syringomyelia.
Protection of the Central Nervous System

4. White matter
Gray matter
Protection of the Central Nervous System
3. The Meningies
The brain and spinal cord are covered and protected
by three layers of tough, connective tissue called
meninges. From the outermost layer inward they are:
the dura mater, arachnoid mater, and pia mater.

5. White matter
Gray matter
The dura mater is a strong, thick membrane that
closely lines the inside of the skull. It has two layers,
the periosteal and meningeal dura, which are mostly
fused, but do separate to form venous sinuses.
Protection of the Central Nervous System
3. The Meningies

6. White matter
Gray matter
The dura creates little folds or compartments. There
are two special dural folds, the falx and the tentorium.
The falx separates the right and left hemispheres of
the brain and the tentorium separates the cerebrum
from the cerebellum.
Protection of the Central Nervous System

7. White matter
Gray matter
The arachnoid mater is a thin, web-like membrane
that covers the entire brain. The arachnoid is made of
elastic tissue. The space between the dura and
arachnoid membranes is called the subdural space.
Protection of the Central Nervous System
3. The Meningies

8. White matter
Gray matter
The pia mater hugs the surface of the brain following
its folds and grooves. The pia mater has many blood
vessels that reach deep into the brain.
Protection of the Central Nervous System
3. The Meningies

9. White matter
Gray matter
The space between the arachnoid and pia is called the
subarachnoid space. It is here where the
cerebrospinal fluid bathes and cushions the brain.
Protection of the Central Nervous System

10. Blood is carried to the brain by
two paired arteries, the internal
carotid arteries and the
vertebral arteries. The internal
carotid arteries supply most of the
cerebrum. The vertebral arteries
supply the cerebellum, brainstem,
and the underside of the
cerebrum. After passing through
the skull, the right and left
vertebral arteries join together to
form the basilar artery.
Arterial Blood Supply of the Brain

11. The basilar artery and the internal
carotid arteries “communicate”
with each other at the base of the
brain called the Circle of Willis.
The communication between the
internal carotid and vertebral-
basilar systems is an important
safety feature of the brain. If one
of the major vessels becomes
blocked, it is possible for
collateral blood flow to come
across the Circle of Willis and
prevent brain damage.

12. The venous circulation of
the brain is very different
than the rest of the body.
Usually arteries and veins
run together as they supply
and drain specific areas of
the body. In the brain, the
major vein collectors are
integrated into the dura to
form venous sinuses - not to
be confused with the air
sinuses in the face and nasal
region.
Venous Blood Drainage of the Brain

13. The venous sinuses collect
the blood from the brain and
pass it to the internal jugular
veins. The superior and
inferior sagittal sinuses drain
the cerebrum, the cavernous
sinuses drains the anterior
skull base. All sinuses
eventually drain to the
sigmoid sinuses, which exit
the skull and form the jugular
veins. These two jugular
veins are essentially the only
drainage of the brain.

14. The Blood Brain Barrier is a physiological mechanism
that alters the permeability of brain capillaries so that
some substances are prevented from entering brain
tissue, while other substances are allowed to enter
freely.
The Blood Brain Barrier

15. A key aspect of the blood-brain barrier are the thin,
flat cells known as endothelial cells which form the
walls of capillaries. In most parts of the body, the
endothelial cells in the capillaries overlap at what are
called junctions. These junctions are leaky enough to
let a lot of different materials move through the wall
of the blood vessel into the tissue and back again.

16. However, in the brain there's a different arrangement.
The endothelial cells meet at what are called tight
junctions. These tight junctions block the passage of
most things except for small, hydrophobic molecules
like O
2
, CO
2
, & hormones. Cells of the barrier also
actively transport metabolic products such as glucose
molecules.

17. In addition to tight junctions, the "end feet" of
astrocytes (see Neuroglia; slide 3; Astrocytes)
surround the outside of capillary endothelial cells. The
reason for this endothelial-glial connection is unclear,
but may reflect an influence of astrocytes on the
formation and maintenance of the blood-brain barrier.
Astrocyte
Capillary
“End Foot”
of
Astrocyte