Anatomy of the vertebral canal

Narrator: When they say to watch your back
you really should, because there is

something pretty important in there called
the Spinal Cord.

Now, if you think of a nerve as a road
then the spinal cord is huge highway.

It's a major reflex center and holds many
neural tracks that connect the brain to

the rest of the body, allowing for
important communication to occur.

The Spinal Cord starts at the
Foramen Magnum, where it is continuous

with the Madula Oblongata, which is the
most coddle portion of the brain stem.

It then extends inferiorly through the
vertebral canal.

In adults it usually ends at the first or
second Lumbar vertebra.

In infants it usually ends at the second
or third Lumbar vertebra.

The tapered end of the Spinal Cord is
called the Conus Medullaris.

If we look at a transverse section of the
spinal cord we can see the anterior

median fissure that extends along the
midline of the Spinal Cord anteriorly,

similarly, the posterior median sulcus
extends along the midline of the

Spinal Cord posteriorly.

The spinal cord can be divided into spinal
cord segments.

One spinal cord segment gives rise to the
anterior and posterior nerve roots

which comes together to form a spinal
nerve on each side of the Spinal Cord.

There are thirty-one Spinal Cord segments:

eight Cervical, twelve Thoracic,
five Lumbar, five Sacral,

and one Coccygeal.

A spinal nerve pair shares its name with
the spinal cord segment it arises from,

so, for example, the spinal nerves arising
from the C1 spinal cord segment are named

the C1 spinal nerves, logically the number
of paired spinal nerves is the same as the

number of Spinal Cord segments.

Now, not all regions of the Spinal Cord
are the same size, there are two regions

that carry more fibers and are therefore
wider, resulting in

two Spinal Cord enlargements.

The cervical enlargement spans the C4
through T1 segments.

The anterior rami of the spinal nerves
that arise from most of these segments,

form the Brachial Plexus, which provides
innervation for the upper limbs.

The Lumbosacral enlargement spans the T11
through S1 segments.

The anterior rami of the spinal nerves
that arise from most of these segments,

form the Lumbar and the Sacral Plexuses,
which provide innervation

for the lower limbs.

Let's take a closer look at spinal nerves.

Spinal nerve roots travel laterally
from their spinal cord segment through

the vertebral canal to their respective
opening which for most spinal nerves is

an intervertebral foramen.

Just before reaching the opening,
the roots unit to create a spinal nerve,

which then exits the vertebral canal.

Now, let's look at how each pair of spinal
nerves exit the vertebral collum.

The C1 spinal nerve exits above the C1
vertebra.

Nerves C2 to C7, exit through the
Intervertebral Foramina superior

to their corresponding vertebra, while
C8 exits through the

intervertebral foramina between the C7
and the T1 vertebrate.

Nerves T1 to L5 then exit through
intervertebral foramina inferior to

their corresponding vertebra.

The S1 to S4 spinal nerves branch into
anterior and posterior rami and then exit

through the anterior and posterior
sacral foramina inferior to their

corresponding vertebra.

And finally, the S5 and Coccygeal nerves
exit through the Sacral hiatus, which is

inferior to the S5 vertebra.

Now, since the Spinal Cord only extends to
the L1, L2 vertebral level,

the Lumbar, Sacral and Coccygeal nerve
roots travel inferiorly through the

remaining vertebral canal to reach their
respective openings.

In doing so, they form a bundle of nerve
roots inferior to the Spinal Cord called

the Cauda Equina.

Fun fact, Cauda Equina is Latin for horses
tail due to the fact that this bundle

of spinal nerve roots resembles the hair
of a horses tail.

There is also another important
structure within the distal portion of the

vertebral canal called the Filum Terminale.

It extends from the Conus Medullaris and
descends among the nerve roots of the

Cauda Equina to attach to the Dorsum
of the Coccyx, acting as an anchor for the

spinal cord within the vertebral canal.

Okay, now let's take a short break, can
you label these images?

Now, the Spinal Cord and the spinal nerve
roots are covered by three membranes

collectively called the Spinal Meninges;
the spinal Dura Mater,

Arachnoid Mater, and Pia Mater.

The Spinal Meninges are continuous
with the Cranial Meninges through the

Foramen Magnum and they function to
support and protect the Spinal Cord,

they also contain the Cerebrospinal Fluid
or CSF for short

in which the Spinal Cord is suspended.

Now, let's talk a little bit more about
each meningeal layer.

The Dura Mater is the outer most
layer of the spinal meninges.

This fibrous layer forms the tubular
sheath along the vertebral canal called

the Dural Sac.

The space that separates the Dural Sac
from the bony walls of the vertebral canal

is called the Epidural Space and it
contains the internal vertebral

Venus Plexus surrounded by epidural fat.

Superiorly, both the Dural Sac and
epidural space extend to the foramen

magnum where the spinal Dura Mater
is continuous with the cranial Dura Mater.

Inferiorly, the Dural Sac and Epidural
Space terminates at the

level of the S2 vertebra.

The Dural Sac is also anchored too the
coccyx via the Filum Terminale,

The Dura Mater also has tapered lateral
extensions called the Dural root sheaths,

which cover the anterior and posterior
spinal nerve roots that arise from the

Spinal Cord segments.

At the intervertebral foramina, these
sheaths merge with the outer covering of

the spinal nerves called the epineurium.

The spinal Dura Mater is innervated
by branches of the spinal nerves called

the recurrent meningeal nerves.

Next is the Arachnoid Mater, which lines
the spinal Dura Mater and

the dural root sheaths internally.

It encloses the subarachnoid space
which is filled with CSF.

Inferiorly, the subarachnoid space extends
beyond the Conus Medullaris

of the spinal cord.

The space inferior to the Conus
Medullaris which extends to the L2 vertebra

to the S2 vertebra, is called the Lumbar
Cistern.

It contains cerebral spinal fluid and the
Cauda Equina.

Now, the Arachnoid Mater is not
actually attached to the Dura, but rather

the CSF within the Subarachnoid space
presses the Arachnoid Mater

against the Dura.

Usually there is no space between the two
layers, however, if bleeding occurs, then

blood can push them apart creating a
potential subdural space.

In an anatomy lab, you might notice that
the Arachnoid Mater is separated

from the Dura mater, this happens in
cadavers because there is not CSF in

the Subarachnoid space is to press the
arachnoid against the Dura.

The Arachnoid Mater is separated from
the Pia Mater by CSF, but connective

tissue strands called Arachnoid Trabeculae
make connections between the

Arachnoid and Pia Mater through the
Subarachnoid space.

The Pia Mater is the inner most meningeal
layer which covers the Spinal Cord,

spinal nerve roots and spinal blood vessels.

A fine thread of the spinal cord Pia Mater
forms the Filum Terminale,

extending inferiorly from the Conus
Medullaris.

Laterally, the Pia Mater forms extensions
along the Spinal Cord

called the denticulate ligaments.

These ligaments arise between the anterior
and posterior nerve roots on both sides

of the spinal cord.

The Denticulate Ligaments have triangular
processes that extend laterally and pass

through the Arachnoid to attach to the
Dura Mater.

Superiorly, the Denticulate Ligaments
attach to the Cranial Dura Mater.

The Denticulate Ligaments along with the
Filum Terminale help suspend the

spinal cord in the Cerebrospinal Fluid of
the Subarachnoid Space.

Okay, now that we've discussed the
Meninges, we can take a short break.

Can you recall which structures anchor
Spinal Cord?

The blood supply for the spinal cord comes
from a few sources.

First, there are three long longitudinal
arteries, one anterior spinal artery,

and two posterior spinal arteries.

The anterior spinal artery originates
within the cranium from branches

of the vertebral arteries.

It extends inferiorly along the
interior Median Fissure.

Its branches, called the Sulcal arteries,
enter the Spinal Cord through the

anterior Median Fissure.

The two posterior spinal arteries
also originate in the Cranium

either as branches of the Vertebral
arteries or the posterior inferior

Cerebellar arteries.

The three longitudinal arteries can only
supply the superior parts of the

Spinal Cord by themselves.

The circulation to the majority of the
Spinal Cord depends on the

posterior and anterior Segmental
Medullary arteries and radicular arteries

that run along the spinal nerve roots.

The posterior and anterior Segmental
Medullary arteries arise at various

vertebral levels from the spinal branches
of the following arteries:

Ascending Cervical, Deep Cervical,
Vertebral, Posterior Intercostal,

and Lumbar Arteries.

These arteries supply blood mainly
to the Cervical and

Lumbosacral Enlargements.

Additionally, a branch of an Inferior
Posterior Intercostal or

Upper Lumbar Artery, called the Great
Anterior Segmental Medullary Artery,

also reinforces the blood supply to the
lower two-thirds of the Spinal Cord.

Lastly, branches also give rise to the
Radicular arteries which are small

arteries that supply blood to the
spinal nerve roots and their coverings.

Unlike the segmental arteries, they do not
anastomose with the anterior and

posterior spinal arteries.

Finally, let's discuss the veins of the
Spinal Cord.

The veins of the Spinal Cord usually
follow the arteries.

There are typically three anterior
and three posterior spinal veins.

They drain into anterior and posterior
Medullary and Radicular veins which

communicate with the Epidural Venus
Plexus.

From the Epidural Venus Plexus,
blood flows to the Dural sinuses of the

Cranium, the Vertebral veins and the
External Venus Plexuses.

Alright, as a quick recap, the Spinal Cord
has thirty-one spinal segments and pairs

of spinal nerves: eight Cervical, twelve
Thoracic, five Lumbar, five Sacral, and

one Coccygeal.

The two enlargements of the Spinal
Cord are the Cervical Enlargement and

Lumbosacral Enlargement.

The Spinal Cord and the spinal nerve roots
are covered by the spinal meninges, which

consist of the Dural Mater, Arachnoid
Mater and Pia Mater.

The Dura Mater is separated from the bony
walls of the Vertebral Canal

by the Epidural Space, the spinal
Arachnoid Mater is pressed against the

Dura by the Cerebrospinal Fluid
of the Subarachnoid Space.

The spinal Pia Mater directly covers the
Spinal Cord, the spinal nerve roots

and spinal blood vessels.

Inferior to the Spinal Cord, it
continues as a fine thread

called the Filum Terminale.

Laterally the spinal Pia Mater forms
extensions along the Spinal Cord called

the Denticulate Ligaments.

Finally, the Spinal Cord blood supply
comes from the anterior spinal artery,

the two posterior spinal arteries, and the
posterior anterior segmental medullary

arteries, including the Great Anterior
Segmental Medullary Artery.

Venous blood drains through the three
anterior spinal veins

and three posterior spinal veins.

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