Anatomy of the vertebral canal

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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 a huge highway. It's a major reflex center and holds many neural tracts 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 medulla oblongata, which is the most caudal portion of the brainstem.
It then extends inferiorly through the vertebral canal. In adults, it usually ends at the level of 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 the posterior nerve roots which come together to form a spinal nerve on each side of the spinal cord, there are 31 spinal cord segments, eight cervical, 12 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 c one spinal cord segment are named the c one 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 C four through T one 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 T 11 through S one 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 the 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 unite 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 column.
The C one spinal nerve exits above the arch of the C one vertebra nerves C two to C seven, exit through the intervertebral foramina superior to their corresponding vertebra.
While C eight exits through the intervertebral foramen between the C seven and the T one vertebrae nerves T 1 to L5, then exit through intervertebral foramina inferior to their corresponding vertebra.
The S one to S four spinal nerves branch into anterior and posterior remi and then exit through the anterior and posterior sacral foramina, inferior to their corresponding vertebra.
And finally, the S five and coccygeal nerves exit through the sacral hiatus, which is inferior to the S five 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 a horse's tail due to the fact that this bundle of spinal nerve roots resembles the hair of a horse's 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.
Ok. 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 outermost layer of the spinal meninges. This fibrous layer forms a 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 venous 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 terminate at the level of the S two vertebra.
The dural sac is also anchored to 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 from the L2 vertebra to the S two vertebra is called the lumbar cistern.
It contains cerebrospinal 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 no CSF in the subarachnoid space 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 innermost meningeal layer which covers the spinal cord, spinal nerve roots and spinal blood vessels, a fine thread of the spinal 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.
Ok. Now that we've discussed the meninges, we can take a short break.
Can you recall which structures anchor the spinal cord? The blood supply for the spinal cord comes from a few sources.
First, there are three 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 anterior 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 part 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, spinal 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 venous plexus from the epidural venous plexus.
Blood flows to the dural sinuses of the cranium. The vertebral veins and the external vertebral venous plexuses.
All right. As a quick recap.
The spinal cord has 31 spinal segments and pairs of spinal nerves, eight cervical, 12 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 dura 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 and anterior segmental medullary arteries including the great anterior segmental medullary artery, venous blood drains through three anterior spinal veins and three posterior spinal veins.