Anatomy of the vertebral canal

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: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 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 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 C1 spinal nerve exits above the arch of the C1 vertebra.

Nerves C2 to C7 exit through the intervertebral foramina superior to their corresponding vertebra, while C8 exits through the intervertebral foramen between the C7 and the T1 vertebrae.

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 “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 S2 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.