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A lateral view of the vertebral column, spinal cord and spinal nerves.
Ever heard of a lumbar puncture? What about an epidural? Yup, we’re talking about a doctor sticking a MASSIVE needle in someone’s spine. And although a great many horror films would suggest, otherwise, the doctors are not out to get you. They know what they’re doing and they’re actually not even touching your spinal cord when they prick you in the back. Want to know how that’s possible? Stay with us to find out while we investigate the interrelationships of the vertebral column and the spinal nerves.
So, what are we going to talk about in this tutorial? Our main aim is to understand the relationship between the spinal nerves and the vertebral column. First of all, we’ll have a look at different vertebral regions and the general skeletal side of things. We’ll then move on to the spinal cord, run through a few features, and see how the spinal nerves anatomically relate to the vertebral column.
We’ll also focus on the spinal cord’s position in the vertebral column, and how that is influenced by fetal development. Then, we’ll discuss the meningeal coverings of the spinal cord as they, too, developed quite an interesting pattern due to the changes during fetal development. Finally, we’ll take a look at some clinical notes and wrap up for the day.
Let’s jump right in and learn about the vertebral column. The quicker we start, the sooner we’ll finish.
So, when you think of the vertebral column or the spine, you probably imagine something a little like what you can see on your screen right now. Well, why don’t we take a bit of a different spin on it and look at the midsagittal section instead. Much better. This makes it a lot easier to understand how the vertebral column relates to other structures.
But before we study any of these interrelationships, let’s look at the different regions of the spine. You probably already know that there are five distinct regions which can be identified by the different characteristics of their vertebrae. Starting most superiorly, we find seven cervical vertebrae in the neck. Right below it are twelve thoracic vertebrae, which span the thorax. Lower still in the lower back, we find the lumbar spine with its five lumbar vertebrae, and it is followed by five sacral vertebrae which are normally fused into one bone called the sacrum. Right at the inferior tip of the vertebral column is a funny little structure called the coccyx, also known as the tailbone. It consists of three to five coccygeal vertebrae fused to a variable extent. Together, these thirty-three vertebrae form the vertebral column, which stretches from the foramen magnum superiorly right to the tip of the coccyx inferiorly.
Something that you always need to keep in mind when learning about the vertebral column are the intervertebral discs between each vertebra, but their structure and function are discussed in more detail in some of our other tutorials. And that’s the bony elements of this tutorial pretty much covered. That was easy, wasn’t it? Now, let’s turn our attention to the basic gross anatomy of the spinal cord.
The spinal cord is a long cylindrical structure which starts off as an extension of the medulla oblongata of the brainstem and occupies the vertebral canal along much of the vertebral column. It is forty-two to forty-five centimeters in length depending on the size and sex of the individual. A bilateral pair of spinal nerves is associated with each vertebra of the spinal column down to the coccyx which only has one pair of nerves associated with its three to five segments. These spinal nerves exit the vertebral column through the intervertebral foramina and go on to supply motor and sensory innervation to most of the body. As we go through the various regions of the spinal cord, we’ll highlight whether the spinal nerves exit through the intervertebral foramen above or below their corresponding vertebrae. This is an important component of the relationship between the spinal cord and the vertebral column.
Speaking of innervation, the number of structures the spinal region innervates actually affects the size of the spinal cord in that region. So more complex structures, such as the limbs, which carry out functions like movement and interacting with what’s around us require a greater neural output to innervate it. This means we have more wiring coming from the parts of the spinal cord responsible for those structures. Makes sense, right?
So, with that in mind, there are two enlargements worth to mention. The first is the cervical enlargement which contributes to the innervation of the upper limb. It generally stretches between C4 to T1 spinal levels; however, you’ll find that these values may vary from source to source many describing it as extending from C3 to as far down as T2. It would make sense that there would also be an enlargement in the region of the spinal cord that innervates the lower limb, and indeed there is.
The lumbar or lumbosacral enlargement is, of course, in the lumbosacral region of the spinal cord and supplies the lower limb. It stretches between the L1 and S3 spinal levels and again these values are subject to variation with the lumbar enlargement occasionally reaching up as far as T11 in some cases depending on what level the spinal cord terminates.
Many of the structures in the body are innervated by spinal nerves so let’s take a look at them starting right at the top of the cervical level. In this area, spinal nerve C1 to C7, leave the vertebral column above their respective vertebrae. Here we can see spinal nerve C3 exiting above the third cervical vertebra.
So you know how there’s generally one pair of spinal nerves associated with one vertebra? Well, in the cervical region, we actually have eight pairs of spinal nerve even though we’ve only seven cervical vertebrae. So, the extra pair, C8, leaves below the C7 vertebra.
We’re moving on to the thoracic region now, where you can see the twelve pairs of thoracic spinal nerves highlighted in green. Nothing odd happens here, really. Just like C8 spinal nerves, the thoracic nerves T1 to T12 leave below their respective vertebrae through the intervertebral foramina.
We’re now in the lumbar region, which you can see highlighted on the image, and it’s the same story as the thoracic region in terms of exit location. We have five pairs of lumbar spinal nerves, L1 to L5, which leave below their respective vertebrae.
The sacral region is a little bit different. The S1 to S5 spinal nerves technically still leave below their respective vertebrae, but it’s important to remember that all five sacral vertebrae are normally fused together. So instead of the intervertebral foramina, being the final exit space, their sacral foramina that parts of the spinal nerves travel through. The sacral foramina are on both the posterior and anterior aspects of the sacrum
What actually happens in the sacral region is that the spinal nerve enters the sacral foramen as a single structure and then splits into anterior and posterior rami. The anterior rami of S1 to S4 nerves exit through the anterior sacral foramina and the smaller posterior rami exit through the posterior counterpart. The S5 spinal nerves exits through the sacral hiatus which is the inferior opening of the sacrum.
Lastly, we have the coccygeal region. This is another unusual arrangement as only one pair of nerves leaves the vertebral column over the anterior aspect of the coccyx in spite of it having three to five segments.
In case you haven’t been counting, let’s add up all the spinal nerves. We have eight cervical spinal nerves, twelve nerves in the thoracic region, five in the lumbar, five in the sacral region, and just one coccygeal spinal nerve which makes thirty-one pairs of spinal nerves in total.
So, looking at the spinal nerves, you might have noticed that something quite odd goes on in the vertebral canal. Although, we’ve corresponding regions in the vertebral column and the spinal cord, they don’t quite line up. The spinal cord terminates as the conus medullaris at around the L1 or L2 vertebral level, but the root of many spinal nerves continue below that level in the vertebral column and still leave through the intervertebral foramina below their respective vertebral levels.
Collectively, the spinal nerve roots that continue inferiorly beyond the conus medullaris are called the cauda equina. I guessed it must have reminded someone of a horse’s tail because that’s what it translates to from Latin.
So, you might wonder why this happens. It’s all due to changes during fetal development. Although the cord in the spine are the same size until the third month of fetal development, the vertebral column then grows longer than the spinal cord. The end of the spinal cord, the conus medullaris, at birth is around the level of L3. Eventually, as the individual continues growing, the vertebral column continues to grow longer than the spinal cord, so the conus medullaris is around the L1 to L2 vertebral level. Some individual variation exists and flexion and extension of the spine also affects the position of the spinal cord within the vertebral column.
Now we’ll quickly go over the meningeal coverings because they have quite an interesting relationship with the spinal cord and the vertebral column.
The first is the dura mater. It is the tough external layer which surrounds the spinal cord. Superiorly, it’s attached to the foramen magnum and is continuous with the cranial dura mater which surrounds the brain. Inferiorly, you can see the effect of the different growth rate of the spinal cord and vertebral column because the dura extends down to S2 instead of terminating at L1 to L2 like the spinal cord. Here it creates the dural sac around the cauda equina. The sac has extensions creating dural root sheaths on spinal nerves exiting the sac.
The next layer is the arachnoid mater, which is pushed tightly against the inner aspect of the dura mater. The innermost layer is called the pia mater and it closely adheres to the spinal cord and extends over the spinal roots.
The space between the arachnoid mater and pia mater is known as the subarachnoid space and it’s filled with cerebrospinal fluid. This space extends below the conus medullaris from the L1 to L2 vertebral level to the S1-S2 vertebral level. This part of the subarachnoid space is known as the lumbar cistern. It is filled with cerebrospinal fluid and is enclosed by the dural sac.
Now that we know this information, the lumbar puncture and epidural are starting to make sense. The needle for lumbar punctures is normally inserted between two vertebrae in the range of L3 to S1. We know that the conus medullaris is found around L1 to L2 so a lumbar puncture only pierces the dural sac to draw some cerebrospinal fluid, but doesn’t actually touch the spinal cord.
The patient is either in the neofetal position or sitting up with the spine curved. In these positions, the L4 vertebra is in line with the iliac crest, which can easily be palpated. The needle is normally inserted into the intervertebral space above or below the L4 vertebra. An epidural is slightly different as it never pierces the dural sac. It injects the anesthetic into the epidural space between the vertebral column and the outer edge of the dura mater. Still, it is normally performed at lower lumbar levels to minimize the risk of damage to the spinal cord.
Below the conus medullaris, the pia mater becomes continuous with the thin fibrous structure called the filum terminale. It pierces the dural sac, picking up a dural layer, and stretches as far down as the coccyx anchoring to its posterior surface.
The filum terminale can be divided into two parts based on its relation to the dural sac. The part contained within the dural sac extends down to S2 level and is called the filum terminale internum. The part outside of it is called the filum terminale externum. This part picks up a dural layer from the dural sac and anchors to the coccyx.
And this wraps up the gross anatomy part of this tutorial.
Before we completely finish up this tutorial, something we should consider is how the vertebral column can affect the spinal cord. In this case, how it might damage it. Quite an interesting example is the cauda equina syndrome. First of all, what are the symptoms? Lower back pain, weakness in the lower extremities, urinary and bowel disturbance, and more rarely, sudden sexual dysfunction are all associated with cauda equina syndrome. I saved the most interesting for last, though. With cauda equina syndrome, you can develop what is known as saddle anesthesia.
We just can’t get away from horses today, it seems.
Saddle anesthesia refers to the loss of sensation on the buttocks, perineum, and inner thighs, so the areas which would be touching the saddle if you sat on one. It is associated with the S3 to S5 dermatomes which are innervated by spinal nerves in the cauda equina. Most of these symptoms tend to develop over time rather than acutely and tend to be asymmetric.
So, do you have any ideas about what could cause this? Let’s see.
The most frequent cause of cauda equina syndrome is posterior herniation of an intervertebral disc in the lower lumbar region which compresses some sacral nerve roots. It can also occur due to compression from spinal tumors, trauma or spinal stenosis, which is narrowing of the vertebral canal. And how can we help affected individuals? Well, the general consensus seems to be surgical intervention, and the sooner, the better. That way permanent damage to nerve roots can be avoided or at least minimized. If nerve roots are damaged permanently, urinary and bowel control may be lost as well as sexual function. In more extreme cases, complete lower limb paralysis can occur.
Well done guys! You’ve made it to the end of this tutorial. Let’s remind ourselves what we’ve learned today.
We started by looking at the bony elements of the vertebral column – seven cervical, twelve thoracic, five lumbar, five sacral vertebrae fused to form the sacrum, and three to five tiny coccygeal vertebrae forming the coccyx. That makes thirty-three vertebrae all together with the intervertebral discs between the vertebral bodies.
We moved on to learn about the spinal cord which is the main structure filling the vertebral canal. We learned that it starts as an elongation of the medulla oblongata and has a cervical enlargement and a lumbar enlargement. The spinal cord gives rise to bilateral pairs of nerves corresponding to each vertebral level for a total of thirty-one pairs of spinal nerves. There are eight pairs of cervical spinal nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves.
We saw that spinal nerves C1 to C7 exited at the vertebral column above their respective vertebra. The C8 spinal nerve exited below the C7 vertebral. The spinal nerves in the thoracic and lumbar regions exited below their respective vertebra. Sacral nerves S1 to S4 exit separately as anterior and posterior rami through the sacral foramina. S5 exits through the sacral hiatus, and the single pair of spinal nerves associated with the coccygeal region sits on the anterior aspect of the coccyx.
We then saw that the spinal cord terminates at the L1 to L2 vertebral level as the conus medullaris. The nerve roots below this level are collectively called the cauda equina.
Finally, we looked at the meningeal coverings of the spinal cord especially the structures they form near the inferior end of the spinal cord. We saw the dural sac formed by the dura mater and the filum terminale, which is an extension of the pia mater.
Lastly, in our clinical note section, we looked at how different conditions of the spine can cause cauda equina syndrome, which can present with saddle anesthesia.
And that’s a wrap! Thanks for watching this tutorial and see you next time.