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Structure and function of the vertebral column.
Hello everyone! It's Megan from Kenhub here, and in today's tutorial, we'll looking at the vertebral column. Before we begin, let me quickly go over what we're going to cover in this tutorial. So first we're going to look at the vertebral column as a whole focusing on its general functions and structure and you can see the entirety of this structure from a posterior perspective highlighted in green in the image on the right. Once we're comfortable with the vertebral column as a single entity, we'll break it down further and take a closer look at the vertebrae which are the bones that form it.
So the vertebral column is commonly referred to as the spine or the backbone. It extends from the inferior aspect of the occipital bone of the skull to the tip of the coccyx. The vertebral column has several important functions. For one, it houses and protects the spinal cord and helps to support the weight of the upper body. It also plays a role in maintaining posture and facilitating movement.
Now the vertebral column can be divided into five regions – each of which is characterized by a different type of vertebra. So first we have the cervical region which is made up of seven cervical vertebrae and is found in the neck. Next we have the thoracic region which is made up of twelve thoracic vertebrae. This should be easy to remember because they correspond with the twelve ribs. Then we have the lumbar region which is made up of five chunky lumbar vertebrae and inferior to the lumbar region is the sacral region made up of five sacral vertebrae that fused together to form the sacrum. Lastly, we have the tiny coccygeal region made up of four coccygeal vertebrae that fused together to form the coccyx.
If we add this number together, we can see that the vertebral column is made up of 33 vertebrae in total. It's worth noting that each vertebra is usually abbreviated using the initial of its region followed by a number signifying its order within the region. For example, the C7 vertebra refers to the seventh vertebra of the cervical region.
So between the vertebrae we find these fibrocartilaginous structures you can see highlighted in green and contained within the dark gray circle on your screen which are called the intervertebral discs. The discs make up roughly one-fourth of the spinal column's length forming a cushion between every two vertebrae except between the first two cervical vertebrae and between the sacrum and the coccyx. These discs do literally act as cushions serving as a spinous shock-absorbing system and cushioning the effect of shock and stress produced when an individual walks, runs, bends or twists. The intervertebral discs also prevent friction between two moving vertebrae by preventing vertebral bodies from grinding against each other.
So despite how things may look when we view the vertebral column from an anterior perspective or a posterior perspective as is the case with our image on our right, the spine does not just run straight up and down. If we view it from a more lateral perspective, we can see that it's actually made up of four distinct curvatures. Firstly, we can see two forward facing curves in the cervical and lumbar regions of the spine. These cervical and lumbar curvatures are referred to as lordoses or singularly as a lordosis. So we have a cervical lordosis and a lumbar lordosis.
We can also see two backwards-facing curves in the thoracic and sacral regions. These thoracic and sacral curvatures are referred to as kyphoses or singularly as a kyphosis. So we have a thoracic kyphosis and a sacral kyphosis. These four curvatures function together to increase the flexibility of the vertebral column and to distribute body weight more evenly among the vertebrae. So, hopefully, you'll remember from earlier that the vertebral column is divided into five regions – each containing a specific number of vertebrae – and we're now going to look at this vertebrae or the buildings blocks of the spine.
Each vertebra generally consists of a vertebral body anteriorly and a vertebral arch posteriorly. These then enclose a space called the vertebral foramen which houses the spinal cord. The vertebral arch is formed by two pedicles which we can see on either side and two flattened laminae that complete the arch posteriorly. Each vertebra also has a spinous process, two transverse processes and four articular processes. You've probably noticed that we can only see two articular processes in this image. This is because there are two superior articular processes and two inferior ones. In this image, we're viewing an isolated thoracic vertebra from above so we can see the two superior articular processes and if we flip this vertebra, we'd see the inferior ones.
As we look at the vertebrae in the different regions of the vertebral column, we'll see that they not only differ in size but also have their own special features. So let's begin by looking at the vertebrae found in the cervical region of the spine.
As I mentioned earlier, seven cervical vertebrae make up the cervical portion of the vertebral column. These bones are of course labeled C1 to C7. Of all the vertebrae that make up the spinal column, the cervical vertebrae are the smallest and as you can see from this image, they are located just inferior to the base of the skull and superior to the thoracic vertebrae. The first, second and seventh cervical vertebrae have unique characteristics while the others are similar in structure. So first let's focus on the characteristics of a typical cervical vertebra and we'll do this using this image showing a single cervical vertebra as viewed from above.
A typical cervical vertebra has a relatively small body and a large triangular vertebral foramen. Its transverse processes each possess a hole called the foramen transversarium which allows for the passage of the vertebral artery. Note that the vertebral artery passes through the transverse processes of C1 to C6 but not C7. Another distinguishing feature of the cervical vertebrae are their small, bifid spinous processes.
Now let's move on to look at the atypical cervical vertebrae starting with the first cervical vertebra. The first cervical vertebra is also known as the atlas or C1. It supports the skull hence the name atlas which is derived from the Titan Atlas who was condemned to hold up the sky in Green mythology. The atlas is unique because as you can see in this image here of the atlas from above, it has an anterior arch and a posterior arch but no vertebral body and no spinous process. Superiorly, the atlas articulates with the occipital condyles on the occipital bone of the skull and with the second cervical vertebra inferiorly.
The second cervical vertebra also known as the axis or C2 is located just inferior to the atlas. This vertebra is also unique because it possesses an odontoid process on its superior surface which is also referred to as the dens. Finally, the seventh cervical vertebra or C7 is atypical because its spinal process is not bifid. It's also known as the vertebra prominens due to its large spinous process which protrudes from under the skin and is actually visible to the naked eye. In this image, we can also see how the cervical vertebrae articulate with one another with the superior articular facets facing upwards and the inferior articular facets facing downwards.
Now let's move on to the thoracic vertebrae. There are twelve thoracic vertebrae which articulate with the twelve ribs to form the bony thorax. Again, the thoracic vertebrae have characteristics that allow us to distinguish them from other types of vertebrae. Let's take a look at these features now. I am using an image of a thoracic vertebra from the side and one showing a thoracic vertebra from above. Their vertebral bodies are medium in size and heart-shaped and they increase in size as you move down the spine. Their vertebral foramina are small and circular and their spinous processes are long and angled sharply downwards. Furthermore, the thoracic vertebrae exhibit costal facets which articulate with the ribs.
Before we look at the costal facets of the thoracic vertebrae, let's first look at the articular processes. Now, do you remember me briefly showing you how the cervical vertebrae articulate with one another with the superior articular facets facing directly upwards and the inferior articular facets facing directly downwards? Well, the thoracic vertebrae do this slightly differently. Their superior articular processes have facets that face posteriorly and laterally as you can see represented by the two arrows on your screen whereas the inferior articular processes have facets that face anteriorly and medially. The inferior articular facets articulate with the superior articular facets of the vertebra below in a jigsaw-like manner forming the zygapophyseal joints.
Now we can move on to the costal facets and there are three that we need to distinguish. The first two are seen here from a lateral perspective. On our left, we have the superior costal fovea or facet and on our right, we have the inferior costal facet. Keep in mind that now as you see them separately they're actually called demi-facets meaning that they're half because they're only going to become a full articular surface when these two vertebrae come together and the superior and inferior costal facets come together to form a fully articulating surface that will articulate with the head of a certain rib. The first and tenth to twelfth thoracic vertebrae display variation in terms of the size, location and number of these facets. For example, some possess whole facets instead of demi-facets.
Now moving on to another type of costal articular surface that is known as the costal fovea or the costal facet of the transverse process. The first to tenth thoracic vertebrae exhibit these costal facets on their transverse processes which articulate with the tubercles of the ribs. We can see this clearly on our image on the right where we can see a thoracic vertebrae and two ribs from above.
So far we've looked at the structures of the cervical and thoracic vertebrae paying particular attention to their unique characteristics. Now let's move on to look at the lumbar vertebrae.
There are five lumbar vertebrae which are found inferior to the ribcage and superior to the pelvis and the sacrum. Now in terms of their features, the vertebral bodies of the lumbar vertebrae are large and kidney-shaped as we can see in our superior view of a lumbar vertebra. In fact, their vertebral bodies are the largest of all the vertebrae of the spine due to the fact that they support the weight of the upper body. Each lumbar vertebra has a triangular vertebral foramen and a short flat quadrangular spinous process which we can see here in our lateral view on the left. Let's take a look at how the lumbar vertebrae articulate with one another.
As we know, vertebrae generally have four articular processes. The lumbar vertebrae have two superior articular processes and two inferior articular processes. Their superior articular processes have facets that face medially as you can see here represented by our arrows and their inferior articular processes have facets that face laterally. In the image on the right, we can see a nice example of how the lumbar vertebrae fit together.
Moving further down the spine, we'll now discuss the five fused vertebrae that make up the sacrum. This wedge-shaped bone is situated at the base of the spine and it forms the links between the iliac bones and the vertebral column. So now let's go over some of the key features of the sacrum. If we look towards the superior aspect of the sacrum, we can see the area highlighted in green which is known as the base of the sacrum. This is where the sacrum articulates with the fifth lumbar vertebra via the superior articular processes of the sacrum seen here in our gray circles and of course the inferior articular facets of L5. This articulation forms the lumbosacral joint.
The inner surface of the sacrum which is known as the pelvic surface is concave in shape and possesses four paired sacral foramina found on either side of the midline. These sacral foramina transmit the ventral rami of the first four sacral spinal nerves S1 to S4. On the posterior aspect of the sacrum running along the midline is the jagged bony ridge seen here highlighted in green which is known as the median sacral crest. This crest is palpable and is formed by the fusion of sacral spines S1 to S4. The landmark of the sacral hiatus which is caused by the lamina of S5 failing to meet in the median plane resulting in an exposed dorsal surface is located just below the S4 tubercle.
The triangular sacral canal is formed by the sacral vertebral foramina and descends from the opening on the basal surface represented by the triangle now drawn on your screen to the sacral hiatus. On either side of the sacral crest, we can see the sacral cornua. Since the spinal cord finishes at around L2, the sacral canal doesn’t actually carry the spinal cord. Instead, it carries the filum terminale, the anterior and posterior roots of the sacral and coccygeal spinal nerves and fibrofatty material.
Lateral to the sacral foramina on the posterior aspect of the sacrum are a pair of longitudinal ridges known as the lateral sacral crests. These ridges are formed by fused transverse processes of the sacral vertebrae. Moving on, there's an auricular surface on the lateral aspect of the sacrum that articulates with the ilium to form the sacroiliac joint. We can see these joints highlighted in green in our image on the right. Finally, the apex of the sacrum is the most inferior segment. It's formed by the fifth sacral vertebra and has an oval-shaped facet for articulation with the coccyx forming the sacrococcygeal joint.
Finally, the coccyx which as we have just seen, articulates with the fifth sacral vertebra to form the sacrococcygeal joint is made up of three to four bones. Here, we have a posterior view of the coccyx on the left and an anterior view on the right. The first coccygeal vertebra is the only coccygeal vertebra that shows some of the typical structures of a fully formed vertebra. Quickly going back to the sacrococcygeal joint, it should be noted that the presence of this joint allows for a passive backward motion of the coccyx. This is important during childbirth as it increases the anteroposterior diameter of the pelvic outlet.
Towards the beginning of our tutorial, we saw that the spine has four distinct curvatures that increase the flexibility of the vertebral column and distribute body weight more evenly among the vertebrae. However, the vertebral column can also exhibit some abnormal curvatures, one of which we'll focus on in our clinical notes.
So, scoliosis is a deformation of the vertebral column in which lateral curvature is present either on one side forming a C-shape or on both sides forming an S-shape. In our x-ray here, we can see that a lateral curvature is present on both sides of the spine resulting in a slight S-shape. Severe scoliosis can be debilitating leading to decreased mobility, back pain and respiratory issues.
Before we bring our tutorial to a close, let's quickly summarize what we've learned today. First, we looked at the general functions and structure of the vertebral column. We saw that it protects the spinal cord and supports the weight of the upper body. We also saw that it's divided into five regions namely the cervical region, the thoracic region, the lumbar region, the sacral region and the tiny coccygeal region. We saw that these regions make up a total of thirty three vertebrae and between these vertebrae are the intervertebral discs. It's also worth remembering that the spine is not a straight up-and-down structure but one that exhibits anterior and posterior curvatures.
Once we were happy with the spine as a whole, we looked at the individual vertebrae that form it. First, we saw the cervical vertebrae. There are seven cervical vertebrae and generally they exhibit some defining characteristics including a small body, a triangular vertebral foramen, a foramen transversarium in each transverse process and a bifid spinous process. We then saw the thoracic vertebrae. There are twelve thoracic vertebrae and they also exhibit some unique characteristics including a medium-sized heart-shaped body, a small circular vertebral foramen and their spinous processes are long and angled sharply downwards. Furthermore, the thoracic vertebrae exhibit costal facets which articulate with the ribs.
Next, we had the lumbar vertebrae. There are five lumbar vertebrae and again they have some special characteristics including a large kidney-shaped body, a triangular vertebral foramen and a short flat quadrangular spinous process. Inferior to the lumbar vertebrae, we find the sacrum which we can see highlighted in green. The sacrum is made up of five fused sacral vertebrae. And finally, we had the coccyx which is made up of three to four bones. We can see the coccyx highlighted in green forming the tip of the vertebral column.
So that brings us to the end of our tutorial on the vertebral column. I hope you find it useful and thanks for watching!
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