Hello everyone! This is Megan from Kenhub, and welcome to another anatomy tutorial. In today's tutorial, we're going to be looking at some of the external features of the spinal cord. As you may already know, the spinal cord is the vital link between the brain and the rest of the body, so, it's important to know the location of the structure and its relation to surrounding structures. In order to describe these features, we're going to explore this image that you see now on your screen.
In this image, we see the spinal cord inside the vertebral column from a posterior view, and we have removed the posterior part of each vertebrae by making a cut at the lateral parts of the vertebral arch and also we have removed all of the intervertebral discs. So, we have exposed this longitudinal canal which is formed by the unity of each of the vertebral foramina. In this way, we can see the whole length of the vertebral column from the foramen magnum at the top here down to the inferior most part – the sacrum and the coccyx.
To help you better orientate yourself, I would like to show you some helpful and important landmarks. In the next image, we can see the first cervical vertebra just inferior to the occipital bone. Another landmark that you can see is the first rib. The important thing about this rib is that it articulates with the first thoracic vertebra. And here you can see the twelfth rib which is also referred to as a floating rib due to the fact they articulates only with the vertebral column and not with the sternum or its cartilage. This rib articulates with the twelfth thoracic vertebra and marks the end of the thoracic spine. Finally, here, at the inferior most part of the vertebral column, we see this small bone – the coccyx – which is the final portion of the vertebral column.
So, now, that we've seen these bony landmarks and it's easier for you to orientate yourself, we can now move on and see some of the external features of the spinal cord. The spinal cord is the continuation of the medulla oblongata and extends from the foramen magnum here at the top of the image to approximately the level of the discs between the vertebrae L1 and L2. As you can see in this image, the spinal cord does not fill in all the length of the vertebral column. This is caused by the fact that during embryological development, the vertebral column outgrows the spinal cord, resulting in the caudal nerve roots to descend below the terminal point of the spinal cord forming a structure known as the cauda equina. We will discuss more about this later on in the tutorial.
But, for now, I'd just like to remind you that the spinal cord is a cylindrical structure of nervous tissue and, as you can see in this transverse section of the spinal cord, it's composed of white matter externally and gray matter internally. The gray matter contains mainly the cell bodies and glia of the various nuclei while the white matter corresponds to the ascending or the descending fiber pathways.
There is a tiny central canal in the central point of the gray matter that is not visible at this magnification but contains cerebrospinal fluid. I will not discuss the nuclei and pathways of the spinal cord in this tutorial but I would like to mention that the nerve roots from the ventral and dorsal horns of the gay matter join together outside the spinal cord – as you can see here – forming thirty-one pairs of spinal nerves which innervate the whole body. We're going to see the spinal nerves in more detail later on in this tutorial.
As I mentioned earlier, the lower end of the spinal cord is found approximately at the level of the intervertebral discs between the first and second lumbar vertebrae at which point it begins to taper off. This cone-shaped distal end of the spinal cord is known as the medullary cone.
The pia mater that surrounds the spinal cord continues downwards all the way to the coccyx as a thin filament known as the filum terminale. This provides for a tight connection between the medullary cone and the back of the coccyx and, thereby, stabilizing the entire spinal cord.
As I mentioned earlier, the spinal cord gives rise to thirty-one pairs of spinal nerves named according to their position with respect to associated vertebrae. The most superior spinal nerves that you see here highlighted in green are the cervical nerves. There are eight pairs of cervical spinal nerves, C1 to C8. The first pair of these nerves seen here emerges between the occipital bone of the skull and first cervical vertebra. So, it follows that the cervical nerves C2 to C7 also emerge from the vertebral canal above the respective vertebrae; however, the eighth cervical spinal nerve, C8, emerges between the seventh cervical vertebra and the first thoracic vertebra. As a consequence, all the remaining spinal nerves beginning with the first thoracic spinal nerve, T1, emerge from the vertebral canal below the respective vertebrae. This is an important point to remember in order to understand the exact position of each spinal nerve.
Another important point to remember is that once the spinal nerve emerges from the intervertebral foramen, it gives off two major branches – an anterior ramus or a ventral branch and a posterior ramus or a dorsal branch. The anterior rami of the cervical spinal nerves form the cervical and brachial plexuses. To be more specific, the ventral branches of the first four spinal nerves form the cervical plexus, whereas, the ventral branches from C5 to C8 of the spinal nerves form the brachial plexus. The posterior or dorsal branches of the cervical spinal nerves supply the nuchal muscles and the skin of the lateral nuchal region.
Next stop, let's look at the thoracic nerves. There are twelve pairs of thoracic spinal nerves each emerging just below the corresponding thoracic vertebrae. In the same way as the cervical spinal nerves, the thoracic spinal nerves have anterior and posterior branches or divisions. The anterior division or branches of the T1 to T11 thoracic spinal nerves run as intercostal nerves between the ribs. The anterior ramus of the spinal nerve T12 – also known as the subcostal nerve – runs below the twelfth rib. whereas each of the thoracic spinal nerves from the posterior division or branch divide into a medial and lateral branch both of which supply motor fibers to the deep autochthonous muscles of the back.
From the lumbar segment of the spinal cord come the lumbar nerves. The lumbar nerves – like all the nerves we have discussed so far – are divided into posterior and anterior divisions. There are five pairs of lumbar spinal nerves that emerge just below each of the respective lumbar vertebrae – as you can see here. The posterior division or branches of the lumbar spinal nerves supply the autochthonous back muscles as well as giving off cutaneous branches, whereas, the anterior division or branches of these spinal nerves form the lumbar plexus specifically the branches from L1 to L3 and part of L4. The rest of the branches from L4 and L5 form part of the lumbosacral trunk. We will discuss this later on in this tutorial.
As we move further down the vertebral column, we see another five pairs of nerves. These are the sacral nerves which also have an anterior and posterior branch or division. The sacral nerves emerge from the spinal cord at the level of L1 or L2 vertebrae – as you can see in our illustration – and descend into the sacrum. The anterior and posterior branches or rami exit the vertebral column through the anterior and posterior sacral foramina respectively.
The anterior branches of the S1 to S4 spinal nerves joined with the lumbosacral trunk that originates from the L4 and L5 spinal nerves and together they form the sacral plexus. This plexus supplies the skin and muscles of the pelvis and the lower limb. The anterior division of the fifth sacral nerve exits from the hiatus between the sacrum and the coccyx.
The posterior branches of the sacral spinal nerves are small and diminish in size from top to bottom as they emerge. The posterior branches with the exception of S5 emerge through the posterior sacral foramina and provide motor innervation to the multifidus muscle. They also provides sensory innervation to the skin over the posterior part of the buttocks.
The coccygeal nerve is the thirty-first pair of sacral nerves and it's the last of the spinal nerves. The nerves that originate from this plexus are known as the anococcygeal nerves and they supply sensory fibers to the skin over and between the coccyx and the anus.
Having covered the spinal nerves, let's move on to look at the structures seen here highlighted in green which is known as the cauda equina. The term cauda equina comes from the Latin meaning "horse tail". As I mentioned at the beginning of this tutorial, during embryological development, the vertebral column outgrows the spinal cord resulting in the end of the spinal cord moving further up in relation to the surrounding vertebrae.
In adults, the lower end of the spinal cord as we saw previously is found approximately at the level of the L1 or L2 vertebrae. Therefore, the caudal nerves of the spinal cord need to travel a longer distance to reach the respective vertebral foramina and its these descending nerves below the spinal cord that are referred to as cauda equina. So, basically, the cauda equina is comprised of three or four lower lumbar nerves, the sacral nerves, and the coccygeal nerves. Here, we can see a zoomed-in illustration of the cauda equina.
If we look carefully at the posterior surface of the spinal cord, we can just barely see this structure here which is a shallow groove situated at the midline along the posterior aspect of the spinal cord and it's known as the posterior median sulcus. This median longitudinal groove is located between the right and left posterior funiculi.
The next structures that we can see from this posterior view are these thin, short fibers that emerge from the posterior surface of the cord. These are the posterior roots of the spinal nerves. If you remember from the beginning of this tutorial, I mentioned that these roots emerge from the posterior or dorsal horns of the gray matter of the cord and to help you understand this, I'll show you another image.
So, this image depicts the transverse section of the spinal cord. As you can clearly see here, the posterior roots of the spinal nerve exit from the posterior aspect of the spinal cord and, very often, these roots emerge from the spinal cord as six or eight thin fibers known as rootlets that come together and form the posterior root. Distal to the posterior roots that we've just seen, we see this spindle-shaped bulges that are located inside the intervertebral foramina. These are the sensory ganglia of the spinal nerves.
Again, I'm going to show you a transverse section where you can see the ganglion just lateral to the posterior root and inside the intervertebral foramen. These two structures, namely, the posterior root and the sensory ganglia are the sensory part of the spinal nerve, meaning that they carry information towards the spinal cord. If you look carefully in this image, the posterior part we just saw at the end of the intervertebral foramen joins with the anterior part which is the motor part of the spinal nerve and, together, they form the spinal nerve. So, to make a long story short, the anterior motor root and the posterior sensory root join together to form the mixed spinal nerve.
Shortly after the spinal nerve exits the intervertebral foramen, it splits into two major rami – the ventral and the dorsal rami. If we zoom in on this image, we can partially see the dorsal or posterior rami. Once again, in this image, we can see the posterior ramus as it divides from its anterior ramus. It then further divides to give off medial and lateral branches that you see here which supply the skin of the back as well as the autochthonous muscles of the back. And, finally, let's look at this membrane that covers many of the structures that we've seen in this tutorial, and this is called the dura mater.
The spinal dura mater is the outermost meningeal membrane and it's separated from the bones forming the vertebral canal by an extradural space. Superiorly, it's continuous with the inner meningeal layer of the cranial dura mater. Inferiorly, the dural sac dramatically narrows and the dura mater forms part of the filum terminale. As the spinal nerves and the roots pass laterally, they are surrounded by tubular sleeves of dura mater as you see here which merges with and becomes part of the epineurium of the nerves.
Now that we've covered the overall structure of the spinal cord as well as the spinal nerves, let's move on to discuss some arteries that are visible from the posterior aspect of the spinal cord starting with the ascending cervical artery. The ascending cervical artery is a small branch of the inferior thyroid artery that lies medial to the phrenic nerve and the scalenus anterior. This small artery runs up the neck close to the tips of the transverse processes of the cervical vertebrae. It supplies the neighboring muscles, the bodies of the cervical vertebrae, and partially, the cervical spinal cord.
Another important artery that we see here near the upper part of the spinal cord is the vertebral artery. This is a paired artery, meaning, that there are two vertebral arteries – one on the right and one on the left. These two arteries are branches of the subclavian arteries at the level of the anterior scalenus muscle. After emerging, they interdate and pass through the foramina of the transverse processes of the cervical vertebrae from C6 to C1 – as you can see in this illustration – then pass cranially coursing over the atlas. Finally, the two arteries come together in the midline and unite. This unity gives rise to the basilar artery that we can see by looking at this inferior view of the brain. So, in this way, the two vertebral arteries are critical because they are responsible for the posterior circulation of the brain.
The next arteries that we can see when we zoom in in this image are the posterior radicular arteries. These arteries run along the posterior root of the spinal nerves supplying them with oxygenated blood. The final arteries we are going to cover in this tutorial are the posterior spinal arteries. There are two posterior spinal arteries that supply the spinal cord. These two arteries which run the length of the spinal cord on either side of the midline supply the posterior horns of the gray matter and the posterior funiculi.
Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.
Now, good luck everyone, and I will see you next time.
