Origins, insertions, innervation and functions of the lateral tract of the intrinsic muscles of the back.
Hello, everyone. This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, I’m going to be talking about the intrinsic back muscles.
On this tutorial, I will be focusing on a particular group of the intrinsic back muscles known as the lateral tract.
Now, before I talk about the intrinsic back muscles, I would just like to give you a brief introduction to these muscles.
So these muscles are found on the posterior or dorsal side of the trunk wall. They can be divided into the group that we’re going to be talking about now, which is the lateral tract muscles and also another group which is the medial tract muscles.
Knowing this information, we’re now ready to move on and talk about the lateral tract muscles, which you can see here represented on this image of the posterior view of the trunk.
The muscles of the lateral tract group of the intrinsic muscles is comprised of five muscles, and the muscles consist of the iliocostalis or the longissimi muscles, which belong to a group known as the sacrospinal system muscles.
We’re also going to be talking about the splenii muscles which belong to the spinotransverse system muscles. We’re also going to be talking about a group known as the intertransversarii and the levatores costarum, which belong to the intertransverse system muscles.
We’re going to start off with the very first one on the list, the iliocostalis, which you see here complete.
And this is a long muscle, as you can see, that is made up of three segments, specifically a cervical segment, a thoracic segment, and a lumbar segment, which we will divide throughout this tutorial.
We’re going to also start talking about the different functions associated to the iliocostalis.
Now, when we talk about the three segments of the iliocostalis muscle, it’s important to note that these muscles can either work bilaterally or unilaterally.
And together, these three segments of the iliocostalis function to extend, as you can see here indicated by this arrows, and also bend the spine.
For example, the bilateral contraction of the entire muscle will be extending the spine that is basically straightening of your spine. And in unilateral contraction of the muscle on either side, we can see bending of the spine laterally to the same side of the contraction.
Now, I would like to give a brief word on the innervation of the iliocostalis. All three segments of this muscle are going to be innervated by the lateral branches of the dorsal rami of the spinal nerves, from C8 to L1.
We’re now going to move on and talk about the different segments here that I’m now highlighting. This is one known as the iliocostalis lumborum. And if you remember, then, this is the lumbar segment of the iliocostalis.
This one will have, then, different origin points. One is going to be then the sacrum, the iliac crest, and the thoracolumbar fascia.
And as we’ve seen on the origin points, we’re also going to see different insertion points.
The iliocostalis lumborum muscle has more than one insertion point. One is from rib six to rib twelve.
And also, the muscle is going to be using the transverse processes of the upper lumbar vertebrae to insert.
We’re now moving on to the next segment that you see here highlighted. This one is the thoracis, and the iliocostalis thoracis segment of the iliocostalis muscle is located as… you can see here in the thoracic region and has its origin points on the angles of the ribs, specifically from the seventh to twelfth ribs.
The muscle is going to, then, insert on the ribs as well, but this time, from the first all the way to the sixth.
Now, a quick word on the last segment that you see here of the iliocostalis. This one is the iliocostalis cervices, and this one makes up the cervical segment of the iliocostalis muscle.
This portion has its origin point at the third to seventh ribs, and then the muscle is going to be inserting on the transverse processes of the cervical vertebrae, so from C4 to C6.
We’re now going to move on and talk about another set of muscles known as the longissimi muscles, which are also muscles of the sacrospinal system.
Very similar to the iliocostalis, the longissimi muscles are going to be divided into three segments, the thoracis, the cervices, and the capitis this time. And as you can see here, this muscle also stretches along the length of the trunk wall.
Now, a quick word on the different functions associated to the longissimi muscles in general. Now, the longissimi function to, then, facilitate the lateral and backward flexion of the spine as well as rotating the face to the ipsilateral side.
So in a similar way to the iliocostalis, the bilateral contraction of the longissimi will, then, extend the spine. And in unilateral contraction, the spine bends to the same side of the contracted muscle.
Now, a quick word on the innervation of the longissimi muscles. Then, the innervation of this muscle is provided by the lateral branches of the dorsal rami of the spinal nerves from C1 to L5.
And now that we covered the function and innervation of the longissimi muscles, I want to go on and cover the three segments of this muscle, beginning with this one that you see here highlighted on the image, which is known as the longissimus thoracis, which is the thoracic segment of the longissimi muscles.
And keep in mind that the longissimi muscles are usually referred to as a group as the longissimi, meaning plural, but sometimes, they’re also referred as a whole unit known as the big longissimus muscle.
Now, a quick word on the origin of the longissimus thoracis, it also has more than one origin point. One on the sacrum, also the iliac crest, and we will see origin points coming from the spinous processes of the lumbar vertebrae and the transverse processes of the lower thoracic vertebrae.
Just like this muscle has different origin points, it will also have different insertion points.
It goes to insert on to the ribs from the second to the twelfth ribs and the transverse processes of the thoracic vertebrae.
We’re now ready to move on to the next segment, this one that you see here highlighted in green.
You probably guessed. Yes, this one is the longissimus cervices, which as the name suggests, is the cervical part of the longissimus, but it has its origin points on the transverse processes of T1 to T6, so on the thoracic vertebrae.
This section or this segment will then go on to insert on the transverse processes of the cervical vertebrae, specifically C2 all the way to C6.
Moving on to the next segment, as you see here highlighted in green, this time, we’re looking at the longissimus capitis. This segment has its origin points on the transverse processes of the thoracic vertebrae. So T1 to T5. And also the articular processes of the cervical vertebrae, C4 to C7.
Now, this portion of the longissimus muscle will be inserting on the occipital bone, more specifically at the mastoid processes of the occipital bone, which you can see here on this image.
The next group of muscles that we’re going to be looking at now, this is known as, then, the splenii muscles or the splenius muscle if you want to look at it as a unit.
And in the same way as the previous muscles that we already talked about, the splenii muscles or the splenius muscle can be divided into two segments or two muscles. One is the splenius cervices, and the other one is the splenius capitis.
Now, in terms of the function of this muscle, this is similar to two muscles we already discussed.
Bilateral contraction of the splenii muscles will then extend the cervical spine and the head as you see here indicated by these arrows, whereas unilateral contraction of the muscles will then flex and rotate the head to the same side.
For example, when the left side of the muscle contracts, this flexes and rotates the head to the left side.
Let’s start off with the very first segment or the very first muscle of the splenii muscles. This one is the splenius cervices. And in terms of origin points, this muscle will start at the spinous processes of T3 to T6. So, on the thoracic vertebrae.
The muscle goes all the way to, then, insert on the transverse processes of the cervical vertebrae, specifically C1 and C2.
The next segment or muscle of the splenii muscles that you see here, this one is the splenius capitis or the upper portion of the splenius muscle, as you can see here.
And a quick word on the specific functions of the splenius capitis, that the contraction of this muscle results in backward flexion and rotation of the head, which you see here indicated by these arrows.
Now, a quick word on the origin points for the splenius capitis, this muscle has its origin points on the spinous processes of C4 all the way to T3 or T4.
Again, a bit of variation because these muscles are relatively different from person to person. And also you see different information between literature.
Also, the nuchal ligament will serve as an origin point for the splenius capitis.
Now, this muscle will have its insertion points on the lateral side of the nuchal line as well as the mastoid process.
We’re now moving on to the next group of muscles that you see here on the screen. These are known as the intertransversarii.
They are located between the transverse processes of the vertebrae, and they belong to, then, the intertransverse system of the lateral tract muscles.
And as you have probably guessed by now, this muscle can also be divided into three segments. One is known as the intertransversarii medialis lumborum. The other ones are going to be the intertransversarii lateralis lumborum. There’s also the intertransversarii posteriores cervices and the intertransversarii anteriores cervices.
We’re going to briefly cover the different origins and insertion points of these muscles, but before we do so, let’s cover the different functions associated to the intertransversarii.
When the muscles contract bilaterally, they function to then stabilize and extend the cervical and lumbar portion of the spine.
So it goes to show that unilateral contraction will be then causing the muscles to bend the cervical spine and the lumbar portion of the spine as well to the contracted side.
Let’s talk about the very first segments here that you see highlighted in green. We’re highlighting two different segments, the medialis lumborum and also the lateralis, but we’re going to now focus on the intertransversarii medialis lumborum.
As you probably guessed, this is the portion of the intertransversarii lumborum that is closer to the midline of your body. Hence the name “medialis.”
Now, in terms of the origin or insertion points, the mamillary processes of the lumbar vertebrae will be serving as insertion points and origin points for the intertransversarii medialis lumborum.
Now, when it comes to the innervation of this muscle or these muscles, this small group of muscles is innervated by the dorsal rami of the spinal nerves.
We’re going to, now, talk a little bit about the intertransversarii lateralis lumborum, so found a bit to the sides when compared to the medialis portion.
And in terms of origins and insertion points, the transverse processes of the lumbar vertebrae will be, then, serving as origins and insertion points for these muscles.
A quick word on the innervation of these muscles, and these muscles will be then innervated now by the ventral rami of the spinal nerves.
Let’s talk about the different group of intertransversarii muscles. This time, the intertransversarii posteriores cervices.
And I’m showing here also the anteriores, so keep that in mind because these muscles are very, very hard to distinguish on an image that we included on the same image here.
But in terms of the origins or insertions points, these muscles will be originating from the transverse processes of the cervical vertebrae, C2 to C7.
So, they attach from one cervical vertebra to the next one and so on, as you can clearly see here also on this image.
Now, a little word on the innervation of these muscles. They are then innervated by the dorsal rami of the spinal nerves.
And last but certainly not least, the intertransversarii anteriores cervices.
In terms of origin and insertion points, these muscles connect to the anterior tubercles of the transverse processes of the cervical vertebrae, C2 to C7. And they are innervated by then the ventral rami of the spinal nerves.
We’re going to talk about the last group of the lateral tract that you see here on the image. These are the levatores costarum.
There are two types of levatores costarum. The levatores costarum brevis and also the .
And they have their names based on their length. As you can see here, the muscle is represented as a group, but you notice that there are shorter segments and also longer segments. And hence the names here that we’re going to be discussing on the next slides.
Now, I would like to give you a quick note on the innervation of the levatores costarum. Now, it is important to note here that both the dorsal and also ventral rami of the spinal nerves will be then innervating these muscles.
As for the functions of these muscles, they function to extend the thoracic spine when they are contracted bilaterally, as you can see here indicated by these arrows.
And during a unilateral contraction of the muscles, this will result into bending of the thoracic spine to the same side while, at the same, rotating the spine to the opposite side.
We’re moving on and talking about the different segments here or the different parts of the levatores costarum, and we’re going to start off with the brevis. Now, these are the short fibers of this muscle that I am point right now that you can see here on the screen.
They have their origin points at the transverse processes of C7 all the way to T11.
Then their insertion point is on the costal angle of the next lower rib to the vertebra of origin.
And now moving on the longi or the long portion of this muscle, the long fibers which you can also see here on this image, in the same way as the levatores costarum brevis, they are also having their origins on the transverse processes of C7 to T11 vertebrae.
But unlike the brevis muscles, the levatores costarum longi, they have their insertions on the costal angle of the next rib from the brevis to vertebrae from the longi’s origin.
It is important to remember this difference and insertions of the two muscle groups and not to confuse them.