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Intrinsic muscles of the back: medial tract

Origins, insertions, innervation and functions of the medial tract of the intrinsic muscles of the back.

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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 and focusing on the medial tract of these muscles.

And before I do so, I would like to clarify what is or just remind you of what is the intrinsic back muscles.

They are basically a group of deep layer muscles that you find on the posterior part of the trunk wall, and then we usually split them into two big groups. One is the lateral tract muscles. Another one is the medial tract muscles, which we will be talking about on this tutorial.

If you would like to learn more about the lateral tract muscles, I suggest you watch another tutorial here at Kenhub where we will be talking about them.

Continuing on to the medial tract muscles which you see here represented on this image, and you can see where we get this name from “the medial tract,” because they’re found medially when compared to the lateral tract muscles.

You also notice here that the medial tract muscles are originating and inserting on the different vertebrae.

The medial tract muscles is a big group, but we can split them into smaller groups which we will talk about: the interspinales and the spinales muscles, which belong to a group known as the spinal system, and the rotatores brevis and longi, the multifidus, and the semispinales.

Now, they belong to the transverse spinal system.

We’re going to start off with the very first one, the first group that you see here on the image. They are known as the interspinales muscles.

These muscles are divided into three groups, specifically the interspinales cervices, the interspinales thoracis, and the interspinales lumborum.

A quick word on the functions associated to the interspinales muscles. When they contract, they will result in dorsiflexion and extension of the cervical and lumbar spines.

We’re moving now onto the innervation of the interspinales muscles. And the interspinales muscles are then innervated by the dorsal rami of the spinal nerves.

We’re now going to look at the different interspinales muscles, specifically seeing here now the interspinales cervices and the different origins and insertions that you see here.

Combined, they are then—these muscles are extending between the spinous processes of C1 to C7.

And as you can see clearly on this image, these muscles are paired muscles.

Now, a quick word on this segment here. This is, then, the interspinales thoracis.

The interspinales thoracis muscles are not always present. However, when they are present, the same way as the other two groups of interspinales muscles, the interspinales thoracis muscles are then paired, and they will be coursing between the spinous processes of the thoracic vertebrae on either side of the interspinous ligament.

We’re now ready to move on to this part here. This is known as the interspinales lumborum, which is found, as the name indicates, on the lumbar region.

These muscles will be coursing between the spinous processes of the L1 to L5. These bands of muscles are especially strong.

The other group of muscles found in the spinous system is the spinales muscles that we see now on the image.

These muscles attach to the spinous processes of the cervical and thoracic vertebrae, and therefore, we can divide them into two segments: the spinales cervices and the spinales thoracis.

A quick word on the different functions of the spinales muscles. The function of these muscles is then back flexion. So bilateral contraction of the spinales muscles extend the cervical and thoracic spines.

And then unilateral contraction of the muscles will bend the cervical and thoracic spines to, then, the same side as you see here indicated by these arrows.

Now, a quick word on the innervation of these muscles. They are innervated by the dorsal rami of the spinal nerves.

Now, let’s focus on this set here that you see highlighted in green. These are the… or this is the spinales cervices.

And in terms of origin points, the spinales cervices will have the origin on the spinous processes of C5 to C7 and T1 to T2 vertebrae.

In terms of the insertion points of the spinales cervices, now, the muscle will be inserting on the spinous processes of C2 to C5 cervical vertebrae.

We’re going to look now at the origin points here of the spinales thoracis, as you can see. Now, they will be originating… the fibers of this muscle will be originating on the lateral side of the spinous processes of T10 all the way to L3 vertebrae.

The fibers go all the way to then insert onto the lateral surfaces of the spinous processes of T2 all the way to T8.

So now that we’ve covered the spinal system of the medial tract of the intrinsic back muscles, it is time to move on to then the transversospinal system, beginning with then these muscles that you see here, the rotatores brevis and longi.

This is the deepest layer of muscles of the transversospinal system.

Bit of functions here of the rotatores brevis and longi, contraction of these muscles bilaterally will then extend the thoracic spine as you see here indicated by these arrows.

While unilateral contraction will be rotating the spine to the opposite side.

So for example, when these muscles on the right side contract, this causes the spine to rotate to the left side.

A quick word on the innervation of the rotatores brevis and longi. These will be innervated by the dorsal rami of the spinal nerves.

I’d like to now focus on the… just a quick word on the rotatores brevis which you see here mixed with the rotatores longi. So these are the brevis, and these will be then the longi.

So as the name indicates, the short ones and the long ones.

These will have their origin points on the transverse processes of T1 all the way to T12 and then insert on the spinous processes of the next higher vertebrae.

Moving on to now to talk a bit about the rotatores longi. Just like the brevis, the longi have their origin point from the transverse processes of T1 all the way to T12.

However, they will be inserting on the spinous processes of the vertebrae two levels higher than of the origin.

We’re moving on to this muscle here that you see highlighted in green, which is known as the multifidus, and this muscle will have or will be coursing between the transverse and spinous processes of all cervical vertebrae all the way to, then, the sacrum.

As this muscle courses along your spine, it can skip two to four vertebrae at a time. It is most developed at the lumbar spine as you can clearly see here on this image.

Like the other transversospinal system muscles, the multifidus will be innervated by the dorsal rami of the spinal nerves.

I’d like to give a quick word on the different functions associated to this muscle. Now, bilateral contraction will cause extension of the spine while unilateral contraction rotates to the opposite side and flexes at the same side as you can see here indicated by these arrows.

When the muscles on the left side contract, this flexes the same side, meaning the left side, but the spine rotates to the right side or the opposite side. Important point to make here.

We’re moving on to the next muscle that you see here. This is the semispinales, which is the longest and most superficial of the transversospinal system.

This muscle functions to, then, extend the thoracic and cervical spines, as you see here indicated by these arrows, and also the head, specifically when the muscle contracts bilaterally.

Now, unilateral contraction of the semispinales rotates the cervical spine, thoracic spine, and the head to the opposite side while at bending, also bending them to the same side.

Now, the semispinales will be innervated by the dorsal rami of the spinal nerves, and this is also, we can split these into different muscles or different segments known as the capitis, the cervices, and the thoracis.

A quick word on the first one on the list here. The capitis has its origin points on the transverse processes of C3 all the way to T6.

Then this muscle will be inserting on the occipital bone between the superior and inferior nuchal lines, as you can also see here on this image.

This is the occipital bone. We’re looking at it from a dorsal view, and here’s the insertion of the semispinales capitis.

A quick word on the next muscle that you see here highlighted in green. This is the cervices, which has the origin points on the transverse processes of T1 all the way to T6. Then the muscle is going to be inserting on the spinous processes of C2 to C7, so the cervical vertebrae.

Last segment or last muscle of the semispinales. This one is the thoracis, which will be originating on the transverse processes of T6 all the way to T12. Then the muscle will be inserting on the spinous processes of C6 to T4.

Now that we have covered all the medial tract muscles, let’s very quickly go over the muscles and their subgroups. The intrinsic back muscles of the medial tract can be divided into two subgroups or systems.

Firstly, the spinal system includes the interspinales and the spinales. The transversospinal system includes the rotatores brevis and longi, the multifidus, and the semispinales.

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