Origins, insertions, innervation and functions of the superficial muscles of the back.
Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we’re going to be talking about the superficial back muscles. So what we’re going to be doing is looking at the different muscles that define your back. So they’re found a bit more superficially because there is also the intrinsic muscles of the back which are going to be covered on a separate tutorial here at Kenhub.
Now, before we move on and talk about the different origins, insertions, innervation, and functions of these muscles, I would like to list them, and we’re going to be talking about the levator scapulae, the serratus posterior muscles, the rhomboid muscles, the trapezius, and the latissimus dorsi muscle.
Let’s start off with these that you see here highlighted in green. These are known as the levator scapulae muscles.
And the levator scapulae is a long muscle, as you can see on the image, and in terms of origin points, there are several that we need to talk about.
One is going to be the transverse process of the atlas, as you can see here on this image.
Another one will be also the transverse process, but this time, of the axis or the second cervical vertebra.
And you also see here that the transverse processes of C3 and C4 are going to serve as an origin point for the levator scapulae.
When it comes to the insertion of the levator scapulae, you see here that the muscle is going to go all the way down to, then, insert on the upper edge of the medial border of this bone here, this triangular bone known as the scapula.
Now, when it comes to the innervation of the levator scapulae, this muscle is going to be, then, innervated by the dorsal scapular nerve, which you see here on this image, highlighted in green. And notice that we are… on this image, we have the brachial plexus here, and superiorly, you have, then, the cervical plexus. And you can see how the dorsal scapular nerve is a branch of the brachial plexus.
Also, the cervical nerves C3 to C4 are going to be innervating the levator scapulae.
Now that we covered the innervation of the levator scapulae, it is time for us to briefly cover the functions or actions associated to this muscle.
And as the main name indicates, the main function of this muscle, “levator,” means that it’s going to be elevating the scapula. And during that movement, it pulls the entire scapula medially. That movement is also helpful when bringing the elevated arm back to the neutral position.
In addition, the muscle also moves the inferior angle away from the back (this angle here of the scapula as you can see on both sides). So it moves away from the back, causing a short tilt, upward tilt, of this bone, of the scapula.
And if the scapula is fixed, a contraction of the levator scapulae will, then, lead to bending of the cervical vertebral column or cervical spine, as you can see here, so bending to the sides, what we call a fancy word, we call it then lateral flexion—so when you move your neck to the sides.
And if you remember from the previous list that we talked about, we’re going to, now, move on to these two muscles that you see on the screen, which are known as the serratus posterior muscles.
On the top, we’re looking at the serratus posterior superior, while on the bottom part, we see, then, the serratus posterior inferior.
Now, the serratus posterior muscles are thin, superficial back muscles—that’s why we’re including in this tutorial—which will be lying above the intrinsic back musculature. And both muscles are known to be among the secondary back muscles, meaning that they migrated to the back in the course of embryogenesis.
We’re going to move on to the very first one on the list that you see here highlighted in green. This is known as the serratus posterior superior.
Now, this muscle is going to be arising from a ligament known as the nuchal ligament and the spinous processes of vertebrae C7 through T3.
Now, the muscle goes to, then, insert into the upper borders of the second, as you can see here, third, fourth, and fifth ribs.
The serratus posterior superior muscle is going to, then, be innervated by the intercostal nerves or the second to fifth intercostal nerves.
When it comes to the functions associated to this muscle, the serratus posterior superior elevates the rib, this way supporting inspiration (so it’s an accessory muscle of inspiration). And furthermore, the activation on both sides extends the thoracic vertebral column, while a unilateral contraction rotates it in the opposite side.
So the next muscle we’re going to be covering now is the inferior one, so the serratus posterior inferior.
This one will be originating from the thoracolumbar fascia as well as the spinous processes of the lower thoracic and upper lumbar vertebrae—so from T11 all the way to L2. Then, the muscle is going to ascend craniolaterally to insert on the ninth to twelfth ribs, as you can see here on this image, from the ninth all the way to the twelfth rib.
The serratus posterior inferior muscle is going to be innervated by the intercostal nerves T9 to T12, which are seen here highlighted in green on this image.
This image is an image of the structure of the spinal cord where you can, then, see the spinal nerves, then, branching out into, then, the intercostal nerves, which are highlighted here and will be, in this case, T9 and T12 will be the innervation of the serratus posterior inferior.
Now, a word on the different functions or actions associated to this muscle, the serratus posterior inferior helps during expiration. So it is going to be depressing the ribs, as you can see here, indicated by these arrows. So they are accessory muscles of expiration.
Now, through bilateral contraction, the muscle can, then, extend the spine, and when it only contracts on one side, it’s then going to rotate the spine.
And by bilateral contraction, when these two muscles are contracting at the same time, they’re going to, then, cause what is known to be as extension of the spine.
Unilateral means that only one side will be contracting, and for that reason, it will cause rotation of the spine.
Now, the next group of muscles that we’re going to be talking about is seen now on the image. These are known as the rhomboid muscles.
And we can distinguish two rhomboid muscles: the rhomboid major, which is found inferiorly (this one here, so a larger one) and there is the rhomboid minor, which is found superiorly (this thin one here).
Now, the rhomboid muscles are two diamond-shaped muscles, as you can see on the image, and they are both palpable. Usually, there is a small space between both rhomboid muscles. However, in some cases, one may find one, single, blended muscle instead.
And before we talk about these muscles separately, I would like to say or give a quick word on the innervation of both rhomboid muscles.
And as you probably guessed, with this highlight here that we’ve seen before, this is the dorsal scapular nerve, which is a branch of the brachial plexus coming from C5, as you can see here.
Now, we’re going to focus on this one here, highlighted in green. This is the rhomboid major. In terms of origin points, this muscle is going to run from the spinous processes of the first to the fourth thoracic vertebrae, as you can see here. So from the first all the way to the fourth thoracic vertebrae.
Now, it’s going to fan out and insert into the medial border of the scapula, as you can see here on both sides.
Let’s take a look, now, at the minor, so the rhomboid minor. This muscle is going to originate further up at the spinous processes of the sixth and also seventh cervical vertebrae.
And as you probably already guessed, in terms of insertion point, this muscle is also going to be inserting on the medial border of the scapula, but this time, a bit above the insertion point of the rhomboid major.
We’re, now, going to move on and have the two rhomboids here next to one another, and you see here that the muscles look slightly different from the previous images because they’re contracting. So we’re now going to talk about the different functions or actions associated to the rhomboids.
Now, both rhomboids perform the same motions as their fiber muscles run parallel towards the same direction. So contraction of these muscles will cause craniomedial movement of the scapula—so adduction and elevation of the scapula.
Now, at the same time, the inferior angle of the scapula is moved towards the vertebral column, which is known as, then, rotation of the scapula, and that movement mainly supports lowering of the elevated arm.
Another function of the rhomboid musculature is stabilizing the scapula during both rest and arm movement.
And that’s it. We’re ready to move on to the next muscle that you see here highlighted in green: the trapezius.
Now, the trapezius muscle is a triangular, flat muscle, and we can divide it into three parts: a descending or superior part, a transverse or middle part, and an ascending or inferior part.
So if I had to show you more or less where these parts are, so you have a descending or superior part of the muscle, you have a transverse or middle part, and then these fibers here are known as the ascending or inferior part of the trapezius muscle.
And all of these parts have different origin and insertion points which we will talk about separately. So the descending or superior part of the trapezius muscle will be originating from the external occipital protuberance—so all the way up here on the occipital bone on the skull.
It’s also going to originate from the external occipital crest and the nuchal ligament.
Now, the descending or superior part of the trapezius muscle is going to, then, insert on the lateral third of the clavicle.
Now, let’s take a look at the transverse or middle part of the trapezius muscle, and in terms of origin points, the spinal processes of the first to fourth thoracic vertebrae will be serving as an origin point for this part of the muscle. Then, it goes all the way to insert on the acromion of the scapula.
Now, the last part of the trapezius muscle, if you remember, this is the ascending or inferior part.
In terms of origin points, this part of the muscle is going to come from the spinal processes of the fifth to twelfth thoracic vertebrae… and then goes all the way to insert at the spine of the scapula, which you can clearly see here on this image.
So this is the spine of the scapula, which is clearly seen from a dorsal view, and then you can see how the muscle is inserting, or the ascending or inferior part of the trapezius muscle is inserting on the spine of the scapula.
I would like to now talk about the innervation of the trapezius muscle, which we will see now, the accessory nerve as well as spinal nerves C3 to C4, which are seen here highlighted on both of these images.
Now, these will serve as the innervation for the trapezius muscle. So accessory nerve and spinal nerves C3 to C4.
Let’s talk about the different functions associated to the trapezius muscle.
Now, the trapezius stabilizes and secures the shoulder blade at the thorax and fulfills numerous tasks. It will be moving, then, the shoulder blade medially and rotates it outwards.
And in addition, the descending part of this muscle will be causing elevation of the scapula, while the ascending part will be causing, then, depression of the scapula.
Furthermore, a unilateral contraction bends the head to the ipsilateral side to what we call, then, a fancied expression for, then, lateral flexion of your head, whereas a bilateral contraction will be raising the head and the cervical vertebral column to what we call, then, the dorsal flexion of the head and cervical spine.
Now, from one large muscle to another one, the famous latissimus dorsi muscle. This muscle is the widest muscle in your body, and it is relatively thin and covers all muscles of the back at the posterior trunk except the trapezius.
And together with the teres major muscle, it forms the posterior axillary fold in the armpit where it is thickest and therefore easy to palapate.
Now, as a large muscle that the latissimus dorsi is, it should have different origin points, and for that reason, we can separate or divide it into different parts.
We’re going to be talking about these parts: the first one is the vertebral part, which has an origin point on the spinous processes of the seventh to twelfth thoracic vertebrae. But a main origin point is the thoracolumbar fascia.
Now, there is an iliac part which will, then, originate from the iliac crest, a costal part of this muscle which will originate from the ninth to twelfth ribs.
And finally, a scapular part which originates from the inferior angle of the scapula.
Now, all fibers of this muscle will come together to attach on one point only, as you can see here on the image. This will be, then, the insertion point for the latissimus dorsi, and that is the crest of the lesser tubercle of the humerus—so between the pectoralis major and teres major.
And during its course, the fibers turn spirally around the teres major, what we call torsion. And that way, the lower part of the latissimus dorsi inserts proximally at the humerus, and the upper part will be, then, inserting more distally.
Now, we’re going to look at the innervation of the latissimus dorsi, which is this nerve that you see here, highlighted in green. That is the thoracodorsal nerve… which is also a branch of the brachial plexus.
Now, as a large muscle, it will have also different functions in your body. Now, it supports various, different movements of the shoulder, as you would probably guess due to its insertion point.
Now, the muscle contraction leads to inward rotation of the humerus, which you see here represented by this arrow.
In addition, the muscle is going to be pulling the humerus closer to the trunk to what we call, then, adduction, seen here, indicated by this arrow.
And it will also be slightly pulling back the humerus to what we call, then, retroversion.
By performing these three movements at the shoulder joint, the latissimus dorsi is referred to as one of the swimming muscles of the human body.
Another important function of this muscle is to provide support during breathing. So the activation of the latissimus dorsi muscle on both sides causes a compression of the rib cage and then facilitating expiration. That’s why we also call it an accessory muscle of expiration.
And for that reason, the muscle is particularly strained during coughing attacks, to what we also sometimes refer to as the coughing muscle.
And for that reason, the muscle is also particularly strained during coughing attacks.
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