Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we’re going to be talking about the scalene muscles.
And before I talk about this group of muscles, I need to show you where you can find them. To do so, I’m showing you right now an image of the anterior view of your neck with some of the muscles that you find in this region of your body.
And I’m going to, now, just remove all of these muscles to just expose the group that we’re going to be talking about, which is the scalene muscles.
Now, on this tutorial, we’re going to be talking about their anatomy, so we’re going to be talking about their origins, insertions, innervation, functions, as well as some related clinical aspects.
And in addition to their kinetic functions on the neck, they are important accessory muscles of inspiration. So it is important to learn and understand how they work.
Now, the scalene muscle group contains three muscles that are located at the lateral neck, as you can see here on this image (so, on the sides of your neck). And these are the anterior scalene, the middle scalene, and the posterior scalene.
Sometimes, there is a small fourth muscle which is known as the scalene minimus.
And generally speaking, the scalene muscles, as you can see here on this image, they run from the vertebrae on the neck and then go all the way to the ribs, and they’re able, when they contract, to lift the thorax.
Let’s start talking about these muscles, and the good news here, in terms of innervation, you only need that all scalene muscles have the same innervation coming from the cervical plexus, seen here highlighted in green from this image, as well as some parts of the brachial plexus.
And now, let’s talk about the first scalene muscles that you see here. These are the two anterior scalene muscles (one on each side of your neck).
In terms of origin point, they will originate from the anterior tubercle of the transverse processes of C3 all the way to C6 – so, the cervical vertebrae.
Then, the muscle is going to run caudally to insert at the first rib, as you can see here on the image. They insert at a tubercle which is known after the muscle as the scalene tubercle of the first rib.
A bit further behind, we’re going to find these other two which are known as the middle scalene muscles, and they originate at the posterior tubercles of the transverse processes of C3 all the way to C7, as you can see here on the images. So the posterior tubercles of the transverse processes of the cervical vertebrae, C3 or third cervical vertebra, all the way to the seventh cervical vertebra.
And just like the anterior scalene muscle, the middle scalene runs caudally towards the ribs, as you can see here, to insert on the first rib as well. And it will be inserting dorsally from the subclavian artery and the anterior scalene muscle.
And even a bit further posteriorly, we’re going to be finding these two or these pair known as the posterior scalene muscles, which will be originating at the posterior tubercles of the transverse processes of C5 all the way to C7 as you can see here.
Now, the muscles progress caudally towards the ribs to insert at the second ribs, as you can see here on the image.
We’re now going to move on and talk about the different functions associated to the scalene muscles.
We now have here an image of the lateral view of the scalene muscles.
And as mentioned before, the scalene muscles have an important function. So when they contract on both sides, or what we call then bilateral contraction, it will lead to elevation of the first rib, especially when the anterior scalene and the middle scalene muscles will contract, then it will cause elevation of the first rib because if you remember correctly, these two muscles will be inserting on the first rib.
And then when they elevate the first rib, the thorax… the rest of the thorax will follow through.
So this makes the scalene muscles accessory muscles of inspiration.
Now, when there is unilateral contraction (so when one of the muscles is contracting) you’re going to be seeing lateral flexion of the cervical spine – so movements side to side as you see here indicated by this arrows.
And furthermore, a bilateral contraction of the anterior scalene muscle causes bending of the cervical spine to the front to what we call, then, ventral flexion, seen here indicated by this arrow.
And here’s a tip on how to understand the functions or actions of these muscles. You can compare the inspirational function of the scalene muscles to the departure of a train.
The front locomotive or the scalene muscles will be pulling the first wagon, which is then this here, the first rib. Then, the rest of the ribs, being the other wagons of the train, as you can also see here on this image.
And when the first rib is pulled up, the intercostal muscles between the ribs—like I mentioned before, there are all these connections happening between these ribs—now, these connections will help tense and also pull the rest of the train wagons (a.k.a. ribs) along with them until the whole train or the thorax moves or elevates.
Now that we covered the different origins, insertions, and functions, and even innervation related to the different scalene muscles, I would like to add an important point that is also related and usually mentioned whenever we talk about the scalene muscles.
As you can see here, there’s an important structure being formed by the scalene muscles, an important structure where other important structures will be passing through.
So notice here, this is the anterior scalene muscle, which is forming with the middle scalene muscle and also the first rib, a triangle which is known as the interscalene triangle, and I’m highlighting it right now on the screen.
Now, this is an important structure because, here, the subclavian artery, as you can see here, and the brachial plexus will be passing through this triangle, or it will be passing… these structures will be passing through a gap that is formed by these muscles.
I would like to add a very important point here, because this is usually an exam question (so write this on your notes) that the subclavian vein does not run through the interscalene triangle, and I just added the two subclavian veins on each side, and you notice here how it’s running in front of the anterior scalene muscles.
So the subclavian veins are going to be running between the anterior scalene muscles and these muscles that I’m adding right now, the sternocleidomastoid muscles.
Before I end this tutorial, I would like to add some clinical anatomy points. One of them is related to the previous structure that I just talked about.
So when the interscalene triangle is too narrow, it will be compressing these structures that pass through it which are the brachial plexus and the subclavian artery if you remember correctly.
When that happens, we call it the scalene syndrome or thoracic outlet syndrome. However, the thoracic outlet syndrome can have other causes other than those related to the interscalene triangle.
Now, in terms of the different causes for the scalene syndrome, maybe, amongst others, a hypertrophy of the scalene muscles due to chronic overload or variations or deformities of the first rib.
Now, back to the clinical points, I would like to also add that compression of the brachial plexus will lead to paresthesia, or tingling, or tickling, prickling—this is what we call, then paresthesia—and also pain. Whereas the compression of the subclavian artery can lead to circulatory disorders, such as edema or ischemia.