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Origins, insertions, innervation and functions of the muscles of the shoulder joint.
Hello, everyone. This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we’re going to be talking about the shoulder joint muscles.
What we’re going to be doing here on this tutorial is discussing the different origins, insertions, innervations, some of the functions, and also related clinical anatomy of the muscles of your shoulder. And right now, we’re looking at them from an anterior view here on this image.
Now, the list that I’m just going to give you right now may vary a little bit depending on how you learn these muscles, but I suggest you go and watch other tutorials where you might find those muscles that are not included on this particular list.
Now, the shoulder muscles include the coracobrachialis, the deltoid, the teres major, the latissimus dorsi, the pectoralis major, the supraspinatus, the infraspinatus, the teres minor, and the subscapularis.
And notice here that these last four muscles belong to a group that is commonly known as the rotator cuff.
And let’s start with the very first one that we saw here on that list that you now see highlighted in green. This is known as the coracobrachialis muscle.
Now, the coracobrachialis is located at the upper and medial part of your arm, and right now, here on this image, we’re looking at it from an anterior view.
Now, the origin point for this muscle (this is an easy one) it’s going to be as the name indicates, the coracoid process. This projection that you see here of the scapula.
The muscle is inserted through a flat tendon into an impression at the middle of the medial surface of the humerus, as you can also see here on this image.
The coracobrachialis muscle is going to be innervated by the musculocutaneous nerve, as you can see here. This nerve highlighted in green on the image.
Now, we’re ready to move on and talk about the different functions or actions associated to the coracobrachialis. And for that, we’re looking now at an image of a lateral view of the coracobrachialis with some movement going on as the muscle contracts.
And the action of the coracobrachialis is to flex, also known as anteversion of the arm, as you also see here indicated by this arrow.
And as you also see here on this slide already, this muscle is able to medially rotate the arm, and in order to show you a bit of the movement, I’m going to go back to this image of the anterior view of the coracobrachialis and the upper arm, where I can show you now this arrow showing how the movement, medial rotation, is going to happen if the coracobrachialis is going to be contracting and helping with this movement.
This muscle is also responsible for adduction of the arm towards your torso, as you see here indicated by this arrow.
And another important function of the coracobrachialis is stabilizing the humeral head within the shoulder joint, especially when your arm is hanging freely on your side.
We’re now ready to move on to the next muscle that we saw on that list, now seen here highlighted in green. Do you remember? One second.
Yes, this one is going to be, then, the deltoid muscle.
This muscle, especially if you do bodybuilding, this is the muscle that you have to work out if you want to form some round contour of your shoulder. So this is the muscle that actually defines the contour of your shoulder.
In terms of origin points—yes, I said “origin points” because there are several origin points that we can distinguish here on this muscle, on the deltoid. There are three distinct sets of fibers that we usually call them also heads, and one of them known as the anterior fibers (also known as the clavicular fibers).
They’re going to originate, as the name indicates, from the clavicle. And you can see here on this image of the anterior view of the deltoid muscle.
Another set of fibers will be, then, known as the lateral fibers or known as the acromial fibers that will be, then, originating from the acromion of the scapula.
And as we go to the posterior fibers to the posterior part of the muscle, we’re going to see the spinal fibers or the spinal part of the deltoid muscle that will originate from the spine of the scapula.
Now, all these heads, all these fibers will converge toward their insertion point on the deltoid tuberosity on the middle of the lateral aspect of the shaft of the humerus.
As you can see here, all these heads are then converging and then inserting distally at the deltoid tuberosity.
Now, a word on the innervation of the deltoid muscle. Now, this is an easy one. All you need to know is that the deltoid is going to be innervated by this nerve seen here highlighted in green, the axillary nerve.
A word on the different functions or actions associated to the deltoid muscle. Now, the different parts of this muscle are able to perform various functions, and you can think of many functions that are associated to the different heads of the deltoid muscle.
So the deltoid muscle is responsible for a lot of the movements on the shoulder.
Now, I’m going to list a few of the main functions. So the anterior fibers are going to be able to perform shoulder abduction when the shoulder is externally rotated.
And you can see shoulder abduction here on this image. So this is when you move your arm up to the side. You can also see here this arrow indicating a bit of shoulder abduction.
Now, the other set of fibers are going to be the lateral fibers, which are responsible for also shoulder abduction (when the shoulder is now internally rotating).
And finally we’re going to be seeing the posterior fibers which are responsible for extension or retroversion of the upper arm at the shoulder joint.
I would like to add a bit of clinical anatomy associated to the deltoid muscle. And one point worth mentioning is axillary nerve injury, which affected patients, they typically show clinical picture of an atrophic shoulder in comparison to the healthy side.
So you’re going to be seeing, then, paralysis of the deltoid muscle, which eventually leads to an atrophic shoulder.
Another classic sign that you can see here is the acromion standing out prominently. This is what we call scaphoid sign.
Even though the limitations of movement are partially compensated by the other shoulder muscles, abduction and external rotation become tremendously difficult in these patients.
We’re ready to move on to the next muscle that you’ll see here highlighted in green. Now, we’re looking at a posterior view of the teres major.
Now, the teres major lies above the teres minor muscle and goes from the shoulder to the humerus as you can clearly see here on this image.
And unlike the teres minor muscle, the teres major does not attach to the capsule of the glenohumeral joint and therefore is not regarded as part of the rotator cuff.
Now, the teres major is going to be originating from the inferior angle of the scapula and also a bit here, as you can see, on the medial border of the scapula.
The muscle goes all the way to, then, inserting at the crest of the lesser tubercle of the humerus. It does so because it goes and runs cranially and also laterally, then inserts at the crest of the lesser tubercle.
Now, we’re going to move on and talk about the innervation of the teres major. And the teres major is innervated by the subscapular nerve coming from the brachial plexus.
Now, a bit on the different functions associated to the teres major. Again, we’re showing here the anterior view of the shoulder and the teres major highlighted in green.
Now, the teres major causes three movements of the shoulder joint. Due to its insertion point at the anterior side of the humerus, it allows then internal rotation as you see here indicated by this arrow.
In addition, this muscle is able to perform, then, adduction as you can see here indicated by this arrow, so when you bring your arm closer to your torso. And also retroversion, so when you move your arm towards your back.
And in case of a fixed humerus, the contraction of this muscle will lead to craniolateral movement of the inferior angle of the scapula. So in other words, this part of the scapula is going to be able to, then, move up and to the lateral side when the muscle will be contracting.
Now, I would like to add a few words here on the teres major to say that…
And just before I continue, I just want to explain what we’re seeing here on the screen. We see the different rotator cuff muscles here, and the teres major that we just talked about, and another muscle here, a muscle of the arm known as the triceps brachii.
We’re looking at them from a posterior view.
I wanted to use this image to say that the teres major muscle plays a role for two anatomical spaces that are frequently asked in your exams. Now, these are known as the triangular space and also the quadrangular space.
So these spaces that you see here, they are formed by all these muscles. These two spaces are, then, important spaces, anatomical spaces where different structures are going to be passing through.
So as I just mentioned, they are gaps in the axillary region where other structures run through.
Now, a few words on the triangular space, it has the following boundaries: the teres major, and laterally, you’re going to see then the long head of the triceps as you can see also here. This is the long head of the triceps.
Now, cranially, you’re going to see the boundary or this border here which is going to be, then, the teres minor.
Now, the triangular space contains the scapular circumflex vessels.
Another space that I talked about was, then, the quadrangular space, and the borders will be then, caudally, the teres major as you can see here. We’re going to be seeing, then, laterally, the bone, the humerus forming also a border.
Medially, we’re going to be seeing then the long head of the triceps being part of this border of the borders of the quadrangular space. And on the cranial border, we’re going to be seeing this muscle here that we talked about, the teres minor.
The quadrangular space contains the axillary nerve and the posterior humeral circumflex artery.
We’re now ready to move on to the next muscle here, a large one known as the latissimus dorsi. This is the widest muscle in the human body while being relatively thin and covering almost all back muscles at the posterior trunk.
An important note here that, together with the teres major, it forms the posterior axillary fold in your armpit.
Now, let’s talk about the different origin points for the latissimus dorsi. It has a lot of origin points, including the spinous processes of T7 to T12, so the spinous processes of the thoracic vertebrae, as you can see here.
Another point of origin will be the iliac crest, as you can see here.
Now, some ribs will serve as origin points for the latissimus dorsi including the ninth all the way to the 12th rib.
The thoracolumbar fascia which is found right about here is also an origin point for the latissimus dorsi and, sometimes, the inferior angle of the scapula is also considered as an origin point for the latissimus dorsi, and you can see that it’s hidden behind the muscle on this image.
And also one reminder here that we’re looking at the latissimus dorsi from a posterior view.
The muscle will go, then, all the way or all the fibers together have an insertion point at the crest of the lesser tubercle, as you can see here, of the humerus on both bones.
The innervation of the latissimus dorsi is going to be carried by the thoracodorsal nerve, a branch of the brachial plexus, and you can see here highlighted in green on this image of the posterior view of the shoulder, with some nerves here of the brachial plexus, and you notice here and highlighted in green the thoracodorsal nerve.
Now, the latissimus dorsi is able to perform different functions. As you would assume, this is a big muscle. But we’re going to be focusing on those functions that are associated to… or movements associated to the shoulder.
When the latissimus dorsi contracts, it can lead to internal rotation of the humerus or internal rotation of the arm as you can see here indicated by this arrow.
In addition, it will pull the humerus towards the trunk, which is known as then adduction, and you can see here indicated by these two arrows.
It also pulls your arm or your humerus to the back, which is known as retroversion, and you see here a representation of this movement on this arrow.
Now, 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. You could really use this muscle to be a good swimmer.
Now, another important function of the muscle is to support breathing.
The activation of the latissimus dorsi muscle on both sides causes a compression of your rib cage, which then facilitates expiration. So you can say that this is an accessory muscle of expiration (so when you breathe out).
For this reason, the muscle is usually strained during coughing attacks. We can also call it the coughing muscle.
A little bit of clinical anatomy associated to the latissimus dorsi is to say that this muscle is often used for surgical reconstruction of the anterior trunk.
One example, so after a mastectomy or larger injuries of the breast, which in that case, it is removed from its origin and implanted at the anterior trunk wall together with skin, subcutaneous fat tissue, and an artificial implant if necessary.
This is what we call the pedicled latissimus dorsi musculocutaneous flap, which is one of the most common methods in breast reconstruction worldwide.
We’re now ready to move on to the next muscle that you see here highlighted, now from an anterior view of the chest. Yes, this is the pectoralis major.
The pectoralis major shapes the surface anatomy of the chest and participates in forming the anterior wall of the axilla.
As for the origin point of this muscle, there are several that you need to know and sometimes varies from literature, that the muscle has its origin widely spread on the medial part of the clavicle as you can clearly see here on the image, will also use the sternum clearly seen here as an origin point, and also the second all the way through the sixth costal cartilages will serve as an origin point for this muscle.
And sometimes, it may vary a bit in literature. You can say that the seventh costal cartilage will also serve as an origin point for the pectoralis major.
Now, the anterior layer of the rectus sheath will serve as an origin point also for the pectoralis major.
Now, the insertion point for this muscle is a bit easier to remember because all fibers insert together at the crest of the greater tubercle of the humerus. Notice here the greater tubercle of the humerus, now the crest of the greater tubercle where the muscle will be inserting.
And this insertion point is located at the proximal humeral shaft.
When it comes to the innervation of the pectoralis major, the innervation is carried out by two nerves: the medial pectoral nerve as you see here highlighted in green and the lateral pectoral nerve which are direct branches of the brachial plexus.
And notice here, we’re looking at it, at these two nerves from an anterior view on both of these images. Notice here, if you remember well, yes, this is the brachial plexus and the nerves just branching out of this plexus.
Now, as you would expect, we’re going to briefly talk about the different functions or actions associated to the pectoralis major, most important muscle for adduction and also anteversion of the shoulder joints. Don’t forget that.
So adduction, indicated here by this arrow (when you pull your arm towards your torso). And anteversion is when you move your arm towards the front of your body, and you see here indicated by this arrow.
Now, the pectoralis major is also able to perform internal rotation, which you see here indicated by this arrow.
If the arms are fixed, the muscle lifts the trunk, which can be helpful in climbing or during inspiration. So we can also consider it as an inspiratory breathing muscle.
Now, I would like to add a few words on clinical, related clinical anatomy to the pectoralis major and would like to briefly talk about aplasias of the pectoralis major muscle, which rank among the most common muscular malformations.
In Poland syndrome, we’re going to see that the development of the breast wall remains incomplete during embryogenesis. And for that reason, the chest muscle may be partially or completely missing.
Now, we’re going to move on to this group of muscles that you see here from a posterior view. We’re going to be talking about the rotator cuff. And the rotator cuff consists of the supraspinatus muscle, the infraspinatus, the teres minor, and the subscapularis.
Let’s start with the very first one that you see here highlighted in green. We just added another couple of muscles here that we talked about before, the teres major and also the triceps, but highlighted, we have the supraspinatus muscle,
Which is a relatively small muscle of the upper back and is one of the four rotator cuff muscle, and the origin point for the supraspinatus will be, then, the supraspinous fossa of the scapula, which is a shallow depression in the body of the scapula above its spine.
So this is the spine of the scapula, and this is going to be, then, the supraspinous fossa where the supraspinatus will be originating from.
Now, the muscle runs from the supraspinous fossa to, then, insert to the greater tubercle of the humerus as you can see here on the image.
Now, the supraspinatus muscle is going to be innervated by the suprascapular nerve as you see here highlighted in green, which arises from the superior trunk of the brachial plexus.
A bit on the different functions or actions associated to the supraspinatus.
A contraction of the supraspinatus will lead to abduction of your arm, as you see here represented on this image. So when you move your arm away from your torso. So abduction of the arm at the shoulder joint.
And the supraspinatus also helps stabilize the shoulder joint by keeping the head of the humerus firmly pressed medially against the glenoid fossa of the scapula.
And you can actually see how the humerus is or the head of the humerus is now forming this joint here with the glenoid cavity of the scapula and how this muscle even helps it hold these two together.
We’re now ready to move on to the next muscle that you see here highlighted in green, which is the infraspinatus. This one is a thick, triangular muscle which occupies the main part of the infraspinous fossa.
In terms of origin point for the infraspinatus, I already mentioned, yes, the infraspinous fossa of the scapula (just below the spine of the scapula) will serve as the origin point for the infraspinatus.
And like we’ve seen with the supraspinatus, the infraspinatus will go laterally to insert at the middle facet of the greater tubercle of the humerus as you can see here.
The innervation of the infraspinatus is going to be provided by this nerve here as well, the suprascapular nerve. So remember that both the supraspinatus and the infraspinatus muscles are going to be innervated by the suprascapular nerve.
A little bit on the different functions or actions associated to the infraspinatus, the infraspinatus is the main external rotator of the shoulder, which you see here represented by this arrow.
And additionally, the infraspinatus reinforces the capsule of the shoulder joint and also assists in both abduction and adduction, as you see here indicated by these arrows.
So abduction is when you extend your arm to the side, away from your torso, and adduction is the opposite. So you bring your arm closer to your torso.
We’re ready to move on to the next muscle that you see here highlighted in green. Yes, this one is going to be, then, the teres minor. The teres minor is a narrow, elongated muscle of the rotator cuff.
It arises from the dorsal surface of the axillary border of the scapula (also known as the lateral border of the scapula) for the upper two-thirds of its extent, and from the two aponeurotic laminae, one of which separates it from the infraspinatus muscle, the other from the teres major muscle.
Now, the fibers of the teres minor muscle run obliquely upward and laterally. The upper ones end in a tendon which inserted… which is inserted into the lowest of the three impressions of the greater tubercle of the humerus, as you can see here.
Now, the lowest fibers of this muscle are inserted directly into the humerus, immediately below this impression.
Now, the muscle is going to be innervated by the posterior branch of the axillary nerve, which you see here highlighted in green, where it forms the pseudoganglion.
The pseudoganglion has no nerve cells, but nerve fibers are present here.
A little bit on the functions associated to the teres minor. Now, the teres minor muscle functions consist primarily of external rotation, which you see here indicated by this arrow and partially retroversion (so when you move your arm to the back, towards your back) and also adduction, which you see here indicated by this arrow (bringing your arm closer to your body).
We’re now ready to move on to the next muscle, the subscapularis, seen here highlighted in green.
Now, the subscapularis is a large, triangular muscle of the rotator cuff. And notice here that this is the only muscle that you can see from an anterior view. We’ve seen all the other three from a posterior view. Right now, we’re looking at the shoulder joint and at the subscapularis from an anterior view.
Now, this muscle arises from its medial two-thirds and from the lower two-thirds of the groove of the subscapular fossa of the scapula of course. And this would be the subscapular fossa where the subscapularis is, then, originating from.
Now, the muscle goes all the way to, then, insert at the lesser tubercle of the humerus as you can see here.
When it comes to the innervation of the subscapularis muscle, this muscle is innervated by the upper and lower subscapular nerves, which are branches of the posterior cord of the brachial plexus.
And notice here the posterior cord of the brachial plexus where the two subscapular nerves are then branching off from.
As you’re expecting, now, we’re going to move on to talk about the different functions and actions associated to the subscapularis.
The subscapularis is responsible for internal rotation of the head of the humerus, as you see here indicated by this arrow. And when your arm is raised, it draws the humerus forward and also downward.
This is an important muscle as you can see here because it’s going to be able to stabilize the front of the shoulder joint by preventing displacement of the head of the humerus, as you could probably guess by just noticing how it is placed over the shoulder joint.
This muscle also supports the upper arm during abduction and adduction, as you can see here indicated by these arrows.