Origins, insertions, innervation and functions of the muscles of the arm.
Hello, hello everyone! This is Joao from Kenhub. Welcome to another tutorial. I’m so glad you joined us because we’re going to be talking, on this tutorial, about the muscles of the arm, as you can notice right now on your screen.
Now, according to their position and also location within the upper limb, we can divide two types of muscle, and you can see them on your screen right now. One, the one you see on your left, is found a bit more distally on the upper limb, and we call it—you probably have guessed it already—these are the forearm muscles. On the other hand, you have on your right side an image showing part of the muscles that we’re going to be talking about today on this tutorial, and these are the arm muscles, which are found a bit more proximally on the limb.
Now, as I mentioned on this tutorial, we will be focusing on the arm muscles which are positioned within the arm region, within the upper arm region, more specifically around the humerus. Now, we’re going to be focusing on origin and insertion, and also innervation of these muscles. And we’re not going to define them or include in this group specifically based on their function, because if we look at the function of muscles that work on movements of the arm, of course, we could have a larger list. But for this reason, for the fact that we’re just going to focus on location, then we’re going to define four muscles. And the four muscles are the biceps brachii, the triceps brachii, the brachialis, and finally we’re going to look at a very small muscle but needs some attention as well known as the anconeus muscle.
Now, let’s talk about the very first muscle here on our list: the biceps brachii seen here, highlighted in green. When we talk about the surface anatomy of the anterior side of the upper arm, this is essentially defined or formed by the biceps because the biceps is a large and thick muscle that definitely defines the anterior portion of the upper arm.
Now, also important to mention that it has two origin points or two heads—one that is known as the long head, and of course, the opposite would be a short head. So we have two origin points for the biceps brachii that we’re going to look at right after. Also, important to mention that these origin points are covered right here by the deltoid, so it’s really hard to feel them on your body or even impossible because the deltoid is covering them. But right here at the crook of your arm, you have the insertion point of the biceps, which you can feel especially when the muscle is contracted.
Let’s start by covering the origin points of the long head of the biceps. Now, there is only one origin that you need to remember, and this is the supraglenoid tubercle of the scapula. Now, this is above the socket of the scapula, and it lies within the interscapular space but still being extrasynovial. Now, this long tendon, then, will make a sharp turn here on the humeral head and continues its course right on the bicipital groove of the humerus. This is also known as the intertubercular sulcus. Now, this turning point is secured by ligaments at the capsular area and also known as the biceps pulley.
Now, let’s quickly have a look at the origin point of the short head of the biceps. Now, in terms of origin, you need to remember one, and this is the coracoid process of the scapula right about here. Also, it’s important to mention that this is where it partially blends or this muscle’s tendon is going to partially blend with the origin tendon of the coracobrachialis, another muscle that will be originating from the coracoid process.
Now, let’s talk about the insertion points of the biceps brachii. Well, as you notice, the long head and also the short head of this muscle will unite into this larger belly, as you can see here, which will then have a common tendon that inserts on the, what is known here as the radial tuberosity. So this is the bony point for insertion of the biceps brachii. But keep in mind that there is a fibrous membrane that emerges from the distal part of the muscle. This portion right here. You can see it. And this is known as the bicipital aponeurosis which inserts at the deep faschia of the forearm. So this is also another insertion point that you can consider for the biceps brachii.
Now, the last topic that we need to cover about the biceps brachii, what you need to know is about the innervation of this muscle. Now, and in terms of innervation, this muscle is supplied by the musculocutaneous nerve, C5 to C6, which is a branch of the brachial plexus.
Now, moving on to the second muscle on our list: the triceps brachii, which is a large three-headed muscle—very important. It has three origin points which you find also on the upper arm. But this time, we’re looking at the dorsal portion of the upper arm, because due to its superficial course, the triceps determines the contour of the dorsal upper arm. Now, these three heads that we’re going to talk about the origin points are the long head, as you can see here. You can also find a medial head right about here, and then a third head, the lateral head.
Now, I want to give you a quick note on the location and topography of the triceps within the upper arm and also the shoulder. And you can see that the proximal part of the triceps is partially covered by the deltoid, and you can clearly see, especially here on this image of the dorsal view of the deltoid or of these muscles. This is the deltoid. Right here you have, of course, the triceps. Notice that these heads are covered by this large muscle that defines your shoulder, which is the deltoid.
Now, both the lateral and long heads can be easily palpated. As you notice here, you can easily palpate these two heads on the back of your arm. But the medial head is a bit hidden here below these two larger heads and also the deltoid. So you cannot feel the medial head of the triceps.
Now, moving on and talking about the long head of the triceps, we’re going to look at the origin point of the long head. This is one. You can see here this head highlighted in green. Notice here, this is the origin point. And if you can guess it right, yes, it’s the infraglenoid tubercle of the scapula. And then from there, it courses between the teres major and minor and divides the axillary space into two halves.
Now, let’s move on to the medial head of the triceps seen here, highlighted in green on both of these images. Now, in terms of origin points, we need to remember one. And as you notice here, we’re looking at the dorsal view of the humerus. So the medial head is originating from the dorsal portion of the humerus shaft—or humeral shaft, or humeral body, distally from the radial sulcus—very important.
Now its fibers are connected with the medial intermuscular septum, also, another important note for your information.
Now, let’s move on to the last head of the triceps brachii. The lateral head seen also here, highlighted in green on both of these images. In terms of its origin point, it also has one. And similar to the medial head, we’re looking at the dorsal view of the humeral shaft or humeral body. And this is where the lateral head will be originating from, but this time proximally from the radial sulcus where it is fixed to the lateral intermuscular septum.
Now that we covered the three origin points for the triceps, it is time to logically talk about the insertion point. And as you see, the three heads are coming together to form this really thick tendon that will go and insert here on this bone. If you can guess it well, yes, this is the ulna, more specifically on the olecranon of the ulna. Now, it additionally will attach to the capsule of the elbow joint and also the antebrachial faschia.
Lastly, we’re going to be talking about the innervation of the triceps. Now, the triceps is supplied by the radial nerve which is in close relationship with this muscle, specifically in terms of course. So first, this nerve will run within the radial sulcus of the humerus, where it is covered by both the medial and lateral heads of the triceps. Then from there, it courses through the crook of the arm along the lateral bicipital groove between the triceps and biceps.
Now, it is time for us to move on to the third muscle on our list, the brachialis. Now, the brachialis is a relatively long and strong muscle found on the anterior portion of the upper arm. Now, for the most part, the brachialis lies under the biceps brachii. So if we look at this image right here, the biceps, as we talked about before, the two heads connecting here to form this belly, this large belly of the biceps, and right underneath, you find the brachialis a little bit shy here, but you can still see it.
Now, that is why it’s not easy to palpate the brachialis from the surface. The muscle barely has superficial parts which you can find on the lateral border and also distally. And even though it is located deep in the upper arm, the brachialis still contributes indirectly to the surface anatomy as its large belly makes the biceps brachii look much larger on the surface than it actually is. So for that reason, we have to say that behind every great biceps, there is a great brachialis.
Now, let me remove here the biceps brachii for you, exposing the brachialis and highlighting it in green. And also here another image so you can have another perspective of the brachialis, so we can start talking about the origin points for this muscle. Now, you can notice that the origin point happens right about here. You notice here on this image. And this is the distal half of the anterior side of the humerus or the shaft of the humerus. You can also call it the body of the humerus. And in addition the two, or the origin tendon attaches to the medial and lateral intermuscular septa of the arm which are two dividing membranes separating the flexor muscles from the extensor muscles.
Now, let’s talk about the insertion point of the brachialis. There is one that you need to remember that happens right about here. You can notice right here the bony insertion point. And it happens distally where the muscle inserts at the tuberosity of the ulna where its fibers also connected to the joint capsule.
In terms of innervation of the brachialis the nerve comes… or the nerve supply comes from the musculocutaneous nerve, C-5 to C-7. However in 70-80% of the people the muscle has, what is called as double innervation with the radial nerve.
Moving on to the last muscle on our list, this is the anconeus muscle, which is a small triangular-shaped muscle that you find around the elbow region. And the muscle lies superficially and can be easily palpated at the dorsal lateral side of the forearm near your elbow.
Now, in terms of the origin points for the anconeus muscle, it originates at the dorsal side of the lateral epicondyle of the humerus as you can see here clearly. This is the origin point of the anconeus muscle. The fibers of the origin tendon are further attached to the dorsal joint capsule.
Moving on to the insertion point for the anconeus muscle, this will happen on the olecranon of the ulna, right about here as you can also see on this image.
Moving on to the innervation of the anconeus. Now, the anconeus is supplied by the motor branch of the radial nerve. It arises at the radial sulcus of the humerus and then continues through the medial head of the triceps and finally reaches the muscle distally.
So I would like to complete this tutorial with an important note that you may add to your own notes about the anconeus and the triceps. Now, both morphologically and functionally, these muscles not only are innervated by the same nerve, but they are also very often found either partially or completely blended together. So sometimes, we may consider the triceps and the anconeus muscle as one.
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