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Origins, insertions, innervation and functions of the superficial flexors of the forearm.
Hello everyone! This is Joao from Kenhub. Welcome to another tutorial! This time, we’re going to be learning about the superficial flexors of the forearm. Without further ado, let’s start with our first image here where you can see on a superficial layer the superficial flexors of the forearm.
Now, this is a group of five muscles that you can find on the anterior portion of the forearm, so this image that you’re seeing right now is the anterior view of the forearm. And they form the surface of the ulnar side of the forearm where they can also be easily palpated or felt, especially if you flex them, as the name indicates.
Now, their long tendons can be easily followed, especially if you look at your wrist and you flex it. You’re going to see a few bumps, and these bumps are the tendons of some of these muscles. All five muscles arrive from the common tendon of the—located at the medial epicondyle as you can see here, medial epicondyle of the humerus, and some of them are also originating from other points on the radius and ulna. We’re going to be seeing here on this tutorial all the origins, insertion points of these muscles, as well as their innervation.
Now, in terms of the list of these muscles that we’re going to be talking about throughout this tutorial consists of the pronator teres, the flexor carpi radialis muscle, the flexor carpi ulnaris, the palmaris longus muscle, and the flexor digitorum superficialis muscle.
Now, let’s move on and talk about the very first one on our list, the pronator teres, and as you can see here on this image, especially the one on the left, this muscle is the most lateral of all superficial flexors.
Now, in terms of origins, you need to remember that this muscle has two origin points. In other words, we can say that it has two heads. Two origin points equals two heads. And one head is coming from the humerus, and the other one is coming from the ulna. So you have a humeral head and an ulnar head. Now, the humeral head comes from the medial epicondyle of the humerus, while the ulnar head comes from the coronoid process of the ulna.
In terms of insertion points, it’s a bit easier because you only need to remember one, and this muscle courses all the way from those two heads, under the brachioradialis—write this down. It’s important for your exam—and it will attach to the lateral side of the radius. In other words, the lateral surface of the radius.
Now, this image might be slightly confusing because this muscle, right now, is performing its function. So it’s rotating, somehow, the radius in a way that, now, looks like it’s the medial perspective, it’s the medial surface right here, but this is in fact the lateral surface of the radius, and this is where the pronator teres is going to be attaching.
In terms of innervation of the pronator teres, this muscle is going to be supplied by the median nerve C6 and C7.
Moving on to the second one on our list, the flexor carpi radialis. As you can see, this muscle is found a bit more laterally than the pronator teres, and in terms of its origin, it has one origin point that you need to remember. It comes from the common flexor tendon of the humerus located on the medial epicondyle of this bone.
In terms of insertion points, it’s going to go all the way to the bases of the second metacarpal bone, and sometimes, the third metacarpal bone as well. And you can clearly see here that this example is on both or attaching to both these bones.
In terms of the innervation of the flexor carpi radialis, this muscle is also going to be supplied by the median nerve C6 and C7.
Now, moving on to the other muscle on our list, the flexor carpi ulnaris. This muscle is the most medial of all superficial flexors and is mainly responsible for the contour of the ulnar side of the forearm which you can see here on both of these images. By ulnar side, you can also say as the medial side of the forearm.
In terms of origin points, we also have to consider two heads, two origin points: one coming from the humerus, and the other head coming from the ulnar—so a humeral head and an ulnar head. The humeral head is originating from the medial epicondyle of the humerus, while the ulnar head is coming from the olecranon of the ulna.
In terms of insertion points, you need to think about three insertion points on this one, so it’s a bit tricky. But the tendon uses the pisiform bone as a sesamoid bone towards its insertion at the hook of the hamate or the hamate bone and the base of the fifth metacarpal bone. As you can clearly see here on this portion of the image, the three insertion points of the flexor carpi ulnaris.
In terms of innervation of the muscle, this muscle is going to be supplied by a different nerve than the other two that we looked at, and this time, it’s going to be the ulnar nerve from C7 to Th1.
As the fourth muscle on our list, we have the palmaris longus muscle. This is a very slender muscle. It is variable and can be, in some cases, missing on one or both arms and have an alternative course sometimes. In terms of its origin, it has one that you need to remember, and it’s coming from the common flexor tendon, more specifically on a bony portion of the humerus known as the medial epicondyle of the humerus which you can see right here.
In terms of its insertion, you need to remember that this muscle is going to be inserting on two insertion points. One is the flexor retinaculum. The other one is the palmar aponeurosis.
In terms of the innervation of the muscle, this muscle is going to also be supplied by the median nerve C7 to Th1.
Last on our list is this muscle here known as the flexor digitorum superficialis, and this is the deepest muscle of all superficial muscles. And the long tendon, as you can see here on this images, this muscle has a really long tendon which runs through the carpal tunnel, right about here. As you can see, this is where the carpal tunnel is supposed to be, and the tendon of this muscle is clearly passing through.
Now, in terms of its origin, this muscle has, this time, three heads because it’s quite large as you can see here. And one head has its large origin point attached to the medial epicondyle, which is the humeral head. The coronoid process of the ulna, which is the ulnar head. And the third head is going to be originating from the anterior surface of the shaft of the radius distally from the radial tuberosity, and this is what we call the radial head of the flexor digitorum superficialis muscle.
In terms of its insertion point, the muscle has four insertion tendons which will split into two smaller end tendons—each of which then insert on both sides of the middle phalanges of the second to fifth fingers. And you can clearly see here on this image the tendons inserting on both sides of the middle phalanges of the second to fifth fingers.
In terms of innervation of the muscle, this muscle, as most superficial flexors, will be supplied by the median nerve C7 to Th1.
Before we finish this tutorial, I wanted to do a reminder on the innervation of the superficial flexors of the forearm. What I wanted to add is that, as you’ve seen throughout this tutorial, that all superficial flexors of the forearm are supplied by the median nerve that you can see here on this image, highlighted in green right here. Now, there is an exception, and if you remember well, this exception is the flexor carpi ulnaris which is then supplied by the ulnar nerve.