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Origins, insertions, innervation and functions of the superficial extensors of the forearm.
Hello everyone! This is Joao from Kenhub, and welcome to another tutorial. This time, I’m going to be talking about the superficial extensors of the forearm. As you probably guessed, this is a tutorial that will be dealing with the muscles that you find also on your forearm, but more specifically on the posterior side of your forearm, as you can see here on this image.
Now, keep in mind that we have here a collection of all the muscles that you can find on your forearm, on the dorsal side of your forearm, but I'm going to isolate the group that we’re going to be discussing today: the superficial extensors.
Now, as I mentioned, the superficial extensors are a group of three muscles that are found on the posterior side of your forearm. And due to their superficial course, they form the surface of the lateral side of the posterior forearm where they can be easily palpated, especially during hand extension. They are... they all share a common extensor tendon, as you can see right about here, on the lateral epicondyle of the humerus, this bone that you can see right about here. So they share a common tendon on the lateral epicondyle of the humerus, together with the extensor carpi radialis brevis from the radial musculature.
As I’ve mentioned, this is a group of three muscles that we’re going to be covering throughout this tutorial. The first one that I will be talking about is the extensor digitorum, the second one is going to be the extensor digiti minimi, interesting and funny name I must say, and the third one is going to be the extensor carpi ulnaris. So these are the three muscles that we’re going to be talking about and covering throughout this tutorial.
Now, let’s start with the very first muscle here on our list, the extensor digitorum. The extensor digitorum muscle, as the name indicates, is responsible for extending the medial four digits of the hand. Now, let’s talk about the origin points of the extensor digitorum and as you can see here, and as I mentioned on the previous slide, the origin point is right on the lateral epicondyle of the humerus.
Now, in terms of origin points, now it gets a bit more interesting here. Distally, as you can see, the muscle goes from the lateral epicondyle, and distally right about here, it’s going to divide into four smaller tendons, which then will insert on the posterior surface of the middle phalanges of your digits, right about here, as well as the distal phalanges of the second all the way through the fifth finger. So middle phalanges and also distal phalanges of the second all the way through the fifth finger, or your index finger all the way through your little finger.
And here, the insertion tendons build a sheet-like tendinous expansion called the dorsal aponeurosis covering the medial posterior and lateral surfaces of the distal phalanges. Now, the dorsal aponeurosis of the second through fifth digits or fingers serves as a place of insertion for various ligaments and muscles.
Now, let’s move on to the second muscle on our list, the extensor digiti minimi. Now, this is a slender muscle and is found on the ulnar side of the extensor digitorum muscle—so almost in line with your little finger as you can see here on this image. And its muscle belly sometimes blends with the extensor digitorum muscle.
Now, let’s start talking about the origin point of the extensor digiti minimi, and as I mentioned before, that it will come from this common tendon that is originating from the lateral epicondyle of the humerus.
Now in terms of insertion point, now, the thing is that this muscle is going to be inserting, as you can see here, in line with or somewhere on your little finger or your fifth digit—so on the dorsal aponeurosis of the little finger.
Now let’s move on from this muscle here with the funny name to the third one on our list: the extensor carpi ulnaris. Now, the extensor carpi ulnaris is located at the ulnar side of the forearm as you can see here, also in line with you fifth finger or the little finger. It acts to extend and also adduct at the wrist.
Now in terms of origin points, we’re going to look at two origin points here for this muscle specifically. One is coming from that common tendon that we talked about on the other muscles that is originating here at the lateral epicondyle of the humerus. So we will say that this is a humeral head for the extensor carpi ulnaris. So there is a second head that is coming right about here, an origin point that is happening on the posterior surface of the ulna. For that reason, we also say that the extensor carpi ulnaris has an ulnar head.
In terms of insertion points, it’s important to mention that there is one, and this muscle’s insertion is located at the base of the fifth metacarpal bone, as you can see right about here. So the base of the fifth metacarpal bone is serving as our insertion point for the extensor carpi ulnaris.
Now, we’ve been covering the origins, insertions, it is time to move on to the innervation of the superficial extensors of the forearm. Now, very easy here because when they ask you about what are or what is the innervation of the superficial extensors of the forearm, all you need to know is that all three superficial extensors will be innervated by only one nerve, and this is the radial nerve—very easy.
Now, one thing that I would like to add is that the radial nerve has a superficial branch as well as a deep branch at the height of the radial head. While the superficial branch continues along the brachioradialis muscle, as seen here, highlighted in green, the deep branch will penetrate the supinator muscle after which it branches off as the posterior interosseous nerve. There, the posterior interosseous nerve supplies, then, the superficial extensors with motor branches. Now, this is some additional information that you need to know when you learn the innervation of the superficial extensors of the forearm.