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Origins, insertions, innervation and functions of the deep 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 another group of muscles. This time it’s the deep extensors of your forearm.
Now, let me do a quick review and give you an idea of how the dorsal part of the posterior part of your forearm looks like in terms of musculature. It looks something like this. And if we were to remove or strip out that superficial layer that you find there, we can then find the group of muscles that we’re going to be talking about today: the deep extensors of the forearm.
Now, keep in mind that this is, and as I mentioned, this is a group of muscles that can clearly be seen on the posterior side of the forearm, and they are five muscles—and five muscles that we’re going to be covering throughout this entire tutorial. Now, their bellies and tendons form the surface of the distal forearm and wrist where they can be easily palpated, especially this group right here that I was pointing at, and we’re going to name all of these muscles.
Now, in terms of the list that we’re going to be covering, we’re going to be covering the supinator, also talking about the abductor pollicis longus, then we’re going to talk about the extensor pollicis brevis, then the extensor pollicis longus, and we’re going to end up this tutorial talking about the extensor indicis.
And without further ado, let’s move on to the very first one here on our list, the supinator muscle that is a relatively broad muscle which is curved around the upper thirds of the radius as you can see here on both of these images where we highlighted the muscle in green. It functions to supinate. As name indicates, it supinates the arm.
Now, let’s talk about the origin points of the supinator, and keep in mind this is a relatively complex topic, because between references, we end up finding a lot of differences, but we try to keep it simple and connect the dots here. So whatever you’re learning this from or whatever school you’re coming from, you can apply this knowledge.
Now, as I mentioned, this muscle has a complex origin tendon which one part will arise from the lateral epicondyle of the humerus, what we call a superficial part, and then another part, a deep part, will then come from the proximal ulna on the olecranon. Now, it also has two other origin points that I would like to connect here as well and talk about briefly. One of them is the radial collateral ligament, and another origin point that is also commonly seen between the references is the radial annular ligament.
Now that we looked at the origin points of the supinator, it’s time to then move on to the insertion points of this muscle. And from the origin point, the muscle will then curl dorsally around the neck of the radius and inserts at the posterior and lateral surface of the radius, between the radial tuberosity and insertion of the pronator teres. So this is important information that I believe you should write down and collect in your notes, so when you go into that exam, you know exactly what we’re talking about here, about the supinator muscle.
Moving on from the supinator muscle to the second one on our list, the abductor pollicis longus. Now, the abductor pollicis longus is also known as one of the extrinsic muscles of the hand. And as the name implies, its major function is to abduct the thumb at the wrist. And you can also see here on this image how it will function such matter based on its insertion point that we’re going to talk about next.
Now, in terms of origin points, we’re going to look at three origin points that need to be learned related to the abductor pollicis longus. The first one is going to happen on the posterior surface of the radius, this bone right about here. And you’ll notice that the abductor pollicis longus will be arising from the radius. Now, the second origin point is going to be the posterior surface of the ulna, as you can also see here on this image. And the third origin point is known as the interosseous membrane—so this membrane that should be found between these two bones, the radius and the ulna, and where this muscle will be also arising from.
Moving on to the insertion points of the abductor pollicis longus, there is one origin point or one insertion point that you need to remember, and this is the base of the first metacarpal bone, as you can see right about here. So this base of the thumb or the metacarpal of the thumb will serve as the insertion point of the abductor pollicis longus.
Now, let’s move on to the next muscle on our list. This is the extensor pollicis brevis as you can see here on this image. And this muscle lies on the medial side of and also closely connected to the other muscle that we’ve talked about before, the abductor pollicis longus.
Now, in terms of origin points, for the pollicis brevis, we need to remember two origin points for this specific muscle. One that you can see here on this image, one is coming from this bone. If you guessed well, yes, this is the radius, and the extensor pollicis brevis will be originating from the posterior surface of the radius. Another origin point comes from this membrane that is attached between the radius and the ulna. This interosseous membrane will serve also as an origin point for the extensor pollicis brevis.
Next point that we need to make about the extensor pollicis brevis is the insertion point. Yes, I said “point” because there is one that you need to remember and you can also see here on this image that the muscle is inserting on the base of the proximal phalanx of the thumb or the base of the first proximal phalanx. It is also important to mention that, often, the insertion tendon splits into two and additionally attaches to the trapezium.
Now that we completed the extensor pollicis brevis, it is time to talk about its cousin, the extensor pollicis longus. And this muscle is much larger than the extensor pollicis brevis, and also, its origin is partially covering the previous muscle that we talked about, the extensor pollicis brevis. The extensor pollicis longus also acts to stretch the thumb together with the extensor pollicis brevis.
Now, in terms of origin points, there are two origin points that you need to also remember for this muscle. One is coming from the opposing bone here. The posterior surface of the ulna is serving as an origin point for the extensor pollicis longus. And as we’ve seen on the pollicis brevis, we’re also seeing an origin point happening on the interosseous membrane for the extensor pollicis longus.
Moving on to the insertion points of the extensor pollicis longus, this muscle is going to go all the way to insert, now, on the distal phalanx of the thumb. So it’s going to insert more specifically at the base of the first distal phalanx. Still on the insertion of the extensor pollicis longus, I would like to add a few points that, at its insertion point, it forms a dorsal aponeurosis, which covers the thumb medially, laterally, and also dorsally. This sheet-like tendinous expansion serves mainly as an insertion point for other muscles and ligaments similar to what we see on the dorsal aponeurosis of the extensor digitorum.
It is time for us to move on to the last—last— muscle here on our list for this turtorial, the extensor indicis. And this is a narrow and also elongated muscle found medially to and also parallel with the muscle that we talked about on the previous slides, the extensor pollicis longus.
Now, look at the name, extensor indicis says it all. So word origin really helps you understand a lot about anatomy in general and especially when we talk about muscles. So the tendon of the extensor indicis goes to your index finger which then will help extend it. So that’s why we call it extensor indicis. So no rocket science here—very simple and straight forward.
Now, in terms of origin points, we need to remember two as well for this muscle. One, you can see here on this image, and this is the posterior surface of the ulna which will serve as an origin point for the extensor indicis. Another one is seen on other muscles as well, the interosseous membrane, which will also serve as an origin point for this muscle.
Now, let’s talk about the insertions of... or insertion points for the extensor indicis. And, yes, there is one that you need to remember; and this is the dorsal aponeuorosis of the index finger or the second finger as you can see here on this image as well.
Throughout this tutorial we covered the origin points, the insertion points for all the deep extensors of the forearm. Now, it’s time for us to go over the innervation of these muscles, and all you need to know is that all five deep extensors of the forearm are supplied by this nerve here, seen highlighted in green in this image and here at the center of your screen: the radial nerve.
Now, also, I would like to add that the radial nerve has a superficial branch and, of course, a deep branch. At the height of the radial head, these two branches or the radial nerve will divide into these two branches. Now, while the superficial branch continues along this muscle here, highlighted in green, known as the brachioradialis, the deep branch will then penetrate the supinator muscle and branches off as the interosseous nerve. And the supinator muscle is seen here also, highlighted in green, in this image on your right. Now, this interosseous nerve is then responsible for the innervation of almost all deep extensors. Related to the supinator muscle, it’s important to add that this muscle is the only deep extensor that receives a supply of motor branches directly from the radial nerve.
Now, I would like to move on to the last part of this tutorial and add a topic that I'm just going to briefly discuss on a structure that we hear a lot about, especially during our anatomy classes, known as the anatomical snuff box. This is basically a triangular depression that you find at the radial side of the dorsum of your hand which becomes even more evident, especially when you extend your hand, and as you see here on this image. This is known as the anatomical snuff box, or you can also call it the radial fossa. And I wanted to include it here on this tutorial because some of the muscles that we talked about today will also play a role in defining this special, let’s say, structure in your body.
Now, just a quick review of the borders and definition of the anatomical snuff box, this structure or this depression is framed by the radius on the proximal side and also two tendons: one on the dorsal side—the tendons of the extensor pollicis longus—and of the palmer side, it’s framed by the extensor pollicis brevis or the tendon of the extensor pollicis brevis. Now, it’s also important that, at the base, it’s formed... or the base of the anatomical snuff box is formed by two bones: the trapezium and also the scaphoid. It’s an important structure, clinically speaking, because the radial artery courses through the anatomical snuff box, and for that reason, it can easily be felt here.