Origins, insertions, innervation and functions of the thenar muscles.
Hello everyone! This is Joao from Kenhub. Welcome to this training unit. We’re going to be learning the thenar muscles.
Now, this is a group of four muscles that you find on the radiopalmar side of your hand. And if you want to locate and even palpate these muscles on your hand, you just have to turn it to the palmar view of course. Then, on the direction of your thumb, you can feel an elevation, and this is known as the thenar eminence, this elevation. And I’m showing you here also an image on the right side where you can find the thenar eminence, and this is of course where you can find all these muscles. So this is where they’re located in your hand.
Now, as you can see here on this image, they are found here as a group, and you’ll notice that as origins, they are originating from different carpal bones as well as different metacarpal bones, and then they will extend and distally insert to different points of the thumb. But what we’re going to do here on this tutorial is to cover each individual thenar muscle and find out all the good stuff that we need to know about them.
Now, moving on to the very first thenar muscle that we need to cover, and this is the one seen here, highlighted in green. It is found quite superficially, and it is known as the abductor pollicis brevis. Now, the abductor pollicis brevis has three points of origin—very important thing to know, three points. And two of these points are carpal bones. So the first one is the scaphoid. Second one is the trapezium. Now, the third origin point is going to be a very famous, I would say, ligament that is this one here, very strong ligament of the wrist that is known as the flexor retinaculum. And this is where this muscle also originates from.
Now, moving on to the abductor pollicis brevis or the insertion of the abductor pollicis brevis, and you need to know that there are two insertion points, and these are the base of the first proximal phalanx or the base of the proximal phalanx of the thumb, as you can see here. And the second insertion point is going to be the radial sesamoid bone. So two insertion points for this muscle.
Now, moving on to the innervation of the abductor pollicis brevis. What you need to know is that there is one nerve that is going to be supplying this muscle, and it is the median nerve.
Now, let’s move on to the second thenar muscle, and this is the one seen also highlighted in green, and this is the adductor pollicis. And a very, very important thing that you need to know about the adductor pollicis is that it has two origin surfaces or two heads, and these two heads of the adductor pollicis are going to be this one that you see here on the right side, this oblique head right here. And of course, to simplify things, we’re going to call it the oblique head of the adductor pollicis. Now, the other head is this one, a bit more transverse in terms of direction, and then we will call it the transverse head of the adductor pollicis.
Now, let’s talk about the origin point of the oblique head of this muscle, and I can tell you that there are three origin points for the oblique head. The first one is a carpal bone, the capitate. And then the other two are going to be metacarpals. So one is the second metacarpal bone which will serve as an origin point for this head, and also, the third metacarpal bone will be another origin point for the oblique head of the adductor pollicis.
Now, moving on, still on the origin, but this time, on origin points of the transverse head of the adductor pollicis. And I can tell you that there are one origin point for this one, and you can clearly see that it’s the third metacarpal bone. If you notice here, the muscle is occupying the palmar side of the shaft or the body of the third metacarpal bone.
Now, let’s move on to insertion of the adductor pollicis, and as you can see, these two heads will merge into one common tendon and then will insert into two points. And these points are the base of the first proximal phalanx or the base of the proximal phalanx of the thumb, right here. And also, another sesamoid bone. This time, on the ulnar side—so the ulnar sesamoid bone.
In terms of innervation of this muscle, I can tell you that the ulnar nerve is going to be responsible for the innervation of the adductor pollicis, but I have to be very specific here and tell you that it’s going to be the deep branch of the ulnar nerve—so, very careful with this. The adductor pollicis’ innervation is going to be done or the nerve responsible for it is the deep branch of the ulnar nerve.
So let’s move on and talk about the third thenar muscle, this one here highlighted in green, which is known as the flexor pollicis brevis. And this muscle, as the adductor pollicis, also has two origin surfaces or two heads, and these origin surfaces can be found here. The first one is located more superficially, and for that reason, we’re going to call it then the superficial head of the flexor pollicis brevis. This one here on the right, the second one, you can see here an outline of green. Because this head is found underneath the superficial head, for that reason, we’re going to call it the deep head of the flexor pollicis brevis.
Now, let’s move on and talk about the origin points of the superficial head, and there are or there is one origin point, and it is the ligament, the flexor retinaculum. As you can see here, the muscle or this head is clearly or has an origin point on this flexor retinaculum. Moving on to the origins of the deep head of the flexor pollicis brevis, and you can see that there are two origin points coming from the carpal bones, and this is the capitate, and the other origin point is going to be the trapezium.
Now, moving on to the insertion points of the flexor pollicis brevis, there are two insertion points for this muscle. And as you can see, the two heads are coming together into one tendon as well, and they will insert into two points, and these points are the base of the first proximal phalanx or the base of the proximal phalanx of the thumb. And the other one is the radial sesamoid bone.
Now, in terms of innervation, I can tell you that this muscle is going to be double-innervated. So there are two nerves supplying the flexor pollicis brevis, and these nerves are the ulnar nerve and then the median nerve. So very important, this is the only muscle of the thenar muscles, of this group, that will be double-innervated—very important.
Now, the fourth and last thenar muscle, and this is the opponens pollicis, as you can see here also highlighted in green. And this muscle will have two origin points—very important. The first one is the carpal bone, and it is known as the trapezium, and the other origin point is the ligament that we’ve been talking about throughout this tutorial, the flexor retinaculum. So two origin points, and you can clearly see them here, if I show you here these two origin points of the opponens pollicis muscle.
Now, moving on to the insertion points of this muscle, and you can see that it is inserting into one metacarpal, and this metacarpal is, of course, the metacarpal of the thumb or the first metacarpal. In a very specific way, we can say that it is located on the radial side of the first metacarpal bone.
Moving on to the innervation of the opponens pollicis, and the innervation of this muscle is going to be provided by the median nerve.
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