087 The Superficial Extrinsic Shoulder Muscles
Today, in this video, Leslie talks about the superficial extrinsic shoulder muscles — the trapezius muscle and latissimus dorsi. Watch this episode and learn more about their complex origins, insertions, and actions.
Transcript of Today’s Episode
Hello and welcome to another episode of Interactive Biology TV where we’re making Biology fun. My name is Leslie Samuel, and in this video I’m going to be talking about the superficial extrinsic shoulder muscles.
Those superficial extrinsic shoulder muscles are: Trapezius and latissimus dorsi. Those are the two muscles we’re going to look at. We’re going to look at the origins, the insertions, and the actions. Let’s get right into it.
The first muscle is the trapezius. It’s a relatively complex muscle. It’s a relatively large muscle, and we’re going to look at the details of it right now. You can see that it’s kind of triangular shaped. You see it over here, and you can also see it right here over to the right, kind of triangular shape.
Now, when it comes to the origins of trapezius, this is probably one of the longest ones to remember, but we’re going to get it. I believe in us, I believe in you so, let’s make it happen.
The first thing I want to point out is this little bump thing at the back of the skull. That is called the external occipital protuberance. That’s part of the origins, and from there, we have the superior nuchal line that radiates out from the occipital protuberance. But, the trapezius does not originate from the entire superior nuchal line. It’s actually the medial third. You can see this section right here, the medial third of the superior nuchal line.
Then, there’s a ligament that extends from that external occipital protuberance and let’s do that in a different color, let’s do that in blue. That ligament is called the nuchal ligament. It projects down to C7 and that is part of the origin of trapezius. Then, from there, we have the spinous processes of C7, all the way down to T12.
So, for the origin, we have the external occipital protuberance. We have the medial third of the superior nuchal line, the nuchal ligament, or in some places you might see it called the ligamentum nuchae, and then, we have the spinous processes of C7 all the way down to T12.
When it comes to the insertion, let’s look at that now, it projects from the origins all the way down on the front end. You can’t see it here. I should have taken a picture also with anterior view. But, the first part would be the lateral third of the posterior clavicle. Not the entire clavicle, but just the lateral third.
Then, here we have the acromion process, and on the medial aspect of the acromion process, or you can just say the medial acromion. It inserts there also. Let’s do that in blue since we can’t see that. So, the medial acromion. And then, the superior edge of the spine of the scapula, that is also part of the insertion.
So, the lateral third of the posterior clavicle, the medial acromion, and the superior edge of the spine of the scapula. We have this complex origin, we have this complex muscle and this complex insertion. We can take this muscle and we can divide it into three portions. We can have the upper portion, the middle portion that you see here, and then, the lower portion.
It depends on what’s contracting. That’s going to determine the action, and how it’s contracting, how much of the upper is contracting versus how much of the middle is contracting and so on and so forth. But, if you have the upper contracting, what’s that going to do?
Remember, we’re moving from insertion in the direction of the origin. If it’s the upper contracting, that’s going to elevate the scapula. So, the scapula is moving up in that direction.
If it’s the middle section that’s contracting, what’s that going to do? That’s going to retract the scapula, and if it’s the lower portion, that’s going to cause depression of the lower scapula. Now, of course, none of these portions are going to be 100% isolated, so you’re going to get a combination of it.
But, you have these different types of motions. You have elevation, retraction, and elevation. Depending on how it contracts, you can also get rotation of the scapula.
Alright, so that’s the trapezius. Let’s now go into the latissimus dorsi. You can see that over here to the left, and you can also see it over here to the right, both in red. That one, when it comes to the origin, we’re dealing with the spinous processes of T7 to T12. Then, we also get this section here, that’s called the thoracolumbar fascia. We have the crest of the ilium. And, you can’t see it here, but the inferior three or four ribs, that’s also a point of origin.
Then, what happens is that muscle kind of spirals around and comes to the front, and it inserts on the floor of the intertubercular groove of the humerus. And, if you remember on the humerus, we have the greater tubercle, and the lesser tubercle. And, between the two, we have the intertubercular groove. You can see one of the previous videos for more information on that. So, it’s going to spiral around and insert right there on the intertubercular groove of the humerus.
Now, what’s going to happen when that muscle contracts? You can see a wide origin down here and the insertion is up there. So, if that contracts, one of the things that it can do is cause adduction of the arm.
It can also, because of how it spirals around and comes to the floor of the intertubercular sulcus, that can also cause internal rotation, so you rotate the arm internally, and that can also cause, depending on how it contracts, it can cause extension of the arm.
Let’s do our quick review.
This muscle that we’re looking at is the trapezius. In terms of the origin, we have the, let’s have the pointer here, the external occipital protuberance, the medial third of the superior nuchal line, the nuchal ligament and the spinous processes of C7 down to T12.
The insertion, it’s going to insert on the the lateral third of the posterior clavicle, the medial acromion, and the superior edge of the spine of the scapula.
In terms of the action, we can get rotation, retraction, elevation, or depression of the scapula, depending on what portion contracts or how it contracts.
Then, we have the latissimus dorsi. The origins would be the spinous processes of T7 through T12, the thoracolumbar fascia, the crest of the ilium, the iliac crest, and the inferior third of fourth ribs, then, it rotates around and inserts on the floor of the intertubercular sulcus.
So, that’s pretty much it for this video. If you want more videos like this and other resources to help make Biology fun, you know what to do. Head on over to the website, interactive-biology.com. This is Leslie Samuel. That’s it for this video, and I’ll see you on the next one.[table “10” not found /]