Let’s talk about the structure of the first muscle of the four pectoral muscles — pectoralis major. Also, learn about the the origin, insertion and action of the clavicular and sternocostal portions of this muscle. This and much more.
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 inside this video, I’m going to be talking about the origin, insertion, and action of pectoralis major.
Let’s get right into it.
Pectoralis major is the first of the four pectoral muscles that we spoke about and we’re going to look at the structure of that muscle. You can see that muscle here outlined in red.
What you’ll notice about this muscle is it’s a fairly large muscle, relatively speaking and you will notice that it has a clavicular portion or a clavicular head. Then, we have another portion that’s called the sternocostal portion.
That sternocostal portion is here, and the clavicular portion (let’s do that in green), that’s the clavicular potion in green, and then, in blue here, we have the sternocostal portion. You can see a separation in between. If I remove that, you can see that separation here.
When it comes to the origins, we have to distinguish between those two portions. The clavicular portion — it’s called that because it’s attached to the clavicle. The clavicular head, the origin is going to be the anterior surface of the front of the medial half, not the entire clavicle, but just the medial half of the clavicle.
Once again, the origin of the clavicular head is the anterior portion of the medial half of the clavicle.
Then, we have the sternocostal head and the origin here you can see, of course, it’s sternocostal, so it has something to do with the sternum and it has something to do with the ribs. (The word costal comes from the Latin word costa which refers to ribs.) The origin of the sternocostal portion as you can see here, some of it at least, number one it’s going to be the anterior surface of the sternum and the part that you can’t see is that it’s also attached under here to the upper six costal cartilages.
You can see the external oblique muscles here. It’s one of the abdominal muscles, and the aponeurosis, we have this flat tendon-like substance here that we call the aponeurosis. That’s where we’re going to have the final part of the origin of the sternocostal portion.
That’s the origin.
Then, let’s look at the insertion point. You can see it’s going towards the humerus, but it’s going to attach to a very specific portion of the humerus. That’s going to be here. We have the greater tubercle, and then, of course, we have the crest of the greater tubercle which is the lateral lip of the intertubercular groove. It’s going to attach right there on that crest.
It extends from the origin point to the crest of the greater tubercle or the lateral lip or the lateral border of the intertubercular sulcus.
What’s going to happen when this muscle contracts? What is the action?
Well, as we mentioned before in the previous video, when a muscle contracts, you can basically imagine the insertion point or the bone that it’s inserted on to move towards the origin.
So, if we’re dealing with the clavicular head, what’s going to happen if this contracts?
You can just picture it as that contract, it’s going to move the arm in that direction. Basically, what it’s going to do is elevate the arm, and this is not a 3D thing, so I can’t fully show you how that it would be. You’re basically raising your arm.
Now, I want you to do something. Take your left hand and put it on the clavicular portion of your pectoralis major. Then, I want you to tense your arm and move it up.
What happens? Do you feel that muscle contracting?
I’m doing it right now and I can feel that. That’s because that muscle is involved as we tense and move it up sternly. You’re going to feel that muscle, that clavicular portion of your pectoralis major contracting.
What happens when the sternocostal portion contracts?
Well, one of the things that it’s going to do is it’s going to extend the arm. It’s actually working opposite to the clavicular head. It’s going to extend the arm, so we’re moving it back down. That’s one of the actions.
If we take the entire pectoralis major as a whole, what that does is it adducts the humerus. So, if the arm is abducted in this direction and that muscle contracts, it’s going to cause an adduction and move that arm back towards the trunk.
If you take the muscle as a whole, it adducts the humerus, but it also helps because of how it’s contracted. It also causes internal rotation. So, rotate your arm inwards. That is going to be accomplished also by the pectoralis major.
We have a number of different things that this muscle can do because of the structure, because of how the fibers are running, because you have these different portions, depending on how it contracts, you can move the arm in different ways.
Let’s do a quick review. This is pectoralis major. When we’re dealing with the origin, by the way, if you want to quiz yourself, you can turn the volume down right now.
The origin if we’re dealing with the clavicular head, that’s the anterior surface of the medial half of the clavicle. If you’re dealing with the sternocostal head, that’s going to be the anterior surface of the sternum and the upper six costal cartilages, and lastly, the aponeurosis of the external oblique.
The insertion point, it’s going to insert on the crest of the greater tubercle or the lateral border of the intertubercular sulcus.
The action, the clavicular head is going to cause flexion of the humerus; the sternocostal end is going to cause extension of the humerus. If you take the entire muscle together, it adducts the humerus and it also causes medial rotation.
That’s pretty much it for this video. If you’d like more like this and many other resources to help make Biology fun, visit the website at interactive-biology.com.
This is Leslie Samuel. Until next time, I’ll see you in the next video.[table “” not found /]