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Anatomy and function of the muscles of the shoulder girdle.
Hey everyone! It’s Nicole from Kenhub, and in this tutorial, we’ll be looking at the muscles of the shoulder girdle.
So here’s one of the main images that we’ll be looking at in today’s tutorial, and we’ll be looking at the shoulder muscles that you can see here but we’ll also be looking at the muscles deep to that which you can see as we make the superficial layer of muscle transparent.
The muscles of the shoulder girdle help to support the bones that connect the arms, specifically, the humerus, which is highlighted here, to the appendicular skeleton on either side of the body. And, today, we’re going to be looking at eight muscles in total and for each of these, I’ll cover their origin, insertion, innervation and function. I’ll start by looking at the muscles that are viewed best posteriorly as we can see here and then I’m going to flip our body around to talk about the muscles that can be seen best anteriorly. So, without further ado, let’s begin with the muscles of the shoulder girdle that can be seen best from a posterior view.
So, let’s begin by looking at this huge muscle which we can see here in our image, and as you can see, it’s a pretty large muscle and, in fact, it’s the largest muscle of the shoulder girdle, and it’s known as the trapezius muscle. This big triangular muscle of the upper limb is a flat muscle and, superiorly, we can see that it runs from the occipital bone of the skull and inserts into the lateral third of the clavicle. The middle part of this muscle originates from the spinal processes of T1 to T4 vertebrae which are just here, and inserts on the acromion of the scapula which is just here.
Moving down our image again, we can see that the inferior part of the trapezius originates from the spinal processes of T5 to T12 vertebrae and inserts on the spine of the scapula. The trapezius muscle is innervated by the eleventh cranial nerve or the accessory nerve and by nerves of the cervical plexus. The trapezius has a few functions including stabilizing and securing the shoulder blade as well as facilitating lateral flexion of the head as well as dorsal flexion of the head and cervical vertebral column.
If we take a peek underneath the trapezius muscle, we can see that there are some more shoulder girdle muscles underneath, and these are known as the rhomboid muscles and consist of a major muscle and a minor muscle. First, let’s look at the rhomboid major muscle.
The rhomboid major muscle has its origins on the spinal processes of the T1 to T4 vertebrae just here and inserts on the medial border under the spine of the scapula just here. It’s innervated by the dorsal scapular nerve which is a branch of the brachial plexus and, as you can probably guess by looking at the direction of the muscle fibers, functions to adduct and elevate the scapula as well as rotating the inferior angle of the scapula towards the vertebral column.
Moving on up now just superior to the rhomboid major muscle, we have the rhomboid minor muscle. The rhomboid minor muscle has its origins on the spinous processes of the C6 and C7 vertebrae and inserts into the medial border of the scapula just above the spine of the scapula. It’s also innervated by the dorsal scapular nerve and performs pretty much the same functions as the rhomboid major muscle – that is, elevation and adduction of the scapula and rotation of the inferior angle of the scapula towards the vertebral column.
So that’s all the shoulder girdle muscles we’re going to be covering from the posterior view. Let’s now move on to cover the muscles that can be best seen from the anterior view.
The first muscle I’m going to show you can be easily palpated on most people and is known as the sternocleidomastoid muscle. The sternocleidomastoid muscle is a little bit unusual due to the fact that it has two heads and we’re going to talk about both of these heads in the following slides. Let’s begin with the sternal head.
The sternal head of this muscle – circled here – originates from the manubrium of the sternum. The clavicular head – circled here – originates from the medial part of the clavicle. The muscle fibers then combined to form a single muscle belly which inserts into the mastoid process and the superior nuchal line of the skull. We can use the name of these muscles to help us remember its insertions and origins and vice versa and sterno- means “sternum” while –cleido- refers to the “clavicle” and the -mastoid refers to, of course, the “mastoid process”. And, of course, these three words together make the muscle’s name, sternocleidomastoid.
The sternocleidomastoid muscle is innervated by the accessory nerve which is the eleventh cranial nerve and functions to facilitate the lateral flexion and rotation of the head and the neck.
The next muscle we’re going to look at today is the sternocleidomastoid’s somewhat skinnier cousin and is known as the omohyoid muscle. The omohyoid muscle has its origin on the superior border of the scapula medial to the suprascapular notch which is just here and it has its insertion on the lateral and inferior border of the hyoid bone which is just here. It is innervated by the ansa cervicalis of the cervical plexus – that is, the C1 to C3 spinal nerves – and though this muscle might be small, don’t be fooled as it packs a lot of punch as it serves a few very important functions like helping to depress and fix the hyoid bone which is the only bone in the body not connected to another bone. It therefore relies on muscles like the omohyoid to support it along with surrounding ligaments.
The omohyoid muscle also functions to draw the hyoid bone and the underlying larynx downwards during phonation and the terminal phase of swallowing. It also functions in tensing the cervical fascia which helps to ensure that the internal jugular vein remains open and unobstructed.
The next muscle of the shoulder girdle we’re going to look at can be seen peeping out from under the clavicle here, and is known as the subclavius muscle. The subclavius muscle has its origin on the first rib just here and inserts at the lower surface of the clavicle roughly here. It is innervated by the subclavian nerve which is a branch of the brachial plexus and its main function is the active stabilization of the clavicle in the sternoclavicular joint during movements of the shoulder and the arm. It also functions in the depression of the clavicle and the elevation of the first rib.
Moving on now from the smallest muscle of the shoulder girdle, we’re now going to look at one that is quite a bit larger and it sorts of looks like a backwards wing, and this muscle is known as the pectoralis minor muscle – not to be confused with its neighbor, the pectoralis major, which is technically not part of the shoulder girdle, so we won’t talk about it in this tutorial.
The pectoralis minor has its origins on the third, fourth and fifth ribs and inserts on the coracoid process of the scapula. Now this muscle is innervated by the medial and lateral pectoral nerves which are branches of the brachial plexus. The function of the pectoralis minor muscle is the adduction and depression of the scapula as well as elevation of the third to fifth ribs and the expansion of the rib cage during inspiration.
Moving on to another large muscle, we’re now going to have a look at the final muscle of the shoulder girdle and like the trapezius, this muscle can be divided into a superior part, an intermediate part and an inferior part, and this muscle is known as the serratus anterior muscle. As you can see, it has a sort of serrated edge along the ribs just like a saw blade.
The superior part of this muscle has its origins on the first and second ribs and inserts on the superior angle of the scapula which is just here. The intermediate part of the muscle has its origins on the second and third ribs and inserts on the medial border of the scapula back here. The inferior part of this muscle has its origins on the fourth to ninth ribs and inserts on the medial border and inferior angle of the scapula just here.
The serratus anterior muscle is innervated by the long thoracic nerve, a branch from the brachial plexus, and functions to move the scapula ventrolaterally along the ribs. This muscle also acts as an accessory muscle during inspiration elevating the ribs.
Alright, now that I’ve introduced each muscle in detail, I’m going to quickly summarize for you the muscles we talked about today.
We saw that the muscles of the shoulder girdle function to support the bones that connect the arms to the appendicular skeleton on either side of the body. We started by looking at the posterior muscles of the shoulder girdle kicking off with the trapezius muscle, and if you recall, the trapezius muscle is comprised of a superior part, a middle part and an inferior part each with their own origin and insertion points.
Next appearing underneath the trapezius, we looked at the rhomboid major muscle and this muscle has its origin on the spinal processes of the T1 to T4 vertebrae and inserts onto the medial border of the scapula. Next, we looked at the rhomboid major’s smaller neighbor – the rhomboid minor – and the rhomboid minor muscle has its origin on the spinal processes of the C6 and C7th vertebra and, like the rhomboid major, also inserts onto the medial border of the scapula just slightly more superiorly.
Having looked at the muscles of the shoulder girdle that are best viewed posteriorly, we then flipped around to examine the muscles that are best viewed anteriorly. Thus, we looked at the two-headed sternocleidomastoid muscle, and then we looked at the small thin muscle just here known as the omohyoid muscle and then going even smaller, we looked at the subclavius muscle peeping out from underneath the clavicle just here. And then, we went to look at the pectoralis minor muscle which is not to be confused with its larger neighbor, the pectoralis major. Finally, we looked at the serratus anterior muscle which wraps around the rib cage and assists in the expansion of the ribs during inspiration.
That concludes our tutorial today on the muscles of the shoulder girdle. Thanks for joining me!