Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we’re going to be talking about the infrahyoid muscles.
So on this tutorial, we’re going to be covering the anatomy, function, and also clinical… related clinical points that you need to know about the infrahyoid muscles, and right now, we’re looking at an anterior view of your neck where you can see these muscles.
And you notice here that the infrahyoid muscles are a group of four muscles under the hyoid bone (hence the name “infrahyoid”) and then they attach also to the sternum, larynx, and the scapula.
So they are four muscles, including the omohyoid, the sternothyroid, the thyrohyoid, and the sternohyoid.
Now, they have complex names, but don’t worry. Throughout this tutorial, we’re going to clarify their names, and you can even see that they really help you remember the different attachment points that these muscles will have.
Let’s start off with the very first one here on the list that you see also from an anterior view of the neck, highlighted in green. This one is known as the omohyoid.
The omohyoid is divided into two bellies. It has an inferior belly as you can see here and a superior belly. And they are divided by this tendon here that you see in a lighter shade of green. This is a… where these two bellies will meet.
Now, the inferior belly will be originating from the superior border of the scapula as you can clearly see here on this image. This is the superior border of the scapula, and notice how the inferior belly is originating from this point here.
Then the inferior belly will be ascending craniomedially and then merges into the intermediate tendon at the height of the lateral cervical region. So you can see here, this will be technically the insertion point for the inferior belly of the omohyoid.
And as you probably guessed, then, the superior belly of the omohyoid will start at the… this intermediate tendon here. So this is the starting point for the superior belly and then goes all the way to insert on the hyoid bone as you can clearly see here.
So this is the hyoid bone, the anterior view of… or ventral view of the hyoid bone. Notice how it will serve as an attachment point for the superior belly of the omohyoid.
Now, let’s take a look at the innervation of the omohyoid. The omohyoid is innervated by a branch of the cervical plexus that you see here highlighted in green, the ansa cervicalis.
We’re still looking at it from an anterior view on this image.
Now, the inferior belly of the omohyoid will be innervated by branches of the three cervical rami (C1, C2, and C3) that make up the ansa cervicalis. While the superior belly is innervated by the superior root of the ansa cervicalis, which contains only fibers from the first cervical spinal nerves.
Now, a quick word on the different functions associated to the omohyoid. This muscle is going to be depressing the larynx and the hyoid bone, so pulling it down. It also carries the hyoid bone backwards and to the side.
The omohyoid has an additional function due to its attachment to the carotid sheath, and by pulling this sheath, it maintains a low pressure in the internal jugular vein and this way increasing blood return from the head to the superior vena cava.
We’re now ready to move on to the next muscle on the list, now, that you see here highlighted in green. This one is the sternothyroid muscle.
The sternothyroid will be originating from the dorsal surface of the sternal manubrium as you can see, or the manubrium of the sternum.
This is the manubrium of the sternum, and you see here that it’s originating posteriorly because we’re looking at it from a ventral… or this image is a ventral or anterior view of the neck.
Now, in terms of insertion points, this muscle will be inserting on the oblique line of the thyroid cartilage as you can see here. So this is the thyroid cartilage hiding behind this muscle.
And keep in mind that the sternothyroid is the only hyoid muscle or infrahyoid muscle that does not directly attach to the hyoid bone.
When it comes to the innervation of the sternothyroid, the muscle is going to be innervated by the ansa cervicalis that you also see here highlighted in green from an anterior view.
A quick word on the different functions associated to the sternothyroid muscle. The sternothyroid is responsible for positioning of the hyoid bone and also depresses or pulls the larynx down.
We’re going to be talking about the next muscle on the list that you see here highlighted in green, the thyrohyoid muscle, which is a small, quadrilateral muscle, appearing like an upward continuation of the sternothyroid muscle as you can see here clearly on this image.
Now, when it comes to the origin points, this muscle is going to be originating from the oblique line on the lamina of the thyroid cartilage. As you can also see here on the image, how this muscle is almost picking up where the sternothyroid is leaving.
As for the insertion point of the thyrohyoid, as you probably guessed from the last part of the name here, it will be inserting on the body of the hyoid, which is this part of the bone here. So this is known as the body of the hyoid and will serve as an insertion point for the thyrohyoid muscle.
Now, a quick look at the innervation of the thyrohyoid. This muscle is going to be innervated by the cervical spinal nerve number one, as you see here highlighted in green in this image, which joins this one that you see here, the hypoglossal nerve, for a short distance.
Now, as for the functions of the thyrohyoid, the muscle is able to, then, elevate the thyroid and will be able to depress the hyoid bone.
So it all has to do with insertions and origin points because, as you can see here, when the muscle contracts, it will be pulling down the bone, this bone here, the hyoid bone, and then pulling up the thyroid.
We’re moving on to the next muscle that you see here highlighted in green. This one is the sternohyoid muscle.
The sternohyoid muscle is a thin, narrow muscle attaching the hyoid bone to the sternum as you can see here on this image. And it is the most superficial of all infrahyoid muscles.
As for origin points of the sternohyoid, it’s going to be originating from the manubrium of the sternum and also the sternoclavicular joint.
Now, the muscle will be passing upward and medially and is inserted by a short tendinous fiber into the lower border of the body of the hyoid bone.
The innervation of the sternohyoid will be carried out by this one here, the ansa cervicalis.
As for the functions associated to the sternohyoid muscle, its role is to depress the hyoid bone during swallowing or speech.
Now, before we finish this tutorial, I would like to give a quick word on related clinical anatomy to the infrahyoid muscles.
So trauma in the region of the cervical spine can damage the ansa cervicalis. Remember, one of the structures that will be innervating most of these muscles.
This can then result in paresis or paralysis of the infrahyoid muscles, and clinically, those may be presented as swallowing difficulties, a hoarse voice, and throat tightness.