all right hello anatomy friends this is dr. Alsup and in this video we are going to focus on the
infrahyoid muscles aka the strap muscles which are named because of their strap like or ribbon like
appearance so with infrahyoid muscles there are two different layers there will be a superficial
layer and a deep layer we'll talk about which muscles are in which layer we also talked about
what these muscles do what are their actions and lastly what innervates these muscles with three
out of the four sharing a common innervation and one outlier and we love a good outlier alright so
let's get our bearings regarding where we are in the neck and where these muscles are located kind
of the key relationship and where this group gets its name is that all of these muscles are inferior
to the hyoid thus the infrahyoid so let's get a clue of where we're at so here is your hyoid
bone let's take a moment to discuss the hyoid bone because it's a really important landmark
in the anterior neck and particularly a lot of what we're talking about today it is going to be
located around the c3 to c5 vertebral levels and that's gonna be between the mandible and
the thyroid cartilage so I always describe if you palpate your own anterior neck right in the middle
you will first feel your thyroid cartilage because it's the part that projects the most anteriorly
and more to come on the thyroid cartilage later I promise but if you move your fingers a little
more superiorly and sometimes I have to kind of tilt my chin down you can palpate the hyoid bone
certainly more difficult to palpate than thyroid cartilage but it's possible to palpate it this
bone is unique in its isolation from the rest of the skeleton and the fact that it does not
directly articulate with any other bone rather is completely suspended by muscles tendons and
some ligaments so the hyoid's functioning as an attachment site for many anterior neck muscles
including the infrahyoid muscles that we're talking about right now and it can also play
an important role in propping the airway open all right so that's our quick stop to talk about the
hyoid bone and all these muscles will be inferior to it the infrahyoid muscles will also be superior
to the sternum which would be right about here ish so not projecting into the thorax region the
muscles will also be medial to the carotid sheath in its contents so here would be a carotid sheath
here a credit sheath would also be located over here and lastly and importantly these muscles
will all be superficial to the thyroid gland and the larynx so one has to reflect these muscles to
gain access or view these deeper structures now as mentioned at the beginning there are two layers or
planes of muscles the superficial layer is going to be composed of the sternohyoid as well as the
omohyoid which is a personal favorite of these infrahyoid because it's so unique with the deep
layer made up of the sternothyroid and the thyrohyoid and there's that hyoid bone right
there if you have a thyro or thyroid in the name you know you're talking about the deep layer so
let's talk about each one we will start with one of the larger ones and this is the sternohyoid
which you can see outlined here in blue I bolded the attachment points that match up to the muscles
names and you will see that as a common theme for these infrahyoid muscles and many other muscles in
the body this muscle will extend inferiorly from the sternum and medial clavicle all the way up
to the hyoid so here's your highlight here your sternum is right here and your clavicle is right
here it as you can see here completely blocks the view of the deep strap muscles so this has to be
reflected in order to see any of those this muscle like most strap muscles will play an important
role in stabilization of the hyoid so kind of keeping it in place while other things happen
also just looking at the location of this muscle if you think about this muscle concentrically
contracting so shortening it makes sense that it will take the hyoid with it inferiorly thus
depressing the hyoid okay this next muscle is a fun one the Omohyoid muscle this muscle is unique
amongst the infrahyoid muscles because it has two bellies it has a superior belly and an inferior
belly these bellies are going to be connected by an intermediate tendon which is tethered to the
clavicle by this fascial sling which you can see here outlined in this lighter blue doesn't need
to one know what the prefix Omo what does Omo mean because that gives you an indication of where it
attaches it means shoulder so the name Omohyoid indicates that that inferior belly will have an
attachment to the scapula or the shoulder but more specifically the scapula with the superior
belly attaching to the hyoid bone similar to its superficial partner the sternohyoid the omohyoid
play a role in stabilization and depression of the hyoid bone all right moving on to the
deeper infrahyoid muscles let's start with the sternothyroid note that the sternohyoid had to
be removed in this image in order to see these muscles this muscle will extend in fear early
from the sternal manubrium you can see it even is heading a little bit towards the clavicle as
well so there's manubrium here all the way up to the oblique line of the thyroid cartilage
note something important here this muscle does not make its way all the way up to the hyoid
bone so it does not attach to the hyoid so not playing any direct role in hyoid bone movement
based on its location and attachments when it contracts it can play a role in depressing the
larynx if it was previously elevated so you can see if this is your thyroid cartilage right here
think about what that contracts it will pull the larynx and down alright moving on to the last of
the infrahyoid muscles and also the smallest is the thyrohyoid inferiorly from the oblique line
of the thyroid cartilage up to the hyoid bone so there's a hyoid bone again I'm going to keep
labeling it because it is such an important area here so when it concentrically contracts it can
depress the hyoid bone but also based on its other attachment to the the thyroid cartilage
it can elevate or bring up the larynx as well so kind of an opposite of what we had with the
sternothyroid and so it's all about the location of these muscles thyrohyoid is the muscle that
is the oddball in terms of innervation love a unique innervation I would always suggest
putting stars or hearts or whatever works for you next to anytime that you have something unique in
terms of innervation or something unique in terms of actions this is something that almost always
finds its way into some form of assessment it is innervated by fibers from the c1 spinal nerve that
are not associated with ansa cervicalis which is going to be the innervation for the other 3
infrahyoid muscles these fibers of c1 will travel with or what we often refer to as hitchhike with
the hypoglossal nerve and we'll talk a bit more about what we mean by hitchhiking in a moment
all right the rest of the infrahyoid muscles are innervated by the beautifully named ansa
cervicalis one of my all-time favorite anatomical names which is a branch of the cervical plexus
similar to those efferent nerves from the nerve point of the neck or herbs point fibers of c1
will hitchhike similar to what happened with the thyrohyoid so fibers of c1 will hitchhike
along the hypoglossal nerve or cranial nerve 12 so some of those fibers will shoot off to go
innervate thyrohyoid and other c1 fibers will leave that close proximity to the hypoglossal
nerve to join the superior limb or to form excuse me the superior ansa cervicalis sometimes you'll
have fibers of c2 also forming the superior line' the inferior limb of ansa cervicalis is formed by
fibers of c2 and c3 so the ansa cervicalis will send small muscular branches to all the infrahyoid
muscles accepting the thyrohyoid so let's return to this concept of nerve fibers hitchhiking with
other nerves because this is something that will come up quite commonly throughout our discussion
of nerves in this course particularly when we get to the parasympathetics-- so keep in mind
that nerves are just collections of axons wrapped up together by connective tissue in the peripheral
nervous system so your pns axons may travel with or in close proximity with different nerves
particularly if this is a smaller collection of axons on their way to the target tissue
so you can see here in this nice cartoonish rendition that I made the fibers of c1 that
are destined for say the thyrohyoid or to become the superior limb of ansa cervicalis are going to
travel in close proximity to the hypoglossal alright so they're very close to one another
they're traveling together but they still came from different sources so the hypoglossal nerve
it's it originated in the medulla oblongata and of course c1 is from spinal cord level c1 so so they
are coming from different sources but they can travel together for a while and then eventually
move towards their different targets so the c1 fibers hang around the hypoglossal nerve before
pivoting off in different directions so you can see traveling together for a while and then it's
heading off say to either the thyrohyoid or to become the superior and ansa cervicalis so that's
what we mean by hitchhiking like I said this is something that we will talk about over and over
again throughout the course so this is an area that's confusing return to this think it through
and if any remaining questions I always feel free to reach out to us all right so we've had so much
fun with this area I have two questions instead of my usual one so hopefully this one isn't too
tough which infrahyoid muscles has an attachment to the scapula only one of the four will have an
attachment to the scapula is it A. omohyoid B. sternohyoid C. sternothyroid or D. thyrohyoid so
take a moment and hopefully you have landed on the correct answer which is A omohyoid and this omo is
that key here I'm gonna indicate shoulder so the scapula is part of the shoulder joint so that is
going to be one of the attachment sites where the recalling you're reviewing that the infrahyoid
muscles are named for their attachments and this can really help a lot and not only remembering the
attachments we're really thinking through what the actions of these muscles are alright one
more question all right injury to ansa cervicalis would not affect the actions of which muscle is
it A. omohyoid B. sternohyoid C. sternothyroid or D. thyrohyoid pause the video if you need a
moment to think through and when you're ready to move on let's discuss that D is the correct
answer this nerve is innervated by separate c1 fibers not associated with ansa cervicalis so D
thyrohyoid is the correct answer the rest of these infrahyoid muscles are going to be innervated by
the answer servic Alice well that was a lot of fun I do do love talking about muscles as always as
you review and have any questions please do feel free to ask thank you for your time and attention