hello anatomy colleagues this is dr alsup and i am very excited to
talk some basics and clinically important aspects of the muscles of the anterior neck
and these muscles will include the large rope-like sternocleidomastoids
and the smaller strap-like infrahyoid muscles which are a bit more in the midline
starting with the sternocleidomastoid muscle which sometimes we abbreviate and you'll see
often as scm sometimes we even say scm so the sternocleidomastoid muscle is going to
be a very prominent landmark in the neck one that you can actually see pretty well through the skin
and one you can definitely palpate on yourself so just looking at the location as you can see here
in red try to palpate the sternocleidomastoids on yourself and i think you'll find them pretty
easy to locate particularly if you kind of tilt your um or kind of twist your face to the side
the fact that's so easy to palpate is the reason this muscle is used to subdivide the neck and to
excuse me into an anterior kind of more the anterior cervical region which will be on
this side of the sternocleidomastoid and then the posterior cervical region will be
on the posterior surface and behind or more posterior to the sternocleidomastoid
there are two superior attachments the mastoid process of the temporal bone which is kind of
right in this region if you place your fingers right under your ear you will feel a bump and that
bump will be the mastoid process and the muscles attachments will continue from the mastoid process
posteriorly along the occipital bone so the occipital bone is that posterior part of the skull
and there are two inferior attachments as well but these are a bit more separated or distinct
than what you have posts or superiorly there will be the clavicular attachment
as you can see right here along the collarbone or your clavicle and a more medial attachment as you
can see right here where i'm underlining on the top part of the sternum the sternum is your breast
bone right here in the midline region and the top part of the sternum is called the manibrium
and you can see this muscle is crossing multiple cervical vertebral joints
basically all of them so it will most directly affect actions at these joints
so let's talk a little bit about these actions if both sternocleidomastoid muscles
are contracting so say so both the left and the right are contracting or bilateral contraction
quite a few things can happen but the main action we are going to focus on
is flexion of the vertebral vertebrae aka neck flexion so this is where the chin would start
to it you would flex or kind of bend down and it would get close to the more distal or inferior
attachments so close to that maneuvrium so kind of been kind of think putting your chin to your chest
if only one sternocleidomastoid muscle is contracting or unilateral contraction there will
be lateral flexion of the neck on the contracting side so say the right this is the right side this
is the left side so kind of patient right and left the right sternocleidomastoid is contracting
the head will tilt to the right side so kind of heading towards that right shoulder region
and rotation can occur as well with the head turning kind of superiorly towards
the opposite side so in this case of the right sternocleidomastoid muscle contracting
the face would rotate to the left so kind of think putting your your head on your shoulder and kind
of tilting your chin up that would be this action the action described right here the innervation
of scm is unique in that one nerve does not have both afferent and efferent fibers which is the
case for most innervation of muscles usually we describe one nerve that innervates a muscle and
it has both afferent and efferent fibers instead for the sternocleidomastoid there are separate
nerves providing efferent or motor innervation and afferent or sensory innervation and this accessory
nerve which will be the efferent innervation is our first introduction into a cranial nerve
it is cranial nerve 11 each pair of cranial nerves will have a name and a number associated with it
and the accessory nerve is entirely efferent so it couldn't provide since re-innervation if it tried
so scm will only so the sorry the accessory nerve will only provide motor innervation to the
sternocleidomastoid muscle and the scm will have to get its afferent fibers or innervation from
fibers from the cervical spinal nerve c2 and c3 so the fact that there are two separate um types
of nerves innervating this muscle is unique in terms of muscle innervation
one last thing i want to mention about sternocleidomastoid before we move on is
that this vein right here as you can see in this image that runs just superficial and obliquely
over the muscle and this is the external jugular vein this image only had it labeled
as jugular vein and i added the uh the appropriate external because there is an internal jugular vein
the ejv as we like to call it so we like to shorten things sometimes so the ejv
is the most dominant superficial vein of the anterior neck and the visibility of the
external jugular vein can serve as a slightly kind of visible gauge of venous pressure
if venous pressure is normal the ejv is typically only visible a little bit above the clavicle
whereas a venous pressure is kind of risen the vein becomes
considerably more prominent and visible sometimes throughout its entire length
okay moving to the infrahyoid muscles or the strap muscles as you may hear them referred
to as they are said to resemble straps or ribbons there are four pairs of muscles
two are going to be superficial so that will be the sternohyoid as you can see right here
and the omohyoid which i actually think you can see better here which will extend all the way to
the scapula and there will be two sets of deep strap muscles these will be the sterno thyroid
and the thyrohyoid which will be the more superior one
as the name infra hyoid would suggest all the muscle bellies will be located
inferior to the hyoid bone which is that kind of floating bone in the anterior neck
it's not really floating it doesn't directly articulate with any other bones but it's very
has very close attachments to muscles and some ligaments so um it will not so that all these
muscles will be inferior to the hyoid and all but one will actually have an attachment on the hyoid
and the great thing about these names of these muscles are they tell you exactly where they
attach so the sternohyoid would attach on the sternum and the hyoid whereas say the thyrohyoid
would attach on the thyroid cartilage and the hyoid you can kind of go from there now based on
the location of the muscles and attachments it can become kind of intuitive as to what these muscles
do when they contract and mainly they serve to depress or lower the hyoid when they contract
or also importantly they can just keep the hyoid bones steady to allow for specific actions like
swallowing and speaking to occur efficiently if a muscle has an attachment on the thyroid cartilage
of the larynx it can play a role in either elevating like the thyrohyoid or depressing
as with the sternothyroid muscle the the larynx because that thyroid cartilage is of the larynx
most of the infrahyoid muscles are innervated by the beautifully named ansa cervicalis which
i'm going to kind of outline here on this image which is this kind of delicate loop of nerve that
will be derived from the cervical plexus the ansa cervicalis is closely associated with the
carotid sheath which covers many of the larger and deeper neurovascular structures of the neck
like the common carotid artery as you can see here or the internal jugular vein not the
external jugular vein that's more superficial but the internal jugular vein now peeking
right through this region right here you can see a little bit of the thyroid gland and i
hope you notice that there are some infrahyoid muscles in fact all the infrahyoid muscles
to some extent will cover the thyroid gland so in cases where a surgeon needs access to the thyroid
or any of its of associated neurovasculature such as in like a thyroidectomy the infrahyoid muscles
often need to be moved or resected to gain access to those deeper structures so that should wrap us
up regarding the muscles of the anterior neck and here's our summary slide for this learning
objective please make sure to review this material and ask questions if there are any areas that
need some clarification thank you for your time and attention and have a great rest of your day