hello all this is dr. Alsup here and in this video we are talking all things sternocleidomastoid
muscle or your SC in it may seem a bit odd to have a whole video devoted to just one muscle
but this is a big one when we're talking about the neck it not only is a large and long muscle
but it's important in forming boundaries for the cervical triangles in the neck so it is a pretty
big deal in addition to understanding location after reviewing reviewing the learning objective
texts in this video we should also understand what actions occur with sternocleidomastoid
muscle contraction as well as what innervates these muscles alright so one of the great things
about the sternocleidomastoid muscle is that it is named based on its attachment sites it
is attached inferiorly by what we like to call two heads meaning that there are typically two
tendons there is the more rounded sternal head which is going to be a little bit lower
down here and the thicker and kind of fleshier clavicular head so that's where we're getting
the sterno and the cleido is meaning clavicle the superior attachments include the mastoid process
which would be right about here ish and of the temporal bones of the mastoid processes of the
temporal bone and a thin aponeurosis will extend into the lateral half of the superior nuchal line
of the occipital bone so it will continue back towards this region the sternocleidomastoid muscle
is efferently innervated by the accessory nerve which I'm tracing right here and you can see it
kind of spanning between the sternocleidomastoid which has been reflected here and the trapezius
muscle here it will innervate both of those but before we get into that really what does
efferent mean it is basically synonymous with motor innervation so messages sent from the
central nervous system to an effector organ so in this case we're talking about a muscle
it will send signals which will lead to muscle contraction the accessory nerve is an entirely
afferent nerve even though there is some research it's just a little wishy-washy there but for the
most part most fibers are efferent fibers most typically a muscle is both afferently
and afferently served by the same nerve meaning that that nerve will have both motor and sensory
fibers wrapped up together but this is not the case for the accessory nerve so we need to have
afferent innervation from another source so let's get into the afferent innervation when discussing
sensory or afferent innervation or in regards to muscles this is the means by which the central
nervous system can understand what is going on with that muscle where where is it located is
it moving things like that so a fancy or a more accurate term for this is proprioception alright
so proprioception will convey information to the central nervous system regarding positioning of a
joint or the degree of tension in a tendon so this is a complex concept so this is the best
way I can think to demonstrate it I want you to put your hands behind your back now wiggle your
fingers do you know that this is happening do you know that your muscles are contracting and your
joints are moving to wiggle your fingers this is proprioception so if the accessory nerve is
entirely composed of efferent fibers where does the sternocleidomastoid get us a fair
and innervation well it's from c-2 and c-3 of course so fibers from C 2 and C 3 the C stands
for cervical so the afferent fibers or it's going to have a thin fibers from the second
and third cervical spinal nerves and you can see a little bit of those right in this region
so also kind of entering into the deep portions of the sternocleidomastoid okay what about what
provides oxygenated blood to this muscle what is the arterial supply to the SCM it gets a supply
from numerous small branches kind of throughout its course of the muscle but its dominant supply
is the sternocleidomastoid branch of the occipital artery and the occipital artery is a branch of
your external carotid artery so you got to have that blood supply to have a fully functioning
muscle lastly let's talk about what this muscle can do when it contracts an important concept
regarding muscles is that a muscle must cross a joint to directly affect actions at that joint
so directly meaning that it will directly play a role in moving that joint so just looking at
where this muscle is located and the fact that we know based on its attachments that it is crossing
between the skull and the sternum and clavicle so it's crossing all these cervical vertebrae right
here as well as the cranio-vertebral jointer that atlanto-occipital joint so not only the Atlanta
occipital joint with the cervical inner vertebral joints so what this muscle can do depends on
whether only one sternocleidomastoid muscle is contraction or unilateral contraction or both
sternocleidomastoid muscles are contracting at the same time or bilateral contraction so with
the unilateral contraction you can get lateral flexion that's what this arrow is indicating
here lateral flexion is in the coronal plane think of the head trying to meet the shoulder
of the same side as well as rotation which is what this arrow is indicating so that chin is
kind of moving up so when you put these together you have a head tilt with the chin facing up kind
of think about looking at the ceiling with a bilateral contraction you get flexion of
the cervical vertebral joints ain't touching your chin to your chest it's a flexion or a bending in
that region there are a few other movements of the sternocleidomastoid can play a synergistic
role with but you are not responsible for these alright so that is the sternocleidomastoid muscle
in a nutshell I told you we would need a full video now let's review a concept and this is
a classic true or false question true or false injury to the accessory nerve or cranial nerve
11 would affect proprioception associated with the sternocleidomastoid muscle what do you think
first stop and remember what proprioception is and after that I hope you have landed
on that this statement is false all right the accessory nerve is entirely efferent or motor
proprioception is associated with afferent or sensory innervation which the sternocleidomastoid
muscle receives from fibers of c2 and c3 alright great this was so much fun as you review please
do reach out with any questions and I will see you in the next lecture video have a great day