Hello anatomy colleagues this is Dr Alsup and we will be discussing brachial plexus basics in this
video and I know you have been introduced to the brachial plexus in quite a few areas already so
I hope this feels like a nice review and prepares you for dissecting in this area and we are going
to go the whole way we're starting proximally with roots and ending with terminal branches as you can
see here. So we have the five main divisions of the brachial plexus and as we walk through this
I hope you gain a better understanding of what a plexus is in terms of how we start with these five
five spinal nerve VPRs and we get to these terminal branches that are going to be complex
they're going to have contributions from multiple spinal cord levels so you can see
that it's going to go through kind of a mixing and adjoining to get to these terminal branches.
I also hope you find a nice mnemonic that works for you to memorize this order. When I was learning
the brachial plexus the mnemonic was Randy Travis Drinks Cold Beer so Randy Travis Drinks Cold Beer
with instructors lamenting that in their time it had been
Robert Taylor Drinks Cold Beer and me thinking I don't know who that is
so I am likely at that stage of my career where you will need to pick a more
current mnemonic that will stick best in your head so always pick what what works for you.
So let's start with roots as the most proximal portion of the brachial plexus and it's important
to note that these are not by definition roots of a spinal nerve. Recall those are what come directly
off the spinal cord as entirely sensory or motor. So to be clear that is not what the brachial
plexus roots are. The roots of the brachial plexus are the ventral primary rami of spinal nerves c5
through t1. So c5 c6 c7 (so c5 c6 c7) c8 remember there are eight cervical spinal nerves even though
there are only seven cervical vertebrae and t1. So those are going to be the ventral primary rami are
mixed nerves those first kind of main branches coming off the spinal nerve that have both motor
and sensory fibers. And you can see that they are here in the neck region deep to the common carotid
remember this there's your vagus right there and these are a significant size so we are
talking large very observable nerves. The roots usually pass through the gap between the anterior
and middle scalene muscles with the subclavian artery. The trunks are located in the inferior part
of the neck and this is where some of those roots of the brachial plexus unite to form
three trunks. You're gonna have c5 and c6 uniting to form the superior trunk
c7 just continues to become the middle trunk and c8 and t1 will unite to form the inferior trunk.
Every trunk will have an anterior and a posterior division I always find that
this area is usually the most difficult to discern in dissection I often work from chords
more proximally but you're going to figure out what works best for you. Anterior divisions will
end up organizing distally so end up in distal terminal branches that innervate anterior or
flexor compartments so that kind of matches up with its name. And the posterior divisions will
innervate posterior or extensor compartments. All right love chords this is where things
start really getting interesting. We'll start with the easiest one which is the medial chord.
The anterior division of the inferior trunk will just continue on as the medial cord.
And the big one is going to be the posterior cord so all three of the posterior divisions
from each one of the trunks will join to form the posterior cord so makes sense that
it's going to be one of the larger ones. And the anterior divisions of the superior as well as
the middle trunks are going to form the lateral cord. You can see these chords all give off
non-terminal branches you can see this throughout in this image here
that and while we are asking you to memorize these specifics for our content currently if
it comes up in other areas or when we discuss a specific muscle these may come back into play.
The chords are named based on their relationship with the axillary artery in anatomical position
but as we typically dissect with the upper limb in kind of an abducted position as you can see
here they typically don't appear the way their names would suggest. So don't be alarmed there.
So you can see here that lateral cord seems kind of superior the medial cord seems kind
of inferior and that's just how we have to kind of put the arm in terms of dissection.
Last but not least are the terminal branches of the brachial plexus which I have always remembered
with the mnemonic MARMU. We will be talking about each one of these as we discuss muscle
compartments and cutaneous innervation but let's review some big picture concepts here particularly
in terms of what they are derived from and just note we aren't getting into sensory specifics
here we will save that for an upcoming learning objective. All right first is the musculocutaneous
I like to start with this one first, it has the longest name its name implies that it
innervates muscles and skin and it certainly does along with all the other terminal branches.
Specifically the musculocutaneous is going to innervate muscles of the anterior compartment
or the flexor compartment of the arm so like biceps brachii. This is going
to be the terminal branch of the lateral cord it often looks like its continuation.
The axillary nerve is on the smaller end for a terminal branch of the brachial plexus
it is a branch of the posterior cord so coming off that larger posterior
cord and it will importantly innervate the deltoid muscle as well as the teres minor.
So if the axillary nerve is smaller the radial nerve is big it is the biggest terminal branch
of the brachial plexus and that makes sense as it is the terminal branch of that posterior cord
and the posterior cord is large it has a lot to innervate. You will often
hear me say radial nerve equals posterior because it is going to be the one that is
innervating these posterior compartments of both the arm as well as the forearm.
The median nerve is another fun one as it is unique in the fact that it gets contributions
from more than one chord. So from both the lateral as well as from the medial chords and you can see
it then it will form the median nerve kind of have this kind of m shape right here and at the
end of the brachial plexus. It will innervate the majority of the muscles in the anterior
compartment of the forearm so that's key here majority of the anterior compartment of forearm
and five intrinsic hand muscles so some of those small muscles in the hand.
The ulnar nerve is the continuation of the medial cord and it really does travel a
long way before doing much. It is the major innervation for the intrinsic hand muscle
so the majority of the intrinsic hand muscles are innervated by the ulnar nerve
as well as the fact that it hits a few of the muscles in the anterior compartment of the
forearm so minor contribution there. And that is MARMU. Okay excellent thank you for joining
me in this tour of the basics of the brachial plexus, hopefully this served as a nice review
of some of the concepts and maybe some new ones. and you've engaged with this
in neuro and we will continue our discussions as we delve deeper into the musculoskeletal
content. Please do reach out with any questions and I hope you have a great rest of your day