hello anatomy friends this is dr alsup and we are going to continue our discussion
of the muscles of the upper limb focusing on the muscles where of
the arms so specifically the muscles that have their muscle bellies associated with the arm
so these are going to be three major muscles there's one other small muscle
we're not going to get into here so this will be the biceps brachii the brachialis
muscle these are both be anterior and then the triceps brachii which is our posterior muscle
okay so we're getting into compartments
now and we will really hone in on some of the compartments when we get to the forearm region
but we are going to have an anterior and a posterior muscle compartment of the arm and
most the time when we're talking arm or upper limbs when we're talking anterior compartments
we can think of these as predominantly flexors and in the case of the muscles with the muscle
bellies of the arm we're talking about the elbow joint so we are flexing the elbow joint
whereas the posterior compartment of the arm will be associated with extension of the elbow joint
now we'll start with the most superficial muscle in the anterior compartment and that
will be the biceps brachii and the biceps brachii is considered a three joint muscle
and this means that it its tendons will cross three separate joints
and so it's unique in that way and will be unlike the deeper brachialis where the um more proximal
tendons are going to cross the shoulder joint the distal tendon will cross not only the elbow joint
or the humeral ulnar joint but also your proximal radial ulnar joint so these will be the three
joints that this muscle will cross so it can play a role in all three of these at earth
all three of these joints specifically in terms of flexion but also supination so we'll kind of
talk about that right now so really what we typically think of when we think of the
biceps brachii's actions we think of flexion of the forearm reflection of the elbow joint
but since it is crossing the proximal radial ulnar joint and is attaching on the radius
and as we recall the radius is the movable bone when we're talking about those radial ulnar joints
it can also supinate the forearm so a lot of times it gets forgotten that biceps brachii
plays a fairly significant and important role in terms of supination of the forearm
it can also particularly the long head play a small role in terms of flexion of the shoulder
joint or more just kind of keeping that joint in place for some of the more distal joints to have
more powerful contractions so it could feasibly play a role in terms of the shoulder joint as well
the proximal attachments are the two different heads of the biceps and that's where the bi
is getting its name you see the two different heads and two different proximal tendons
both of these will have an attachment on the scapula in different places but both scapula and
then those two heads will combine for one common tendon that will attach on the radial tuberosity
so remember that bump kind of just distal to the head of the humerus that was the radial tuberosity
and it is formed by this attachment of the biceps brachii and again uh recall when we're talking
about pronation and supination is the the radius that's the movable bone so any muscle that plays a
role in terms of either pronation or supination would have to have an attachment on the radius
now deep to the biceps brachii will be the brachialis muscle and you can see its proximal
attachment is right around mid shaft region of the humerus so not crossing the shoulder joint
like what we had with the biceps brachii and it will have its distal attachment
on the ulna or the coronoid process and a little bit distal to the coronoid process of the ulna so
it is not attaching on the radius so it will not play a role in terms of pronation or supination
it will also be innervated by the musculocutaneous nerve the as was the
biceps brachii musculocutaneous is one of the five terminal branches of the brachial plexus and this
is a nice example of how one compartment or this anterior compartment is innervated by one nerve
the musculocutaneous and that will be helpful too as you're kind of trying to compartmentalize
your studying of these areas i always try to think of the compartments for the big pictures
particularly in terms of innervation and that way you're not having to memorize separately
say biceps brachii is musculocutaneous and also brachialis is muscular cutaneous you can kind
of think the anterior compartment of the arm is muscular cutaneous and that will help in terms of
that cognitive load there this muscle will cross the elbow joint and actually
it is going to be the workhorse in terms of flexion of the forearm a lot of times biceps
brachii gets a lot of the glory because that's our flexing muscle that's the one that you can see
but this deeper brachialis is really going to be a powerful flexor of the
forearm and particularly in terms of sustained contraction or sustained flexion of the forearm
okay moving to the posterior arm
or the posterior compartment and really we only have one major muscle back here that we're going
to discuss so when we're thinking the posterior compartment we need to be thinking extension
all right so this will be true in terms of the forearm as well so that'll be helpful
anterior for both arm and forearm think flexion posterior for both the arm and the forearm will
be extension it's just what they're extending or what they're flexing will differ um between where
their tendons are going to insert triceps brachii muscle is a huge muscle uh you can palpate your
own triceps brachii and get a pretty good idea that it's spanning a very large area
as its name would suggest there are three heads there is the long the medial is the one that's
a little bit difficult to see oftentimes you need to reflect these other ones to see it a little bit
better and then the lateral and all three of these will come together similar to what happened with
the biceps so you'll have a common tendon that will have a distal attachment on the olecranon of
the ulna so that point of the elbow the one that's kind of going in and out of the olecranon fossa
that is where that triceps brachii is attaching and it makes sense that the
olecranon of the illness is so large because you have this really large muscle attaching to it
um i'll come back to this in just a moment but just again really the major movement of the
triceps it will be extension of the forearm so extension of the elbow joint even though it can
play a role in terms of extension of the shoulder joint because it does cross the shoulder joint at
least with the long head now here is an important concept that i really hope that we get and we'll
continue to discuss as we move distal but the triceps brachii muscle will be innervated by the
radial nerve and the radial nerve is another one of those terminal branches of the brachial plexus
and what is cool about the radial nerve is that it is the largest of your terminal branches of
the radio uh brachial plexus and it just hangs out posteriorly and that's great because there are a
lot of nerves and there are a lot of muscles but what's really cool is you can kind of at
least for the arm and the forearm and there are a lot of forearm muscles when you think posterior
think radial nerve because it is this radial nerve that is going to be innervating not only
this posterior compartment of the arm but also the posterior compartment of the forearm so i always
have people kind of chant back to me radial nerve equals posterior radial nerve equals posterior
i know that seems a little annoying right now but when you are trying to get your bearings in
terms of muscle compartments it's really cool that radial nerve is doing all that posterior portion
so again i told you i'm not messing around i really want you to make sure you get this concept
so radial nerve which is a terminal branch in fact the largest terminal branch of the brachial plexus
is going to innervate which of the two types of compartments in the upper limb is it anterior
or is it posterior so chant back posterior equals radial nerve right so that's something that i hope
that you understand when we get to the forearm muscle video that's something you're already
gonna know because you've you've gotten gotten it down here all right the anterior compartment's a
little bit trickier you're gonna have different innervations um depending on the arm or the
forearm but what's cool is that both the arm and the forearm posterior compartments are innervated
by the radial nerve all right thank you for your time here we will move on to our next video with
all the many for our muscles fun fun thanks for your time and attention and have a great day