Hello anatomy colleagues this is Dr Alsup and we are going to talk
about the anterior compartment of the forearm, so anterior forearm.
Now I am going to warn you the forearm is complex. There are eight whole muscles in
this anterior compartment and that doesn't even compare to the posterior compartment
and so we'll really focus on the big picture concepts regarding innervation and we'll actually
spend a considerable amount of time talking about actions so strap in and let's get into it.
So let's start with the super duper big picture. This compartment is called the anterior
compartment of the forearm but it is also called the flexor / pronator compartment
which really gets at the main actions that can occur here. So flexion of the joints of the wrists
and the digits as well as pronation of the radioulnar joints. As a general rule with of course a
few exceptions the more superficially placed the muscle is this will have a greater effect on the
more proximal joints like the radiocarpal or wrist joints and the radial ulnar so superficial muscles
usually are these more superficial joints whereas the deeper muscles will affect the
more distal joints so the ones out here in terms of like say the interphalangeal joints.
If I were to give you a bird's eye view regarding actions
as mentioned think flexion of joints associated with the wrist and hand and
pronation of those radiocarpal joints both those the proximal and the distal radial ulnar joints.
The median nerve is the predominant supply for this compartment so I put three stars next to
it you know I'm serious here but I do want to note that there is some contribution from
the ulnar nerve so a much smaller role only in terms of one and a half muscles.
Okay let's start to talk about these layers in a bit more detail starting as you might
imagine with the superficial layer. There are four muscles in the superficial layer.
The first is the pronator teres which is always the one I try to find in dissection
first to help guide me in differentiating the others. You'll have the flexor carpi radialis,
the palmaris longus which we won't get into too much detail there, and the flexor carpi ulnaris.
As you can see in this image these muscles are more located on the medial side so if I kind
of outlined it here they're really more on the medial side of the anterior portion of the arm.
This lateral portion over here of the anterior side is actually occupied by the brachioradialis
muscle which is typically grouped in with the posterior compartment based on its innervation so
we'll talk about this muscle in another learning objective. So this arrangement trips people up
they try to make this muscle be an anterior compartment muscle but this is brachioradialis.
All four of these muscles in the superficial layer
are going to have a proximal attachment on the medial epicondyle
which you can see here and it can be palpated on the medial side of your elbow region and
the attachment of these muscles is through a common flexor tendon for all of these muscles.
The actions of these muscles are more associated with the more proximal joints
as we mentioned these are the superficial ones and if you pay attention to the names
they tell you exactly what actions and what joints they can affect so if you're looking at the flexor
carpi radialis and flexor carpi ulnaris these will flex the carpus or wrist
region so they'll flex the wrist or radiocarpal joint just as its name would suggest.
The pronator teres will pronate the radioulnar joints so pronator will pronate
so that kind of helps out right off the bat there. Additionally the flexor carpi ulnaris
excuse me the flexor carpi radialis will abduct the wrist so if you put your palm in anatomical
position and move your hand away from the body this is abduction or radial deviation of the hand
via the wrist joint. Conversely the flexor carpi ulnaris will
adduct the wrist or bring it back towards the body, this is also known as ulnar deviation.
All of these muscles are innervated by the median nerve except the flexor carpi ulnaris
which is innervated I don't want to do that here here we go flexor carpi ulnaris
is going to be innervated by the ulnar nerve I always think
it has ulnaris right there in its name and it tells you what it is innervated by in this case.
Moving to the intermediate layer and this one is a bit easier to remember as there is
only one muscle here which sometimes you see lumped in with the superficial layer
since it has a proximal attachment also on the medial epicondyle
and this is going to be the flexor digitorum superficialis or the FDS so this is a big
muscle. The distal tendons, well before I get into that before I get into the distal tendons I
do want to note that I agree with the intermediate classification of the FDS as its belly
is definitely deep to the superficial layer so you can see a little bit of it here a little
bit of it here a little bit of it here all deep to these tendons of the superficial layer.
The distal tendons of the flexor digitorum superficialis are long tendons that traverse
the carpal tunnel so the carpal tunnel will be right around this region here
more to come on this on the carpal tunnel in an upcoming learning objective.
So the FDS tendons are going to traverse the carpal tunnel and will attach to the intermediate
phalanges of digits two through five meaning it crosses all the joints on the way to its most
distal joint crossed the proximal interphalangeal joints or the PIPs and it is on these PIP joints
of digits 2 through 5 that the most notable actions of the flexor digitorum superficialis
occurs so that will be flexion of the proximal interphalangeal joints or the PIPs.
But note it can affect the actions of any of the joints it crosses along the way playing a smaller
role in flexion of say the wrist joint and the metacarpophalangeal joints of 2 through 5.
The deep layer has a very similar muscle to the flexor digitorum superficialis which is the
flexor digitorum profundus. So you have the flexor digitorum profundus so obviously we're looking at
a deeper dissection here on this illustration. It also has four long tendons that will also
traverse the carpal tunnel so the carpal tunnel has been open in this particular illustration
and it will and the flexor digitorum profundus tendons distally attach all the way on oops
excuse me the distal phalanges so remember these are the distal phalanges all the way
at the end so it will attach on the distal phalanges of 2 through 5. So it will cause
flexion of those DIP joints so remember the DIP joints are between the intermediate phalanges
and the distal phalanges in fact the flexor digitorum profundus muscle is the only muscle
that can flex the DIP joints. Another muscle in this layer is the flexor pollicis longus
and this will be the first of many muscles that are solely affecting joints associated with the
first digit or thumb which makes sense that the thumb would have a lot of muscles associated with
it due to its unique movements it's capable of. The flexor pollicis longus tendon actually
also traverses the carpal tunnel you can see it a little bit heading that way there
and it will distally attach to the distal phalanx which you can't see too well here it will attach
all the way here in terms of the distal phalanx of digit one. So it will cause flexion of the first
interphalangeal joint remember that there's not a PIP or a DIP joint in the first digit
only an interphalangeal joint as there is not an intermediate phalange in the first digit.
Lastly but not least there is the pronator quadratus which is actually the deepest muscle
in this compartment so it's even deeper than those that it shares the deep layer with
and the pronator quadratus is an exception to the rule
as this is a deep muscle actually affecting a more proximal joint so it's going to be the prime mover
for pronation of the radioulnar joints. The pronator teres is more important for pronation
that requires additional speed or power but it's the quadratus that is doing the bulk of the work.
Returning to the flexor digitorum profundus it is unique in that it has innervation from two nerves.
If I were to kind of split this muscle in half
it's more lateral portion or it's more lateral half is innervated by the median nerve
like most the other muscles in the anterior compartment and the medial half is going to
be innervated by the ulnar nerve. All right excellent we have reached the summary slide and as
i mentioned at the beginning this is certainly a bit denser of a learning objective and will likely
take a little longer to fully digest all of this content. I would start with these generalizations
for the whole compartment and kind of work out from there, so kind of adding in detail as you can.
As always as you are working through the material feel free to reach out to me or any
of my anatomy colleagues with questions. Thank you for your time and attention