Hello anatomy friends this is Dr Alsup and in this video we will
be discussing the muscle compartments of the hand.
And they're going to be five muscle compartments in the hand and this area is unique
and the fact that there is a fairly high frequency of both extrinsic as well as intrinsic
hand muscles, and we will discuss the difference between what extrinsic and intrinsic means.
So as I'm fairly sure you understand the hand is the most distal portion of the upper limb
but it's also the most dexterous you can do a lot of small but quite complex movements in this area.
Additionally we have touched on this in a few areas throughout this session but the digits
are numbered for ease of discussion they're going to be numbered 1-5 from lateral to
medial so I always remember you always have to think of the hand in anatomical position
so the thumb is going to be numbered 1 then we go 2, 3, 4 and then the little finger
is 5 and so this would be lateral and this would be medial in anatomical position.
Most intrinsic muscles of the hand are innervated by the ulnar nerve and its branches, recall the
ulnar nerve is the one that travels all the way from the axilla without doing too much it
hits a few muscles in the anterior forearm but the hand is really its place to shine.
But 3.5 muscles are innervated by the median nerve, so most notably the thenar compartment
of the hand if you hear thenar think thumb,
so these three muscles are all innervated by the median nerve and it also innervates two
of the four limbrical muscles in the central compartment the lateral tube.
Before we move to the compartments I want to take a moment to discuss the difference between
extrinsic and intrinsic muscles in general and then specifically regarding the hand.
An extrinsic muscle is a muscle that has a proximal attachment
and muscle belly NOT located in the region it most greatly affects. So for example the forearm
has many many extrinsic muscles of the hand as their proximal attachments and muscle bellies are
not located within the hand they're going to be in the forearm, but their long tendons and even
up past the elbow for those proximal attachments, but their lung tendons are very much located
in the hand and their distal attachments are going all the way towards
say the phalanges so you can see particularly in this case the flexor digitorum profundus tendons
are playing a big role in terms of the hand, so they very much play a role in terms of
the actions of the hand and it's not just in this palmer portion recall that there are all those
extensor tendons on the posterior portion. This differs from the intrinsic hand muscles sometimes
referred to as short muscles. Intrinsic muscles have both their proximal as well as their distal
attachments within the unit it is affecting. And so when we're talking about the hand the proximal
and distal attachments are both located within the hand and will of course affect the actions.
So very prominent,
sorry where am i yeah here we go I'm in the right place. So when we're discussing the
muscle compartments of the hand the the most action or the most I would say bulk in terms of
musculature is on the anterior or palmar side. The posterior side of the hand is almost all tendons
from the forearm and the extensor expansions formed by the extensor digitorum tendons primarily.
The basic setup of the palmar portion of the hand are two eminences and kind of a
depression between. The larger one is the more lateral one and associated with the first digit
and is referred to as the thenar eminence. The hypothenar eminence is laterally located
so and it's smaller and associated with the fifth digit so I think first digit here
fifth digit located if you continue up this way. And in the middle is a central concavity
like a little valley between these two mountains here and the most superficial
thing in this area other than of course skin and superficial fascia and maybe a few veins
is a portion of very thick deep fascia referred to as the palmer aponeurosis.
This will cover as mentioned this is deep fascia it's a thickened set of deep fascia and it will
cover the deeper soft tissues and tendons so an added layer of protection in an area that greatly
interacts with the environment so it needs a bit more protection. If you look down at your own palm
i'm sure you can notice a fairly prominent thenar eminence under your thumb a smaller
eminence under your little finger and a slight concavity between the two. Okay let's get into
the basics of these compartments starting with the big one and the thenar compartment, which is
located right at the base of the thumb. They're going to be three intrinsic hand muscles and one
extrinsic tendon which is the flexor pollicis longus and we will see all of these in lab.
These muscles will only affect the first digit and it's going to allow abduction,
where are you abduction, as well as flexion and the fun one opposition.
You can't see the the opponents here in this image but you can't see it on this illustration.
Opposition is something you hear as being unique to primates and it's a complex
movement meaning it involves movements from more than one joint so both the carpal metacarpal and
the metacarpophalangeal. So it begins with the thumb in an extended position and then it moves
it to an abducted position with the CMC joint and then the thumb is flexed by the MP joint.
As mentioned earlier it is this compartment that is the major exception to the rule
of intrinsic hand muscles being innervated by the ulnar nerve. This compartment is innervated by a
small branch of the median nerve that does branch either within or distal to the carpal tunnel
so can be affected by carpal tunnel syndrome and this is the recurrent branch of the median nerve
and you can see it kind of this itty-bitty thing right here is going to innervate this compartment.
There is a matching but smaller compartment on the opposite medial side called the hypothenar
compartment and they are so matching that the names of the intrinsic muscles are
almost exactly the same as what we had with the thenar but replaces pollicis with digiti minimi
meaning smallest digit. So the actions taking place in this compartment are the same as well
so you're going to have abduction you'll have flexion and you will have a opposition. And this
compartment is innervated as you would expect most intrinsic muscles by ulnar nerve branches.
The adductor compartment is relatively simple as there's only one muscle and the compartment name
tells you exactly what it does it adducts specifically it adducts the first digit so
it's this large muscle right here and it's a little bit deeper you can see a little
bit of it popping out superficially but the majority or the bulk of it's going to be deep.
The adductor compartment that relatively simple one is going to contrast with the
central compartment which is comparatively complex and this is the compartment that
really has all of those flexor long tendons from the forearm, so the flexor digitorum superficialis
and the flexor digitorum profundus remember that flexor digitorum profundus is traveling all the
way to the distal phalanges. So these are quite prominent in this region you can see them here
these are just the profundus the superficialis have been removed you can see them here as well.
Also in this compartment are these funny little worm-like muscles that are unique in
the fact that their proximal attachments are not on bone but instead on a tendon,
and specifically it's these flexor digitorum profundus tendons and these are the lumbricals.
These cross those metacarpophalangeal joints of the digits 2-5 and will play
a role in terms of flexion of those but then they will insert or have a distal attachment on those
extensor expansions on the posterior side of the hand so they'll actually extend the proxima the
PIP and DIP joints or more general generally the IP joints of 2-5 so that the the fact
that it has that attachment on the extensor expansions is the reason why you have a very
big difference between the types of actions that occur here. These muscles are uniquely innervated
the medial two lembracles are innervated by ulnar nerve branches as you would expect most
intrinsic hand muscles to be but the lateral two these two right here are going to be innervated
by the median nerve they're a little bit closer to that thenar region so it makes sense there.
Lastly and deepest there is the interosseous compartment and these literally span the
distance in some areas between the bones specifically those metacarpals
and there are two types they're going to be the dorsal interossei of which there are
four and these are really the only intrinsic muscles that are sometimes
considered part of the posterior hand and there are three palmar interossei muscles.
These are all innervated by the ulnar nerve, excuse me, and these will enact similar actions to those
we discussed for the liberals so flexion of the MP joints an extension of the interphalangeal joints
of digits 2-5 because these also insert on those extensor expansions.
Additionally these muscles play a role in abduction or adduction of the digits. Recall
when discussing the digits of the hand abduction is going to be away from the third digit
and adduction is towards so I think moving away from the third digit
or moving towards the third digit that's why this illustration looks like it's kind of flipping us
off because that is the all-important third digit by which abduction and adduction occur.
So the dorsal interossei are going to be the ones that abduct and the palmar
interossei will adduct I remember this mnemonic by DAB
so dorsal-abs so DAB and PAD
so palmar-add so DAB and PAD help you remember which one does what. And the good thing is that
it's the same for the interosseous of the foot as well so one mnemonic for both. Alright excellent
we have reached the summary slide and we did go over quite a bit so congratulations. I would
suggest starting with the generalizations for this area and then adding in the big picture details
from the compartments next. Thank you for your time and attention and have a great rest of your day