DIP Episode 147 - The Why’s of Anatomy For The USMLE Step 1 (Upper Limbs 1, Brachial Plexus)

Okay, welcome. My name is Divine, I am a resident and this is episode 147 of the Divine Intervention Podcasts. And in this podcast I'm going to be talking about anatomy. Although to be honest, I think I'm going to call this the wise of anatomy. The thing is anatomy is classically a subject that you know people hate, it's a ton of memorization and all that. And don't get me wrong, anatomy is a ton of memorization. But the thing is anatomy is again unfortunately high yield for step one. And it's also high yield in med school, right? It's one of those things, it's almost like your baptism when you're getting to med school. That's the very first thing most people tend to encounter. So this podcast, I'm going to title it the wise of anatomy and this wise of anatomy series I'm going to be doing. I am going to be focusing primarily on the on the limbs, like the upper limbs and the lower limbs. It's going to be in a series and I will try my very best to make most of these podcasts within the next five to seven days. So hopefully if you're taking a med school anatomy course or you're studying for step one and you need to become a master at MSK anatomy. So basically limbs. Hopefully you accomplish those goals with this. And again, obviously I'm not going to be able to cover everything that's covered in a med school course or anything relating to anatomy.

But I really do think that if you listen to this, it will give you most of the high yield things you'll see on step one and also a lot of the high yields that you'll see on a on a med school exam. A med school like limbs anatomy exam. And furthermore, I would also again, like I said, try to because again, many people think, oh, anatomy anatomy anatomy is all memorization. Yes, there's a lot of memorization. But again, anatomy is also quite mechanistic, especially when you look at it in that fashion. So we're possible. I will try to explain my reasoning behind why certain things obtain in certain ways. So in this podcast, I'm going to start on the upper limbs. And really I'm going to focus for the most part on the brichial plexus. I'll just talk for you know, as long as I can. And then we'll pick up from here in the next part of the series. So the thing is really for the most part, right? The upper limb, if you're thinking in terms of like nerve supply upper limb, you're thinking about the brichial plexus, right? And the thing is the brichial plexus, you know, sort of starts from the C5 to T1 nerve roots of the spinal cord. And if you want to be a little more specific, it's more the anterior remi, right? Because if you think about it, right, these nerves, they break up into like anterior remi and posterior remi, the posterior remi for the most part, they go to the back, but the anterior remi go to the limbs, okay? To your appendages, right? Like your upper extremities.

And the anterior remi of these of, so I said like the brichial plexus is like C5 to T1, the anterior remi of like the C5 and C6 nerves, they kind of become bundled by connective tissue. When they become bundled by connective tissue, they form the upper trunk of the brichial plexus, okay? And the thing is, if you actually look at C8 and T1, the C8 and T1 and tear remi, again, they also bunched up by connective tissue to form the lower trunk of the brichial plexus. The thing is C7, it's kind of does its own thing, kind of chills on its own and becomes the middle trunk of the brichial plexus. Now, as the nerve fibers, right, you know, they keep moving from the neck, they begin to enter the axilla, remember the axilla is like the 5 word for armpit, essentially, you know the thing that smells a lot when you're like in very humid weather. So those shrunks, right, they, they, they, keep going and then they divide to form divisions, okay? And really for the most part, each of those shrunks, right? So like the upper trunk, the middle trunk, the lower trunk, they divide to form anterior and posterior divisions. So you may say, um, divine, who cares? Why does this division into like, oh, each trunk become an anterior posterior division? Like, why does it happen? Well, the thing is, if you think about it, if you think about the process of embryology, right?

The way your upper limbs develop is they essentially begin their life, their lives as like evaginations of like ectoderm from the chest to origin of the body, right? And then these evaginations, they are all ectoderm, they ultimately form the skin. But the whole of these evaginate, they're like ectoderm, ultimately gets filled with misoderm. The thing is, these misoderm cells that fill up that fill up those are ectodermally vaginations, they, um, they kind of sought out into like two big groups, right? There's a group that forms a core, and then there's a group that forms around the core. The thing is, the group around the core, like the misoderm, like the misodermal cells that you know, kind of form that core, they are super, super, super well packed together. And the thing is because they are super well packed together, they sort of stay like dead center, because again, like I said, they are well packed together. It should kind of make sense that these should begin to constitute the bones of the upper extremity, right? So like the humerus, the radius, the owner, stuff like that, right? But on the surface of that dense core that super well packed together, we have, you know, some more misodermal cells that you know, kind of arrange more like in a Lucy, Lucy fashion, right? So they're not arranged like super well together. These Lucy, Lucy, messodermal cells ultimately form muscles.

So if you're kind of thinking of it this way, basically, some of the muscles, they form in front of that dense core, that dense, like again, dense core that ultimately gives rise to the bones of your upper extremity. But there are others that form at the back, they form behind that dense, boni, mesodermal core, okay? That is really for the most part why you have anterior and posterior compartments of the upper extremities, right? And remember, again, anterior compartments for the most part, they are flexors, the posterior compartments for the most part, they're extensors, okay? So that's kind of why these upper middle and lower trunks of the brichial plexus have to divide into, have to divide into anterior and posterior divisions. So that is really why, that is really why you, that is really why you have those anterior and posterior divisions, right? So that is exactly why the dichotomy, from those trunks occurs with the brichial plexus, right? So if you really think about it, any anterior division fiber in the brichial plexus, it really does ultimately, you know, end up intervening in anterior compartment, muscle and I guess vice versa. Now, the thing is, after you form these divisions, right? The fibers, they keep moving through the axilla, these divisions, they, like they ultimately, you know, end up forming structures that are named in reference to the relationships with the axillary artery, right?

So for example, like the anterior divisions of like the upper trunk, they're like this, so say for example, right? You take the anterior division of the, of the upper and middle trunk, it's sort of combined together to form the lateral cord of the brichial plexus. The thing is, the anterior division of the lower trunk, right? It just goes on its and forms the medial cord of the brichial plexus, but the posterior divisions of all those trunks, right? So like posterior division of the upper trunk, posterior division of the middle trunk, posterior division of the lower trunk, they all kind of partner up together to form the posterior cord, okay? So we formed the lateral cord, which comes from the anterior divisions of the upper and middle trunk, we formed the medial cord that comes from the anterior division of the lower trunk, and then we formed the posterior cord that comes from the posterior divisions, all three posterior divisions of the upper middle and lower trunks coming together, okay? And again, the accord lateral, medial and posterior cord, because that's how they are oriented in the arm relative to the axillary artery. Now the thing is, as again, these things continue to progress further, the each cord divides into two, right? So like the lateral cord, it forms the muscular coutinious nerve, and then it gives off a contribution to the medial nerve, right?

The medial cord forms the owner nerve, but also again, like I said, gives off a contribution to the medial nerve, okay? And then the posterior cord essentially just, you know, breaks up to two nerves, like the axillary nerve and the radion nerves. So you essentially have five terminal nerves with the brichial plexus, right? You have five terminal nerves. And so like the muscular coutinious nerve, the owner the medial nerve, the axillary nerve, and the radion nerve, right? And the thing is to make things even easier for you, is you kind of make sense based on the early explanation I gave, that the muscular coutinious nerve, the medial nerve, and the owner nerves should only work with anterior division muscles in the upper extremity, right? Because they come from anterior divisions. Remember this whole thing I talked about, like a few minutes ago, relating to those like bony cords and all that that's why that matters, right? So you can already go right off the bat and say that, oh, you know what? Medial nerve, muscular coutinious, owner nerve, they should do only anterior compartment muscles in the upper extremity. But the problem with, I guess maybe more, little more unfortunately, right? The thing is these terminal branches, they sort of come off too late from the brichial plexus, right? They sort of come off in the arm. The thing is we need some other nerves to essentially be given off along the way, right?

So that we can also like innovate the more proximal muscles of the upper extremities, right? Like those muscles you find around the shoulder, those muscles you find around the axilla, right? That's what unfortunately makes the brichial plexus a lot more complicated than it should be. So I guess let's go ahead and focus on these anterior division terminal nerves, okay? So again, like I said, there's three of them, again, muscular coutinious, median, owner. If you think about it, how many major parts are there in your upper extremity? Just look at your upper extremity. How many major parts are there, right? You have your arm, you have your forearm, and then you have your hand. So it should make approximate sense that these anterior division terminal branches ultimately should hold sway over like the inner vision of the anterior divisions of like the arm, the forearm and the hand respectively. So again, work with me here for a second, right? There are three terminal anterior division derived terminal nerves, right? Muscular coutinious, median and owner. Your upper extremity has three parts. Arm, forearm, and hand, okay? So again, it should make approximate sense that these terminal branches should essentially hold sway in that order in a sense. So like the muscular coutinious nerve dealing with the arm, the median nerve dealing with the forearm, the owner nerve dealing with the hand.

Again, these are rough approximations, but you'll see that just having these tools in your toolbox will make it a lot easier to memorize a lot of this material. So again, keep this construct at the back of your mind. It will help you hopefully realize that the brachial plexus is not as bad as everyone says it is, right? Again, as with most things in med school, if you pursue a course of understanding, it will make the process of memorization a lot easier and a lot more constructive, right? So let's jump into the muscular coutinious nerve, right? So the muscular coutinious nerve it c5 c6. The thing is, it essentially innovates all the muscles that constitute the anterior compartment of the arm, like literally that's it, right? You can already begin to see how this should make your memorization super easy, right? Because if you already know that, oh, the anterior compartment muscles in the upper extremities, they are all for the most part flexors, then you know that if a person leans on the muscular coutinious nerve, that should produce problems with flexion of the arm at the elbow, right? And I mean, if you also, I guess maybe you want to add on some extra knowledge, the muscle coutinious nerve, it also, you know, if you leans on that, you'll also have like issues with like supination, right? Remember supination for the most part is controlled by the biceps bricci muscle. Now, if you jump to the median nerve, right? So the median nerve goes all the way from c5 to t1.

You should hopefully be able to remember that it innervates all the muscles of the anterior compartment of the forearm, okay? Remember your forearm starts at your elbow and progresses to your wrist, right? So your median nerve it innervates all the muscles of the anterior compartment of the forearm, but there are some rare exceptions that I will get to. The way I kind of think about it is, for the forearm, the median nerve does essentially everything with some rare exceptions that go to the owner nerve. For the hand, the owner nerve does most of everything with some rare exceptions that go to the median nerve. That's a nice way to think about that. So really, if you lesion the median nerve, right? That should kind of produce with like wrist and like hand flexion, for example, okay? And again, even if they're exceptions, right? The associated nerves should be a lot easier to remember because if you're again, if you're thinking about if you're dealing with a function of flexion, which is like an anterior compartment function, it has to be an anterior division associated nerve fiber that does the job, right? So really, if you actually think about it, it so happens that the only anterior division term in the nerve that we have not talked about yet is the owner nerve, right? I mean, I have not really mentioned the owner nerve here. I've talked about like the median nerve, I've talked about the muscle cutaneous nerve.

The thing is, those exceptions in the forearm that are not carried over by the median nerve, those actually controlled by the owner nerve, okay? So the owner nerve does those those two exceptions in the forearm that are not taken over by by the median nerve. And those two exceptions are like the innovation to flexor capionaris, right? I mean, it has onaris in the name, so that should tell you that it's innovated by the owner nerve. But also flexor digital, digital proponders, your FDP, okay? It's the at least the the owner half half of that muscle because that muscle belly has it has two parts. It has an owner half and it has a medial half. The owner half is innovated by the owner nerve. The median half, the medial half is innovated by the median nerve, okay? So again, as a general construct, your median nerve for the most part deals with, because think about it, right? Your median nerve sort of runs along your median nerve, sort of runs along the lateral part of your forearm, right? So for the most part, as a general construct, your median nerve deals with the most lateral muscles in the hand, while your owner deals with the most medial muscles in the hand, okay? So your median nerve, it does some things in the hand, but for the most part, the the lot of the rings for your hand is the owner nerve. So again, keep these general constructs in mind and you will very likely not lose your way on on exams.

And I mean, I guess I can go ahead and talk about what the median nerve does in the hand, right? The thing is for the most part, the median nerve, it helps with a thumb opposition, right? So like whenever you see the word, polycus, so P O W L I C I S, the anatomy is referred to the thumb, okay? So opposition of the thumb is done by opponent's polycus, that's done by the median nerve. Also, the thin arm muscles, right? The thin arm muscles are controlled by the median nerve. And again, that should make perfect sense because I just told you earlier that things that are lateral, if you're standing in the anatomic position, for the upper extremities are controlled by the median nerve, things that are medial, for the upper extremities, if you're standing in the anatomic position, are controlled by the owner nerve, okay? And then the median nerve, it also controls some movements of the second and third digits, right? So it essentially helps you like flex your MC Ps, right? So you're met a couple phallanjo joints and then it helps you extend your IP joints, okay? It helps you extend your IP joints, so like your PI Ps and your DI Ps, so your proximal interphallanjo joints and then your distal interphallanjo joints. Essentially your knuckles are your MC Ps, those are your metacapal phallanjo joints and then the next sets of joints that come after your knuckles are your PI Ps, your proximal interphallanjo joints and then after that you have your DI Ps or your distal interphallanjo joints.

Now really those second and third digits, you'll see some anatomies refer to them as the second and third lumbar cults, those second and basically second and third lumbar cult just means second and third digit. Again, please, please, please notice, those your second and third digit, right? So like your index finger and your middle finger, those are the most lateral fingers you have in your hand. Those are like the like the, those are like the fingers that are closest to the lateral side of your hand, closest to the median nerve and those are the forwards the way from the owner nerve, okay? Those are the forwards the way from the owner nerve and please don't forget, right? They're essentially your fingers, you have your thumb and then you have four lumbar cults, okay? You have like lumbar cults, two, three, four and five, those essentially correspond to your other four digits, right? So like lumbar cult two is your index finger, lumbar cult three is your is your middle finger, lumbar cult four is your ring finger and lumbar cult five is your pinky, okay? Really, every, if you just remember all these things I said for the median nerve, every other thing in the hand for the most part from a modal perspective is controlled by the owner nerve. So again, uh, uh, owner nerve again, let's, so let's, so I guess I've sort of talked about like the muscular cutaneous nerve, I've talked about the median nerve, so let me now talk about the owner nerve, right?

So again, let's take it out mantra, right? Owner nerve is CH2 T1. Again, for the most part, it does everything in the interior compartment of the hand, okay? With some ray exceptions, which I already mentioned that are handled by the median nerve, okay? And the good thing about the hand, the hand is actually kind of easy because the hand is actually only has an anterior compartment if you're dealing with motor function. There is no such thing as like post, posterior compartment hand muscle, they essentially do not exist. So if we want to be a little more granular for the owner nerve, right? So the owner nerve actually, you know, it controls the muscles that lie between your metacarpal bones, okay? Uh, the lie between your metacarpal bones, those are called your inter-osius muscles, right? Just look at the name, inter-osius, osius means bone. So if you have muscles that lie between bone, they should be called inter-osius muscles. And the thing is because these muscles kind of lie between your digits, it should really make sense that they either bring those digits together, right? So the AD dot, so AWD, UCT, the AD dot your digits, those are done by your palmar, inter-osius muscles, or they take those digits apart, so they AB, AB dot your digits, right? That's actually done by the dorsal inter-osius muscles. And really there's actually a kind of nice numonic that you've probably heard about like the pad and dab numonics. Those really help you remember those things, right?

So pad, so your palmar inter-osius muscles help with AD dot, your dorsal inter-osius muscles help with AB dot, and then if I just told you that the median nerve does the most lateral lumbaricals, so like lumbaricals 2 and 3, in terms of like flexing at the MCP and extending at your inter-volangial joints, so right? That means like your PI Ps, DI Ps, it should again hopefully make sense that your oner nerve does essentially the same thing, but it does that function for your most medial lumbaricals, right? So like lumbaricals 4 and 5. And then really the oner nerve, it also you know kind of helps with like AD dotting your thumb, right? So like a doctor polycus, so AD dot to your thumb, a doctor polycus does done by the oner nerve, your median nerve helps with opposing the thumb, okay? Again, opponent's polycus. That's really it for the anterior compartment muscles. I mean you have your posterior compartment like terminal nerves, remember for the most part there's just two of those, there's like the axillary nerve, there's the radion nerve, I will talk about those next. In fact, let's go ahead and start with the axillary nerve. So the axillary nerve, right? Kind of like the name tells you axillary, right? For the most part, it's like C5 C6, it works in like the axilla and the proximal arm, okay? Really for the most part, it only does two muscles and actually those two muscles are very easy to remember.

Just remember the security company ADT and you're good to go for the axillary nerve, right? So your axillary nerve, that's the A, does your deltoids, that's the D and then your teris minor muscle. The thing is the deltoids are probably the only ones we really care about because your teris minor, your teris minor muscle like the stuff it does, it's done much better by many of the muscles. So the teris minor, no one really cares that much about it, especially on exams, but you need to remember that it's innervated for sure by the axillary nerve. And really your deltoids, the thing they help you with for the most part is they help you AB docked your arm at the shoulder, okay? Essentially they help you AB docked your arm, so like take your arm away from the body, up to like 110 degrees, right? So like from like 15 degrees to like 110 degrees. And I guess as an aside, you should also remember that your axillary nerve also like control sensation on the lateral surface of the arm. Really every other posterior compartment muscle of the arm and forearm, those all get innervation from the radion nerve, okay? Those all get innervation from the radion nerve, okay?

Again, if you notice I'm not just throwing nins and nins and nins of muscles at you, I'm focused more on concepts for a med student, this is what will ultimately help carry you through med school because med school is a crap ton of information, but if you focus on understanding concepts, it just makes it easier to organize information and commit them to memory. So really for the most part, again, every other posterior compartment muscle of the arm forearm is done by the radion nerve because remember that posterior cord of the brachial plexus only gives rise to two or two two nerves, axillary and radial. And if I've already defined what is done by axillary, then that means every other thing posterior compartment wise is done by the radion nerve, okay? So your radion nerve really for the most part, it takes care of like extension of the arm at the elbow, extension of the forearm at the wrist, extension of the finger joints, okay? And I guess as an aside, right? It actually also controls a supination, also controls a supination. So again, to summarize things, the brachial plexus nerves from C5 to T1, they work in a fairly logical means with respect to innervation, right? Like your more proximal muscles in the upper extremities, they are controlled by C5, C6. The more distal muscles in your upper extremities are controlled by C8, T1. So your shoulder should be primarily C5, C6, your hand should be primarily C8, T1, regardless of what nerve it is, right?

And then for your in-betweeners, right? You can say that like your distal arm and your, remember, your arm goes from your shoulder to your elbow, right? So your distal arm, your proximal forearm, as a rough approximation, should be integrated by like C6 to C7, right? And then you're more like, I guess like your distal forearm and I guess your proximal hand should get like C7, C8. So that's it with regards to motor innervation. And again, I will say some things at the end because you may say, hmm, divine, in my class or whatever I learned about a lot more muscles than you're leading out with, I'll say some things at the end, right? So let's, I guess, sort of jump real quick into the sensory innervation of the upper extremities, right? Sensory innervation upper extremities super easy, right? The big things you want to remember with the upper extremities are that your lateral arm is axillary nerve territory, okay? Sensation for your posterior forearm is radio nerve territory for the most part. But really in, with regards to sensation of the upper extremity, the big, big area that is focused on an exams, like med school exams, the USML step one exam, for the most part is the hand. The sensation in the hand is the big one that shows upon tests. And really the critical thing you want to remember here again because many people say like, oh, divine, I've looked at this super complex diagram. I don't know how to complete, commit all this to memory. Let me give you a trick.

The big thing you want to remember, the critical concept you want to remember is that the middle of your fourth digit, so basically your ring finger serves as a big important boundary. I repeat it again. The middle of your fourth digit serves as a big important boundary. The thing is for sensation of the hand, it makes sense to lengthens by the Palmer surface of the hand, right? So the Palmer surface, you're standing in an atomic position, your Palmer surface is the surface that has all those lines, right? Like the line the surface of your hand, if you may, okay? So we lengthens by the Palmer surface and the Dorsal surface. The Dorsal surface is the side where if you're standing in an atomic position, it sort of faces your back. So for the Palmer surface of your hand, right? If you take the middle of the fourth digit as the boundary, we essentially share sensory innervation between the median nerve and the owner nerve. So it should already begin to make sense that the more lateral side of this boundary should be controlled by your median nerve, sensory-wise, while the more medial side of the boundary should be controlled by the owner nerve. So please, please, I've not been saying median, median, median, median, please. Don't mistake the median nerve as being with the medial surface of the body. That is not the kind of knowledge transfer you want to make on your exam. The medial side of your hand is controlled by the owner nerve.

The lateral side of your hand is controlled by your median nerve, okay? And really for the Dorsal surface, again, if you're using the middle of the fourth digit as your boundary, you essentially share sensory innervation between the owner nerve and the radio nerve, okay? So again, it should make sense that the more lateral side of the boundary should be taken out by the radio nerve, while the more medial side, as I've said for the palm or surface, should be taken out by the owner nerve, right? So essentially, the owner nerve controls sensation on the medial side of the hand, on both the palm and the Dorsal surfaces. But for the palm or surface, the median nerve does the lateral side of that magic boundary, middle of the fourth digit. And then, so your median nerve does the lateral side, on the palm or surface, your radio nerve does the other side, right? Like the lateral side on the Dorsal surface. So again, it should be super easy to remember because again, your median nerve is the anterior compartment like bigwig nerve. Your radio nerve is a posterior compartment like bigwig nerve, right? So it makes sense that the anterior, if you're standing in the anatomical position, your anterior hand, your palm or surface, the lateral side should be controlled by the median nerve. But if you're standing in the anatomical position, posterior surface, that the Dorsal surface of your hand should be controlled by the radio nerve, okay? By the radio nerve, by the radio nerve.

So I guess, before I sort of end things here, I guess one super high yield offshoot I should, I guess, sort of bring to your attention is Kapo-Tonel syndrome, right? Kapo-Tonel syndrome is tested a lot. I'll probably talk about it in a later podcast. But the thing is, Kapo-Tonel syndrome does mod, it's very high to know for exams that it does not affect sensation on the palm. Because the thing is, the branch of the median nerve that does sensation on the palm, at least the lateral surface of the palm, right? Is the branch of the median nerve called the the palm mark, cutaneous branch of the median nerve, okay? The thing is, this nerve actually comes off of the median nerve before going through the, before the median nerve traverses the flexorate tenacolabrit, essentially forms the upper boundary of the carpal of the carpal tonel. So really in the next podcast, I will essentially talk about this same material, although throwing some other details in the next podcast, but I will sort of throw it in a, bring it together in a different format, right? So I'll essentially try to work my way backwards. Kind of think of it like learning Okem, where you know you learn the mechanisms and then you then work your way backwards in the process of like organic synthesis from like product to studying region to whatever.

And again, the main goal of this podcast really for the most part, I wanted to demystify the innervation of the upper extremities, aka the brickelplexes, and sort of help you like lay solid foundation that should make things a lot easier to understand. If you listen to this podcast, and then go back and listen to your class like lectures or whatever, or you study for step one or whatever, you see that, oh, this is actually a lot easier to remember if you're learning all those details, but if you're like learning all those details without like a conceptual framework, I find it just much harder for people to work with. So hopefully this podcast has hopefully, you know, laid a solid foundation for you, may things much easier to understand, and again, remember this stuff is certainly tested on the USM list, step one exam, and the other USM L Is, in a more like I guess in a more limited fashion, but believe it or not, I don't mean shows up in step one, step two, ck and step three. So that's just something to keep at the back of your mind. And as I mentioned at the end of every podcast, I do offer one on one tutoring for all the USM L Is exams, right? So step one, two ck, step two cs, step three.

And then also the preclinical like netzko exams, and the third year clerks yourself shelf exams, and then also this thing I call a longitudinal tutoring, where I, so let's say you had newly means that first year or second year, a tutoring for your like your preclinical exams, and then I tutoring during your dedicated peer for step one, but the thing is as I tutoring during your preclinical exams, I also tutor you for step one with each block, and everyone I've done this with, they've been like wildly successful in the USM L Is, same deal with the third, with the third years, I tutor you for your shelf exams, and then I tutor you during step two ck dedicated period, but as I tutor you for your shelf, I tutor you very specifically for step two ck, and again it has worked out super super well.

And then if you're a college student or you have a college buddy, and you need tutoring for many of these are college exams like general chemistry or gynecchemistry physics, biochemistry, physiology, histology, I do offer tutoring for all those things, and then if you're a med student applying to residency, so like an ERAS application or a college student applying to med school, so like an AMCA application, I do offer like one on one advice, or I guess you could call it consulting for that, so like writing rec letters, assistants with personal statements, editing your applications, mock interviews, I've worked with tons of people on those things, and they've all been again for the vast majority of the people I've worked with, they have done really well. So I do hope you get something from this podcast, I will follow up with another podcast that I've very soon, so have a wonderful rest of your day, and I'll see you next time, God bless you. Thank you.

C5 to T1, specifically focusing on the anterior rami.

C5 and C6.

Due to embryological development; flexor muscles (anterior compartment) are supplied by anterior division fibers, and extensor muscles (posterior compartment) are supplied by posterior division fibers.

Thumb opposition (via opponens pollicis) and controlling the lateral two lumbricals (2nd and 3rd digits).

The ulnar nerve.

Abducting/adducting the arm at the shoulder (Deltoid); C5-C6.

Ulnar nerve.

DELT (Deltoid) and Teres Minor.

Musculocutaneous nerve (Arm), Median nerve (Forearm), Ulnar nerve (Hand).

The middle of the fourth digit (ring finger).

Flexor digitorum profundus (FDP); the ulnar half is supplied by the ulnar nerve, and the median half is supplied by the median nerve.

Carpal Tunnel Syndrome (or Kapo-Torzel syndrome).

Ulnar nerve.

DELT (Deltoid) and Teres Minor.

Musculocutaneous nerve (Arm), Median nerve (Forearm), Ulnar nerve (Hand).

The middle of the fourth digit (ring finger).

Flexor digitorum profundus (FDP); the ulnar half is supplied by the ulnar nerve, and the median half is supplied by the median nerve.

Carpal Tunnel Syndrome (or Kapo-Torzel syndrome).