This is the cervical plexus, a network of nerves that branch out from the spinal cord between the first and fourth cervical vertebrae. They help relay sensory information from big swaths of skin, including most structures from your scalp to your upper back and shoulders. They also send motor signals down to a lot of the muscles in that area too, and even go as far as the diaphragm. But it’s also a unique plexus because of how it interacts with the cranial nerves, those nerves that branch directly off the brain. So in today’s video, we’ll learn how this plexus is laid out and what structures it innervates. [This video was sponsored by Kenhub] Hello and welcome. If you’re new here, my name is Patrick and this channel is all about anatomy and physiology. When we’re talking about the cervical plexus (just like when we’re talking about any of the other plexuses) we’re focused on the ventral rami of the spinal nerves. As a refresher, each spinal nerve that pokes out from the spinal cord has a ventral, or anterior, ramus and a dorsal ramus, which is posterior. In our diagram of the cervical plexus, we can easily see the posterior rami — they’re the little nubs on top of each spinal nerve. The ventral ramus is the rest of that root. When naming those spinal roots, the cervical vertebrae have a pretty unique naming system. The spinal root is named after the vertebra it’s superior to. So spinal root C1 pokes out superior to vertebra C1, between it and the base of the skull. Spinal root C2 pokes out between C1 and C2, and so on. In the last video, I gave the analogy that you can think of the spinal roots as flowers that are named after the flower pot that they’re planted in (with the pot being the vertebra). As we’ll see in this video, that’s not a perfect analogy, because there are vines and loops, and maybe it’s more like a trellis than a flower pot necessarily. But this analogy will get you started Let’s orientate ourselves real quick. The entire plexus is deep to the sternocleidomastoid muscle, that big superficial muscle that you can trace from behind your ear to your sternum. In this picture we can see an anterior view of the cervical plexus in green, with the brachial plexus inferior to that. We’ve got the base of the skull up top, and while I know it kinda looks like there’s another vertebrae there, that’s actually part of the occiput of the skull, called the occipital condyle. It articulates with cervical vertebra 1, what’s also called the Atlas. And together, they make up the atlanto-occipital joint — the joint between the atlas and occiput. Inferior to that, we have cervical vertebra 2, also known as the axis, then vertebrae C3 and C4. Any lower than that and we’re getting into brachial plexus territory. So that’s our bony structure. But since we’re interested in where all those nerves are heading, we typically study the cervical plexus with this view, something that’s a little easier to take in. We’ve got the base of the skull up top, with each of these bones representing cervical vertebrae 1 2 3, 4, and 5. When I’m learning any structure in anatomy I need to break up the big bits of information into smaller bits of information. So that’s what I’m going to do with this diagram. My first chunk of information I call The Trimmings. Like, this diagram has some big features that we’ll get into, but it’s got some nerves that look like thorns on a rose stem. Like those 2 little nubs that poke out anteriorly on C2, C3, and C4. Those are actually motor nerves that innervate some of the smaller neck muscles called longus capitis and longus colli. I covered this in my muscles video, but the naming tells us that these muscles are long muscles that insert on the head for capitis, and neck for colli. Again, video linked here and in the description. Other things that could be trimmings include the two longer nerves branching off of C3 and C4. Those nerves innervate the scalene muscles and levator scapulae muscles. And finally, before getting into the bigger guys, those little branches off that loop between C1 and C2. Those are more tiny muscle innervators, going to rectus capitis lateralis, longus capitis, and rectus capitis anterior. Again, the muscle name tells us that they’ll insert on the skull. All in all, the Trimmings group is made up entirely of motor nerves that innervate quite a few of the smaller cervical muscles, but also some of the big ones. The next group I call the Hypoglossal and Handle, which will make sense in a second. Originating superior to the cervical plexus, we see the hypoglossal nerve, which is cranial nerve 12. Glossal is the anatomy word for “tongue”, and the hypoglossal nerve is mostly responsible for controlling the movement of the tongue itself and all these muscles here. I don’t want to get too deep in the weeds here since I already made a cranial nerves video, but since those little nubs overlap with that branch of spinal nerve C1, we’ll cover those before moving on. The nub that’s closest to the split innervates the thyrohyoid muscle while the next one innervates the geniohyoid. We learn the hypoglossal nerve because it merges with one of the coolest features of the cervical plexus, the Ansa cervicalis, which translates to “handle of the neck”. That’s where the group name comes in — hypoglossal, handle. The ansa cervicalis is a loop made up of branches from spinal nerves C1, C2, and C3, plus that contribution from the hypoglossal nerve. The portion made from C1 C2 is the superior root, while C2 and 3 make up the inferior root. Those C1 and 2 fibers merge with the hypoglossal for a bit, sort of tuck under the posterior belly of the digastric muscle, then split inferiorly and anteriorly to become the superior root of the ansa. Now, the whole ansa cervicalis is an anterior structure, so it has loads of motor branches. And the superior root gives off branches to a few muscles, including three of the four infrahyoid muscles — the muscles inferior to the hyoid bone right there. Going from superior to posterior, the first branch is the superior omohyoid muscle, which connects the hyoid bone to the shoulder (that’s the Omo- root). The second is the sternothyroid, which connects the thyroid to the sternum. The third is the sternohyoid, which connects the hyoid to the sternum. Finally, that last branch is the inferior innervation of the omohyoid muscle. So in the loop we can use the acronym OST SHO or O-S-T-S-H-O for omohyoid, sternothyroid, sternohyoid, omohyoid. Now I mentioned that the ansa cervicalis has branches of 3 of the 4 infrahyoids. The fourth was the thyrohyoid, which we saw was innervated by a branch of the hypoglossal nerve, so we’re in the same neighborhood at least. So let’s review our chunks so far. We have The Trimmings and the Hypoglossal and Handle. The next chunk doesn’t get a cool name. It’s just the remaining motor nerves. The good news is that they innervate some of the more well known muscles, so I’m going to call this chunk the Big Muscle chunk. The first one branches off of spinal nerves C1, 2 and 3. And it kind of looks like it’s passing through the loop, but it’s actually off to innervate the sternocleidomastoid muscle. This one is logical since so much of the cervical plexus is defined by its relationship to the SCM, so it makes sense that the one in the middle of the plexus would innervate the reference to the plexus. The next fully formed nerve inferior to the ansa innervates the trapezius muscle. But here’s the unique part. Cranial nerve 11, the spinal accessory nerve, also comes down to innervate the trap muscles. At some point, that branch from the cervical plexus communicates with, or joins up with, a branch of the accessory nerve to contribute to trapezius innervation. Anatomists still aren’t 100% sure about how much the cervical plexus helps the accessory nerve with trap innervation — sometimes only C2 contributes, sometimes C3 and 4 only. The point is, there’s variation. The most inferior of the nerves in this chunk is the Phrenic nerve, which innervates the diaphragm muscle. You can remember this one since phrenic shares some word overlap with diaphragm, which is how I remember it. The phrenic nerve is made from branches from cervical spinal nerves C3, C4, and some of C5, then heads straight down to the diaphragm. But it’s doing a bunch of stuff on its way down south. It gives off some sensory branches to some of the connective tissue in the thoracic cavity, including the pericardium, the connective tissue around your heart. And since the heart isn’t symmetrical, the right and left phrenic nerves aren’t totally even either. Once it pokes through the diaphragm, it provides more sensory innervation to the connective tissue and central tendon of the diaphragm. But its main role is the motor innervation of the diaphragm, which is the most important muscle for breathing. This nerve also lends itself to one of the worst pickup lines in anatomy — “Did you just lacerate my phrenic nerve? Because you take my breath away. If you want a more practical mnemonic for the phrenic nerve, I’ve heard “branching off C3, 4, 5, this is the nerve that keeps you alive”. I like mine better, but whatever. Our final chunk is a group of 4 sensory nerves that branch off of C2, C3, and C4. They’re the Lesser occipital nerve, Greater auricular nerve, Transverse cervical nerve, and Supraclavicular nerves. The lesser occipital nerve branches off of C2 and 3, and runs superiorly towards the back of the skull, which is known as the occiput. From there, some of its branches hook up with the main occipital nerve, while others innervate the skin on the back of the head and a bit behind the ear. Inferior to the lesser occipital, the great auricular nerve branches off and also runs upwards towards the head. As the name implies, the great auricular nerve innervates the area around the auricle, which is the main fleshy part of the ear, but it also lays on top of the parotid gland and the mastoid process. Next is the transverse cervical nerve. As the name implies, this one is going to transverse, or go across the cervical region, or neck. To me, this one kind of looks like a choker from the 90s. But more technically, it pops out from spinal nerves C2 and 3, behind the sternocleidomastoid, then goes anteriorly. From there, it handles sensory innervation for that anterolateral area of the neck. A little further down we’ve got the last of our purely sensory nerves, the supraclavicular nerve. Now the name is a bit misleading. While some of its branches run across the superior clavicle, it also branches out laterally and innervates the rest of the shoulder, even some posterior structures. This one branches off of a junction from spinal nerves C3 and 4, then heads inferiorly. The last thing to point out is that each of those spinal roots has a branch that communicates with the sympathetic nervous system. You know the whole “fight or flight” response thing? That’s an automated response handled by your sympathetic nervous system, which has anatomy of its own. Without going too into it, the sympathetic nervous system is made up of only motor nerves in the peripheral nervous system. And those little branches that we see when studying the cervical plexus all come down from the superior cervical ganglion, which is just a cluster of neuronal cell bodies. You can see the connections to the cervical plexus easily on some diagrams, like this one from Grays. But some are so simplified, you don’t have anything to identify. So putting it all together, we have 4 chunks that help us remember the cervical plexus — the trimmings, hypoglossal and handle, big muscle, and the sensory nerves. If I were taking a quiz where I was identifying features of the cervical plexus, I’d first identify the boundaries of the structure itself. So find the skull, then cervical vertebrae 1-4, knowing that spinal roots C1-4 are the ones we care about. Next, pick out the hypoglossal nerve so that you don’t confuse it with the rest of the plexus. Next, identify the ansa cervicalis and the four infrahyoid nerves with the mnemic OST SHO. From there, identify the big muscular branches to the sternocleidomastoid, to the trapezius, and the phrenic nerves. Now that you’ve got all the motor branches accounted for, you can tackle the sensory only nerves. Use the mnemonic OATS for that one — lesser occipital, great auricular, transverse cervical, and supraclavicular. And there you go! You’ve got the entire cervical plexus. Now, if you want to get some hands on practice identifying the different nerves, then you need to check out Kenhub. Kenhub is an anatomy learning platform that I use all the time when writing and fact checking these videos. They've got an enormous library of in-depth videos and articles about gross anatomy, histology, blood vessels and everything else you'll learn in anatomy class. All the beautiful illustrations you saw in this video came from KenHub's atlas, which often comes with multiple angles and illustrations of each structure, as well as cadaver images and cross sections. In addition to their library of content, I love Kenhub's quiz feature. They allow you to take different types of quizzes with varying difficulties, and they'll give you personalized feedback so you can figure out your weaknesses and quiz on just that material.You can also create your own custom quiz by selecting the structures and topics you want to be quizzed on. You can use most of Kenhub's features for free, but if you want full access to all of their content and quizzes, and there's A LOT of it, you can go to khub.me/corporis for 10% off your subscription. They've also got a no questions asked 7 day money back guarantee, so you can try out the premium version of 7 days and if you don't like it for whatever reason, you can get your money back. Thanks to Kenhub for sponsoring this series. This is my second installment in the plexus series. You can check out the rest of the paylist right here, as well as some of my other nervous system videos here. Thank you to my patrons on Patreon. 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