Some Say Marry Money But My Brother Says Big something Matters More… What was that last B…Big boots, no… big books… no. Big brains, that’s it. Get your head out of the gutter, todau we’re learning the cranial nerves. [intro] Welcome back to another anatomy video, my name is Patrick, and just like you I remember sitting in class, trying to come up with a clever, usuually inappropriate way to remember these cranial nerves. But if I’m honest with myself, I never really understood why I was learning them. I was just like “Ohh ohh ohh to touch and feel…” and did the menomenics and passed the test So my friends, by the end of this video you’ll have a couple mnemonics to help you out on the test, but more importantly you’ll understand /why/ these nerves are so important. As always, here’s the big picture. As the name implies, the cranial nerves come off of your brain and brain stem unlike the spinal nerves you're probably familiar with. Some of them poke out the anterior of your brain while some of them have branches that loop all the way down into your chest. And the majority of them are involved in vital senses like sight and hearing, so as a clinician, there’s a good chance you’ll have to do a cranial nerve exam at some point. Like when I worked in sports medicine, we’d always do a cranial nerve exam as part of our concussion assessments. So let’s get you familiarized here. Here’s the lateral view of your brain. This is the front or anterior, this is the back or posterior. But this doesn’t give us a super clear picture of the anatomy of interest, so we flip the skull side down and get a bottoms-up view. And now that we can see them, we assign them Roman numerals based on their position from anterior to posterior. We’ll go through each nerve’s name and function, but /now/ we can get our front to back, logical numbering and ordering done. Okay, throughout this explanation, remember that anatomists are terrible at naming things. Most of the time, the name is gonna be a giveaway for the function of the nerve itself. The second thing is that some of these are exclusively sensory, exclusively motor nerves, or both. So I’ll keep a running abbreviation over here with the letter you’ll use for the naming acronym and whether it’s a sensory nerve, motor, or both. Cranial nerve number 1, again going anterior to posterior is the olfactory nerve and it’s responsible for the sense of smell. And look at it, look at this thing. It /looks/ like it’s supposed to go down your nose, right? It’s double true when you see where those big olfactory bulbs land in your skull. They go right on top of your nose where they shoot a bunch of smaller nerves down into the nose that then pick up on those odorants. If you had to test this clinically, obviously you’re gonna have someone smell something nice and familiar. Hot coffee usually works since everyone knows what it smells like and it’s not gonna give anyone a headache. Cranial nerve number 2 is the optic nerve, and it’s responsible for your sense of sight. Unfortunately when you’re studying that inferior to superior view, that illustration doesn’t do this nerve justice. Look, your eyes are incredibly important and deal when it comes to dealing with a lot of incoming information, right? So it totally makes sense that they’d have a big thick nerve that connects the sensory receptors of the eye directly to the brain. On that illustration, that little nub of a nerve doesn’t look as important as what’s really happening. But with the eyeballs still attached, the optic nerve finally looks like a big important nerve that transmits sight back to the brain. You can actually see the optic nerve if you look longingly into the eyes of your patient, shine a bright light in there and find that spot at the back of their eye. You’re looking at their optic nerves. The optic nerve is exclusively a sensory nerve — the next few will be motor nerves for the eye. Cranial nerve 3 is the big eyeball mover, the oculomotor nerve. And it's responsible for /most/ of the movement of the eye. Muscles that make the eye look up, down, and in and even raise the eyelid and constrict the pupil. But that leaves a couple of movements out. That’s where cranial nerves 4 and 6 come in. The trochlear nerve, cranial nerve 4 handles this super specific muscle called the superior oblique muscle, this dude right here. Trochlea comes from the Latin root for “pulley” and when you look at this muscle, it acts totally like a pulley. Now, assuming you’re not a total etymology dork, "trochlea" isn’t gonna come as easy as the oculomotor nerve, so here’s how I remember it. When you test this nerve, you’d have the patient rotate their eye downward and side to side. Like they’re looking down at a track, which is the closest thing you’re gonna get to trochlear. That’s right, come to this channel for anatomy topics and stay for the terrible memory devices. Now, to make things more complicated, the oculomotor nerve only innervates one of side to side eye movers, the medial rectus muscles. So cranial nerve 6, the abducens nerve innervates the /lateral/ rectus muscles. So if you’re already good with your terminology of motion, you know that abduction of a joint is when you lift a limb out to the side. Obviously, that doesn’t happen with your eyes, so I remember this one because of the overlap with abduction and lateral movement. On a clinical test though, most clinicians will do the “follow my finger with your eyes” test to take care of cranial nerves 3, 4, and 6 at the same time. Coming back to Cranial nerve number 5, it’s our first double dipper. It’s both sensory and motor. And /that’s/ because this guy, the trigeminal nerve is actually three nerves, hence tri-geminal. The first branch is the ophthalmic nerve which is the sensory nerve for the front top part of your face, the maxillary nerve which senses everything from your upper palate, upper lip, and nasal cavity. Finally is the mandibular nerve, the biggest branch of the three which innervates the mandible or the jaw. The motor part of this nerve comes from the mandibular branch. If you have someone bite down on something like a tongue compressor, their masseter and temporalis muscles will flex, which is usually your test for that. So because your trigeminal nerve splits into multiple branches and does a couple of jobs, it counts as both sensory and motor. Cranial nerve 7 is a doozie, it’s the facial nerve and it’s another big branching nerve root. Now, our trigeminal nerve already took care of a couple of motor functions around the face, but any complex facial expressions are made by muscles innervated by the facial nerve. But check this out, cranial nerve 7 is a Both. We just heard about its motor functions, but its sensory innervation is so frustrating I hate it. It’s responsible for taste only on the anterior two-thirds of the tongue. And because of that, you have to remember it as a Both. Cranial nerve 8 is the vestibulocochlear and it’s responsible for your sense of hearing, which totally makes sense. We’re all the way down at nerve number 8 by now, so that nerve root is close to the ear. Cranial nerve number 8 is exclusively a sensory nerve which makes sense — unlike dogs, humans don’t need to move their ears, so we don’t have a ton of musculature around them to do that movement. There /are/ some very tiny muscles within and around the ear, but they’re innervated by other nerves like the facial nerve. Couple of ways to remember this one. I recommend getting your lab mate, look directly at their ear, lean in and whisper vestibulocochlear. Then they slap you in the face and the ringing in your ears will be an ever present reminder that vestibulocochlear is responsible for hearing. Cranial nerve number nine is the glossopharyngeal nerve, which takes care of sensation for the back third of the tongue through some of the oropharynx /and/ motor innervation of one specific muscle. There was a lot there, so let’s break it down. Anytime you see that root glosso think tongue. Easy to remember since the tongue is covered in saliva, so it’s glossy looking. The rest of the area innervated by the glossopharyngeal is the pharyngeal part. Students trip up thinking that the pharynx is like way down your neck, but it’s actually right below your tongue. It’s what connects your mouth and nasal cavity to your tongue. Now this thing also controls one tiny muscle, the stylopharyngeus muscle — this dude right here. Its called that cuz it connects the styloid process near your ear down to this super wide area in your throat. Another tiny motor function, but it still qualifies cranial nerve 9 as a both. Next up is the Vagus nerve, or number 10 — and it’s the absolute superstar of the cranial nerves since it takes care of some like, life or death oper ations. The Vagus nerve is the main nerve of the parasympathetic nervous system. And if you’ve never heard that word before, go check out my video on it right there. Quick oversimplified answer though: the parasympathetic nervous system calms down a lot of processes while the sympathetic nervous system excites them. So if you wanted your heart rate to speed up, that’s sympathetic, if you wanted it to come back down, that’s parasympathetic, and where the vagus nerve comes through. Vagus nerve is a long boi and has a bunch of different branches, some dipping down through the diaphragm and into the stomach. Either way, it’s a big sensory interface between organs like the lungs and stomach back up to the brain, but it also helps slow down heart rate. The right branch of the vagus nerve partially innervates the sinoatrial node, or SA node, in your heart. This is your major built in pacemaker that keeps your heart pumping around 60 beats per minute. Cranial nerve eleven is the accessory nerve and it’s the motor innervation for your upper traps and sternocleidomastoid. I remember this one because you take your accessories like your purse or backpack, throw it over your shoulder and have to use your accessory nerve to adjust it. Cranial nerve 12 is the hypoglossal nerve, and since you have a beautiful mind you remember that glosso- means tongue, you’re already thinking this must be the motor control of the tongue. So that’s everything from up down and side to side, which you could use to make an H and remember hypoglossal. Now, very importantly, there /is/ a little controversy on whether we’re finished there or not. While I was never taught this in school, enough of the sources I read mentioned Cranial nerve 0, the terminal nerve. It’s this little bundle of nerves near, but not attached to, the olfactory nerve. And if we’re comparing it to the terminal nerve in other animals, it’s the nerve that would let us sniff out pheromones. These are chemicals that, in other animals help regulate sexual behavior but in humans are almost certainly not a thing. But it might have a role in gonadotropin releasing hormone regulation, so it’s worth knowing. Look, you’re probably not gonna get tested on this one and you’re not gonna use it during a clinical exam. Either way, it’s a fun fact. So that’s all 12 or 13 cranial nerves, depending on if we’re counting nerve zero. To help you remember them, you have nearly infinite mnemonics to use. My undergrad professor insisted we use “On old Olympic towering tops, a French and German viewed some hops” which ... That worked for his brain, but never stuck in mine. Plus, some of the memory devices use slightly different name variations. Like my professor called the accessory nerve the spinal-accessory nerve, which changes the acronym So I recommend reading a bunch of them and figuring out which one genuinely sticks better for you. Some are family friendly, some are so dirty they make me want to clean my eyeballs with bleach. I’ll link to the Wikipedia dedicated to cranial nerve mnemonics down in the dooblydoo, but if you’ve got a favorite, write it in the comments and we’ll upvote our favorites. Pro tip, you’ll probably know which day your cranial nerves test is going to be on ahead of time, so when you go into your test with your mnemonic locked and loaded in your brain, write it out in the margins so you can come back to it and not freak out when you’re mid way through the test and under pressure. A lot of these cranial nerves are the input point for four of your five senses, so I recommend checking my nervous system playlist here if you want a deep dive into how those senses work. A big thank you goes out to my Patrons Diana and Jessica for making these videos happen. If you want to join them, click the link there. Otherwise you can support the channel for free by subscribing and liking the video. Have fun, be good. Thanks for watching.