 Hey everybody, Dr. O here. This is a two-part series where I'm going to cover the 12 pairs of cranial nerves. So like I said, there are 12 pairs of cranial nerves. They're usually going to be named using Roman numerals, so I think it's very important that you know the Roman numerals, 1 through 12. That'll be very helpful for you. Cranial nerves can be one of three types. They can be purely sensory, whether it's just somatic sensors or special sensors doesn't matter. They can be purely sensory, they can be motor, or they can be what's called a mixed nerve, which is going to have both a sensory and a motor component. Before we go through the cranial nerves, let me give you the mnemonic device that I used growing up. I was 16 years old when I first learned this and I still remember it, so it seems to work. The mnemonic device that I used, it was on old Olympus towering tops, a freeloading vagrant German view to hop. Please no offense to Germans, it's just how we fit it together. So on old Olympus towering tops, a freeloading vagrant German view to hop. So each of those words is the first letter of the 12 pairs of cranial nerves. You may see some slight variation, like I'll put up on the screen here, on old Olympus towering tops, a fin and German viewed sum hops. So the difference is there is that the vestibulococlear nerve, V, can also be called the auditory nerve, but you don't see that very often. And then also the accessory nerve can be called the spinal accessory nerve. So just keep in mind, you're going to see different mnemonic devices based on the terminology used. Okay, let's go ahead and jump in. So here we see cranial nerve one, the olfactory nerve. This is a sensory nerve involved in your sense of smell, like the name implies. So it starts here with about 20 bundles of nerves and their receptors there in the olfactory epithelium, those nerve, so when a signal gets sent to them, they're going to send that signal up through the olfactory foramina or tiny holes in the crib reform plate of the ethmoid bone, and that's going to travel up to the olfactory bulb and then it's going to work its way back to the temporal lobe, where your olfactory centers are. Anything else about that? These openings, like just a little aside here, these openings are how the brain eating amoeba can get into your brain. That's why water has to be driven into your nose to get infected with the brain eating amoeba. Trust me, you don't want to do that. So if you're testing the function of cranial nerve one, you just have a patient smell things. I would have them smell coffee and other things that had pretty strong smells, and if they could smell them and recognize them, then cranial nerve one was working. Okay, excuse me. Cranial nerve two, the optic nerve. So this is going to be the sensory nerve involved in vision. So the origin of this nerve is the retina. The retina is where the photoreceptors are that allow you to see. And information from the optic nerve, so the optic nerve itself, each optic nerve is going to be a bundle of about a million sensory nerves, so you have these two bundles of one million sensory neurons traveling from the eye as the optic nerve. And at the optic chiasm, you see that some of them cross, and now it's called the optic tract. It travels to the thalamus, specifically the lateral geniculate nucleus of the thalamus, and then back to the occipital lobe. If you want to test the function of the optic nerve, then you have someone look at something, right? Like a, like a snail and eye chart or have them read the paper, these types of things that tells you that they can see. So the optic nerve is functional. Cranial nerve three is the ocular motor nerve. So like the name implies, motor to the eye. So this is going to be a motor neuron, and it's going to control, let's jump ahead here, it's going to control four of the six extrinsic eye muscles as well as your intrinsic eye muscles. So it controls the superior rectus, inferior rectus, medial rectus, and inferior oblique muscle. So it controls four of the six extrinsic eye muscles that control the movement of the eye. And then when I say, so what kind of muscles are inside the eye, the intrinsic muscles are going to be the ones that control the shape of the lens. So you can see both near and far. And then also the muscles that control the iris or control the diameter of the pupil. So if you want to assess the function of this cranial nerve, you have a patient go through the cardinal fields of view. If they can follow your pen or follow your eye and you have them move the aisle over the place, then the, then those four extrinsic eye muscles are working. And then if you want to test the intrinsic eye muscles, you could, you could make sure their pupils are constricting and dilating or you could have them focus on accommodation, have them look at something far away and then something near. So that would be how you would assess the function of this nerve, cranial nerve three. Next, we have cranial nerve four or the trochlear nerve. Trochlea means pulley. This is the smallest cranial nerve of the 12 pairs. And this is a motor nerve. This controls one of the six extrinsic eye muscles. It's called the lateral rect, sorry, it's called the superior oblique, lateral rectus is an abducent. It's called the superior oblique. And the reason this is called the trochlear nerve is that trochlea or pulley-like structure the tendon goes through. So the trochlea nerve controls the superior oblique muscle that turns your eye down and to the side. So if you want to assess the function of the trochlea nerve, have have someone move their eyes or look at something in a way that causes the eyes to go down and to the side. So that's the trochlea nerve, cranial nerve four. Let's forget about the eyes for a moment. The trigeminal nerve, cranial nerve five is called trigeminal because it has three branches that I'll show you in just a second. This is a mixed nerve, meaning it has both a sensory and motor function. So big picture, it's just sensation to the face. The sensation to the entire face is controlled by the trigeminal nerve and motor to the muscles of mastication, the muscles of chewing. So let me show you the next image here. So sensation, just remember there are three branches. So if you want to assess the function of the trigeminal nerve, you need to make sure that people can feel the ophthalmic branches up here above the eyes, maxillary branch. Your maxilla is your upper jaw and the mandibular branch. So you basically touch someone's face with your fingers or a pinwheel or something. If they can feel on both sides of the face, all three of those areas, then the sensory portion of the trigeminal nerve is working. And then you ask them about chewing. I guess you could have them clench their teeth, but or you can talk to them about, are they having any difficulty chewing, these type of things. So the primary muscles of mastication, as you can see on the screen here, are the temporalis and masseter muscles. And then we have the pteragois, but we covered that back with the muscles. So that's going to be the trigeminal nerve, cranial nerve five, so sensation to the face, motor to the muscles of mastication. And we talked about how to assess that. Then the last cranial nerve for this video is the abducens or cranial nerve six. This is going to be a motor and it controls one of the eye muscles that I actually said earlier. So cranial nerve six, the abducens controls the lateral rectus muscle, which allows you to abduct your eye. So it turns your eye to the side. So if you want to assess the function of cranial nerve six, as long as you did your cardinal fields of view correctly earlier, then you've already assessed the function of cranial nerve three, four and six. All right, that's your first six pairs of cranial nerves. I'll come back with the second six in the next video. I hope this helps. Have a wonderful day. Be blessed.