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Cranial nerves

Overview of the 12 cranial nerves.

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Transcript

Check out this juicy beef burger. Doesn’t it look delicious? Eating something yummy like this involves a lot of processes. First, you lay your eyes on it, then you start to smell the crispy bacon, the gooey cheese, and yup! My mouth is watering. Then, you sink your teeth into the bun, and taste your first bite of heaven. Before you know it, it’s chewed up, swallowed and beginning to be digested by your stomach. Crucial to every step of this delicious experience is an interesting and complex group of nerves, known as the cranial nerves. Stick around to learn about their functions now in our introductory video to the cranial nerves.

Before we begin, I’d like to give you a quick overview of what we’re going to be talking about in this tutorial. So we have twelve cranial nerves which are paired structures and part of the peripheral nervous system or the PNS. All, but the eleventh cranial nerve originate from the brain and pass through various foramina and fissures in the cranial cavity. This is why they’re known as the cranial nerves.

In our image, we can see an inferior or ventral view of the brain where the cranial nerves we’re going to be talking about in yellow. Cranial nerves may consist of motor fibers, sensory fibers, or both. So today, we’ll color-code our motor fibers blue and our sensory fibers red.

The cranial nerves can be identified by their names as well as by the roman numerals one to twelve, and let’s run through them quickly now. So, we have the olfactory nerve, the optic nerve, the oculomotor nerve, the trochlear nerve, the trigeminal nerve, the abducens nerve, the facial nerve, the vestibulocochlear nerve, the glossopharyngeal nerve, the vagus nerve, the accessory nerve, and finally, the hypoglossal nerve.

Now on today’s tutorial, we’re going to be whizzing through all twelve nerves learning about their basic anatomy and functions. However, if you want to learn the nitty-gritty details about each cranial nerve, don’t forget that we have an awesome series of dedicated videos for each of the cranial nerves that you can sink your teeth into. So let’s waste no time and get straight into our tour.

So the first cranial nerve that we’re going to be talking about is the olfactory nerve, and this can be abbreviated to CN I. The olfactory nerve carries special sensory fibers for the sense of smell so, of course, this is the nerve that allows you to smell that delicious burger. Be sure to note that sensory nerves transmit information from the periphery to the brain and that they’re known as afferent nerves.

Before we move on, I’ll give you some basic information relating to the anatomical course of this nerve which begins with specialized nerve fibers located in the olfactory mucosa. And these fibers enter the cranial cavity via the cribriform plate of the ethmoid bone and arrive at the olfactory bulb, which is the site of origin for the olfactory tract. Nerve impulses then pass through the olfactory striae to the olfactory areas of the brain.

The next cranial nerve which we can see here highlighted in green is the optic nerve or CN II. The optic nerve carries special sensory fibers for vision and specifically it transmits visual impulses from the retina of the eye to the visual cortex of the brain. As for the course of this nerve, it originates from the bipolar neurons of the retina and leaves the bony orbit to enter the cranial cavity via the optic canal.

The two optic nerves run posteromedially in the cranial vault arriving at the optic chiasm where the optic nerves partially cross, and you should note that when we talk about the optic nerve, we’re referring to the part of the optic pathway between the eyeball and the optic chiasm.

And if you remember our list, the third cranial nerve is the oculomotor nerve or CN III. And true to its name, this nerve provides motor innervation to a number of the extraocular muscles of the eye, namely the levator palpebrae superioris muscle, the inferior oblique muscle, the superior rectus muscle, the inferior rectus muscle, and the medial rectus muscle. The levator palpebrae superioris functions to move the upper eyelid whereas the rest of these muscles are involved in movement of the eye.

The oculomotor nerve also provides parasympathetic innervation to the iris sphincter muscle and the ciliary muscle, and the iris sphincter muscle is responsible for constriction of the pupil and the ciliary muscle changes the shape of the lens of the eye. In this case, remember that the fibers of the oculomotor nerve transmit information from the brain to the periphery and that they’re therefore known as efferent nerves.

Unlike the first two cranial nerves, the oculomotor nerve originates from the midbrain which is the upper part of the brainstem and it leaves the anterior surface of the midbrain and exits the cranial cavity to enter the bony orbit of the superior orbital fissure. And the fibers of the oculomotor nerve terminate in the extraocular muscles of the eye.

Next, we have the trochlear nerve or CN IV, and this cranial nerve provides motor innervation to the superior oblique muscle only, which is an extraocular muscle of the eye, and as you can see in our image, the superior oblique muscle is arranged as a pulley, and the word for pulley in Latin is “trochlea” which is where the cranial nerve gets its name.

As for its anatomical course, the trochlear nerve originates from the midbrain and exits from the posterior surface of the brainstem. It then passes anteriorly to enter the bony orbit via the superior orbital fissure terminating in the superior oblique muscle, and note that this is the only cranial nerve that arises from the posterior aspect of the brainstem.

So we’re going to be a little bit cheeky now and skip the fifth cranial nerve, but don’t worry, we’re going to be coming back to it. For the moment, we’re going to focus on our sixth cranial nerve, which is the abducens nerve or CN VI. So the abducens nerve is also involved with the extraocular muscles of the eye. Specifically, it provides motor innervation to the lateral rectus muscle, and this nerve’s name comes from the fact that the muscle it innervates abducts the eyeball, meaning that it draws it away from the midline.

The abducens nerve originates from the pons which is the middle of the brainstem and it exits the cranial cavity to enter the bony orbit via the superior orbital fissure, as you can see here. And the fibers of the abducens nerve terminate in the lateral rectus muscle.

So remember that the oculomotor nerve, the trochlear nerve, and the abducens nerve – that is, cranial nerves three, four, and six – are all responsible for the movement of the eye.

Oaky, so let’s move on that nerve that we skipped, which is the trigeminal nerve or cranial nerve five. So the trigeminal nerve is a mixed nerve, meaning that it carries both sensory and motor information, and to be more specific, the trigeminal nerve is the primary sensory nerve for the face and anterior scalp, and the motor nerve for the muscles of mastication and other small muscles.

The term trigeminal comes from the Latin word “trigeminus” meaning triplet which is more than appropriate considering that the sensory root of the trigeminal nerve divides into three terminal nerves – the ophthalmic nerve, the maxillary nerve, and the mandibular nerve. The ophthalmic nerve provides sensory innervation to the anterior half of the scalp, the orbital contents, the upper eyelid, the nasal cavity, the dorsum of the nose, and the frontal sinus, to name a few.

The maxillary nerve transmits sensory information from the lower eyelid, the side of the nose, the nasal cavity, the nasopharynx, the cheek, the upper lip, and the upper or maxillary teeth. And finally, we have the mandibular nerve which innervates structures found in the lower region of the face such as the lower lip, the anterior two-thirds of the tongue, the lower or mandibular teeth, and the mandible. It also supplies the anterior part of the external ear, the temporal region, and part of the external acoustic meatus.

Alright, so it’s time to talk about the motor root which we can now see highlighted in green. So this root provides motor innervation to the muscles of mastication which are the temporalis, the masseter, and the medial and lateral pterygoid muscles. So the motor root of the trigeminal nerve allows you to chow down on tasty treats, and it also supplies motor innervation to the mylohyoid, the anterior belly of the digastric, the tensor tympani, and the tensor veli palatini.

Let’s have a look at the anatomical course of the trigeminal nerve now.

So, it exits from the anterolateral surface of the pons as a large sensory root and a small motor root, and the sensory root expands into this structure that you can see here, which is the trigeminal ganglion. Arising from the anterior border of the trigeminal ganglion are the three terminal divisions of the trigeminal nerve – the ophthalmic nerve, the maxillary nerve, and the mandibular nerve. And the ophthalmic nerve exits the cranial cavity via the superior orbital fissure while the maxillary nerve exits the skull via the foramen rotundum, and the mandibular nerve exits the cranial cavity via the foramen ovale.

The smaller motor trigeminal root joins the mandibular nerve distal to the trigeminal ganglion and, therefore, the mandibular nerve is the only division of the trigeminal nerve that contains a motor component.

So, next, we have the facial nerve which can be abbreviated to cranial nerve seven and this cranial nerve is a mixed nerve and we can see it here highlighted in green. As you can see, it supplies motor innervation to the muscles of facial expression such as the orbicularis oculi and the zygomaticus major and minor muscles. These muscles contract to make you smile or frown, and allow you to express all sorts of emotions from surprise to fear. And the facial nerve also provides motor innervation to the posterior belly of the digastric muscle as well as the stapedius and the stylohyoid muscles. It also supplies parasympathetic innervation to the submandibular and sublingual salivary glands, the lacrimal glands, and the mucous glands of the nasal and oral cavities. And the parasympathetic fibers stimulate the secretomotor activity of these glands and arise from the intermediate nerve which is part of the facial nerve.

The seventh cranial nerve also carries special sensory fibers from the anterior two-thirds of the tongue to the brain, and finally, it transmits sensory information from part of the external acoustic meatus and the deeper parts of the ear. The anatomical course of this cranial nerve is rather complex and will be covered in more detail in its own tutorial; however, if we look at our image, we can see that the facial nerve leaves the brainstem from the anterior surface of the pons. It then exits the skull via the stylomastoid foramen to innervate the muscles of facial expression.

Okay, so it’s time to talk now about the vestibulocochlear nerve or CN VIII. So, this nerve carries special sensory fibers for hearing and balance, and can be divided into two components – the cochlear part and the vestibular part. The cochlear part innervates the cochlea and transmits auditory sensory information to the brain, whereas the vestibular part innervates the vestibular system and transmits information about balance to the brain. So let’s have a closer look at the anatomy of this cranial nerve.

The vestibulocochlear nerve is formed by the union of the vestibular nerve and the cochlear nerve and it enters the cranial cavity via the internal acoustic meatus and terminates at the lateral surface of the pons.

Let’s move on to look at the glossopharyngeal nerve or cranial nerve nine which is a mixed nerve. This nerve transmits sensory information from the carotid body and sinus, the posterior third of the tongue, the palatine tonsils, the oropharynx, the skin of the external acoustic meatus, and the tympanic membrane. The glossopharyngeal nerve also carries special sensory fibers for taste from the posterior third of the tongue to the brain and remember that the anterior two-thirds of the tongue is supplied by the facial nerve. Furthermore, it supplies parasympathetic innervation to the parotid salivary gland.

Finally, the ninth cranial nerve supplies motor innervation to the stylopharyngeus muscle which forms part of the pharyngeal musculature.

As for the course associated with the glossopharyngeal nerve, it emerges from the anterolateral surface of the medulla oblongata which is the lower part of the brainstem and it exits the cranial cavity via the jugular foramen and its fibers terminate in the structures that you can see in this image.

The next cranial nerve that we’re going to be talking about is a rather famous one, and that is the vagus nerve. So the vagus nerve is the most widely distributed of all the cranial nerves and, funnily enough, vagus means wandering in Latin. And the vagus nerve is a mixed nerve, meaning that it consists of both motor and sensory nerve fibers.

It provides sensory information to various structures including the heart, the lungs, the palate, the pharynx, the larynx, the trachea, the bronchi, and the external ear. And the vagus nerve also carries special sensory fibers for taste from around the epiglottis and the pharynx to the brain. Furthermore, this nerve supplies parasympathetic innervation to the smooth muscle and glands found in the pharynx, the larynx, the organs of the thorax and the organs of the foregut and the midgut, and remember that the parasympathetic innervation generally results in what’s commonly known as the rest and digest response, meaning that it does things like slow our heart rate and get our stomach and small intestine busy at digesting our recently inhaled burger.

Finally, the tenth cranial nerve provides motor innervation to the palatoglossus, which is a muscle of the tongue as well as muscles of the soft palate, the pharynx and the larynx, and in this way, it assists with speech and swallowing. Like the glossopharyngeal nerve, the vagus nerve originates from the medulla oblongata and exits the cranial cavity via the jugular foramen. It has the longest anatomical course of all the cranial nerves and extends from the head all the way to the abdomen.

Alright, time for our penultimate cranial nerve which is the accessory nerve, also sometimes referred to as the spinal accessory nerve. So this nerve provides motor innervation to the sternocleidomastoid and the trapezius muscle. As for the course of the accessory nerve, the fibers of the nerve originate from the spinal cord and ascends to enter the cranial cavity via the foramen magnum. The accessory nerve then continues along the cranial vault and exits the cranial cavity through the jugular foramen. And when it exits the jugular foramen, it descends to the neck to innervate the two muscles we’ve already mentioned – the sternocleidomastoid and the trapezius.

And finally, we’re going to talk about the twelfth cranial nerve, which is the hypoglossal nerve. As the name suggests, hypo- meaning below and -glossus meaning tongue, the hypoglossal nerve provides motor innervation to the intrinsic and extrinsic muscles of the tongue with the exception of the palatoglossus muscle which is innervated by the vagus nerve. The hypoglossal nerve leaves the anterior surface of the medulla oblongata and exits the cranial cavity via the hypoglossal canal, and as we can see, it then goes on to innervate most of the muscles of the tongue.

Alright, so it’s now that time in the tutorial where I normally talk about some clinical notes relating to the structures that we’ve learned about; however, today, we’re going to do something a little bit different. We’re going to be talking about the thirteenth cranial nerve.

Yes, you heard me right – thirteenth. There’s another rather forgotten member to this family. The terminal nerve, also known as cranial nerve zero or cranial nerve N is located medial to the olfactory nerve and exits the skull via the cribriform plate. And this nerve appears to exist in all vertebrates and that includes humans. However, due to its extremely fine and delicate nature, the terminal nerve is often destroyed during the dissection process and has therefore been long overlooked as a cranial nerve.

Now, what does this mysterious little nerve do? Well, believe it or not, the terminal nerve is thought to play a role in reproductive behaviors specifically relating to the reception and recognition of pheromones. You see, pheromones are chemical signals involved in sexual attraction and mate selection in addition to improved mood and heightened focus of the receiver. They promote synthesis of hormones in our pituitary gland known as gonadotropins, and this in turn regulates the production and secretion of the sex hormones – testosterone and estrogen. And these hormones ultimately command the sexual behaviors controlling the biology of our reproduction. So your thirteenth cranial nerve may be a key to how you select your romantic partner.

Okay, so before I let you go, we’re going to be summarizing what we’ve learned today using a mnemonic. So, there are a variety of mnemonics for the cranial nerves so try to find one that works for you, but the one we’re going to be using today is one that goes ‘Oh, Oh, Oh, To Touch And Feel Very Good Velvet. Absolute Heaven!’ So, let’s break that down.

So we’ve got the first O stands for the olfactory nerve, the second O is for the optic nerve, and the third O is for the oculomotor nerve. The first T is for the trochlear nerve and the second T is for the trigeminal nerve. And then we have A for the abducens nerve, F for the facial nerve, V for the vestibulocochlear nerve, G for the glossopharyngeal nerve, V again for the vagus nerve, A for the accessory nerve, and finally, H for the hypoglossal nerve.

Alright, so that brings us to the end of our introductory tutorial to the cranial nerves. Hope you enjoyed it. Thanks for watching. Happy studying!

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