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Overview of the 12 cranial nerves.
Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the cranial nerves. In this tutorial, we’re going to cover a very important topic in anatomy and, that is, the cranial nerves. As you see here on this image of the basal view of the brain, you can see the different cranial nerves that we’re going to be talking about here in yellow.
Now, our first point before we start is that you should know that there are 12 pairs of cranial nerves and they are part of what we call the peripheral nervous system or PNS. Now, these nerves pass through the foramina or fissures into the cranial cavity and it is for this reason that they are called then the cranial nerves. On the next slides, we will discuss all the cranial nerves in numerical order including their nuclei, the course they take, what each nerve supplies whether it is a motor or sensory nerve, and some clinical aspects that you need to know. Now, keep in mind that I will not be going into great detail about the branches of the cranial nerves, for example. We’re going to go into greater detail on separate tutorials but for now on this tutorial we’re going to be focusing on a good introduction to the 12 cranial nerves.
On the next slide, I would like to just briefly mention that the cranial nerves are actually ordered – so they have a numerical order – and the 12 cranial nerves are numbered using then Roman numerals from I to XII. So, roughly in the order from top or rostral position to bottom or caudal position. Now, the order of the 12 cranial nerves include then as the 1st one, the olfactory; then there is the 2nd one, the optic nerve; 3rd one is the oculomotor; for the 4th one, we have the trochlear nerve; 5th, the trigeminal; the 6th one is the abducens nerve; the next one or the 7th one is the facial; 8th, the vestibulocochlear nerve; the 9th one is the glossopharyngeal; the 10th one is the vagus nerve, 11th, the accessory; and, finally, the 12th one is the hypoglossal nerve.
So, let’s begin with our overview with the first cranial nerve here that you see highlighted – this is the olfactory nerve. So, as you remember from the previous list I showed you, this is the 1st cranial nerve. You can also use the abbreviation CN I. Now, the olfactory nerve is the sensory nerve that carries the sensory information for the actual sense of smell, hence, the name. As for the course associated to this nerve – now, we’re showing you here an open nasal cavity where you can then see highlighted the olfactory nerve to show you here that the olfactory nerve begins with specialized nerve fibers that are located in the olfactory mucosa - so, the olfactory mucosa of the upper parts of the nasal cavity as you can see here on this illustration. Now, these fibers pass upwards through the cribriform plate which you can also see here – the cribriform plate – and arrive at the olfactory bulb which is the site of origin for the olfactory tract.
Now, if I show you here again the image of the basal view of the brain, you can see this structure here, the olfactory bulb – these structures here. Now, they are enlargements of the rostral part of the olfactory tract and it is located on the inferior surface of the frontal lobe. And as I mentioned in the previous slide, the olfactory nerve passes through the cribriform plate of the ethmoid bone into the olfactory bulb which is the synapse site. Now, the impulses then pass through the olfactory striae which are two thin nerves and these impulses will then go to the olfactory areas of the brain in the depth of the temporal lobe.
On the next slide, we’re going to briefly talk about the clinical significance associated to this nerve that we’re talking about, the olfactory nerve. So, the injuries to the olfactory mucosa and/or to the actual olfactory nerve fibers can lead to localized loss of smell. Now, this isn’t so apparent except when the working areas are in a refractory period and there are no free olfactory cells to process the stimuli. This can occur naturally with old age and is reversible when it occurs as a result of a bad cold, for example. Now, total loss of smell occurs in the case of head injuries which is then the fracture of the cribriform plate that we saw before. It may tear the olfactory nerves. This results then to damage to the actual nerves. This results also in what is known to be as post-traumatic anosmia which is total or partial loss of sense of smell.
We’re now ready to move on to the next cranial nerve – if you remember from that list – this is the optic nerve. This is then the second cranial nerve which is also a sensory nerve, the optic nerve. Now, this nerve transmits visual impulses from the retina to the brain. It contains special somatic afferent sensory fibers and please note that sensory nerves can carry information or messages from the periphery to the center.
As for the next topic, we’re going to be looking at the course of the optic nerve. You can see here on this first image of the eye and the highlighted optic nerve, that the optic nerve originates from the bipolar neurons of the retina in the eye and then leaves the orbit via the optic canal, which you can also see here on this image – notice here the highlighted optic nerve just leaving the optic canal and entering this way to the skull. The two optic nerves run posteromedially in the cranial vault arriving at the optic chiasm which you can see here on this image – this connection here which is known as the optic chiasm. You should note that when we talk about the optic nerve, we are actually referring to the part of the optic pathway from your eyeball to this area here, the optic chiasm. We will, however, discuss the optic pathway in more detail in another tutorial because we have a few details that we need to cover there.
On the next slide, I’m going to do something similar that I did with the other nerve. We’re going to then briefly mention the clinical significance or clinical note associated to the optic nerve. Now, damage to the optic nerve when it happens in front of the optic chiasm, for example, as a result of head injury, will only affect the corresponding eye. In other words, it will affect the eye on the same side as the nerve that is damaged. Now, the effect that damage to the optic nerve has on vision or sight be it as a result of injury, disease or trauma depends on where within the optic tracts said damage occurs. For example, when the damage occurs within the optic chiasm, this results in what we call bitemporal hemianopsia, which is the loss of vision laterally in both eyes. Whereas, damage that occurs posterior to the optic chiasm may result in loss of vision in the visual field opposite to the damaged tract.
Next in our slide, we’re going to be highlighting another cranial nerve but this time we’re highlighting here on this image where we see the lateral view of the orbit. Now, notice here the highlighted nerve that we’re going to be talking about, this one is known as the oculomotor nerve which is, if you remember, the third cranial nerve or CN III. This muscle? innervates some of the muscles of the eye and is responsible for certain movements of the eye within the orbit and, as such, it is then a motor nerve carrying two types of fibers. One that is known as general somatic efferent fibers that innervate most of the extraocular muscle fibers, then another set of fibers known as general visceral efferent fibers that are part of the parasympathetic nervous system. And, on this image here, you can see the nerve highlighted but now from a superior view of the orbit.
On the next slide, we’re going to be focusing now on the course associated to the oculomotor nerve. On the top images, you can see here the brainstem cut in half and we’re looking at a medial view here of the brainstem where we highlighted a few of the nuclei associated to the oculomotor nerve. Here, on the bottom, you can see then the image of the basal view of the brain where you can see highlighted in green, the oculomotor nerves. Now, the fibers of the oculomotor nerve, they originate from the nucleus of the oculomotor nerve and the Edinger-Westphal nucleus, which are found in the midbrain and highlighted on the images above. Afterwards, the nerve will be leaving the anterior surface of the brainstem – which you can also see here on this image – and it does so between the midbrain and the pons. Finally, the nerve exits the skull through the superior orbital fissure, which you can see here on this image – so notice the highlighted nerve going through or reaching the orbit through the superior orbital fissure and terminates in the muscles of the eye which we will look at in the next slides. But, before we do so, I just wanted to zoom in here so you can see on this mage, the highlighted oculomotor nerves.
Next, we’re going to be talking about the innervation associated to the oculomotor nerve. So, as I mentioned in the previous slide, the oculomotor nerve will be innervating the muscles of the eye as it follows the superior rectus muscle which you see here highlighted in green, the inferior rectus muscle which you also see here highlighted, the levator palpebrae superioris muscle. There are other two muscles that are going to be innervated by the oculomotor nerve including the medial rectus which you see here highlighted in green from an anterior view of the orbit and the inferior oblique muscle also seen here from an anterior view highlighted in green.
We’re going to briefly talk about the course associated to the oculomotor nerve. We’ve already seen that the oculomotor nerve also carries parasympathetic fibers. Now, these fibers originate from the Edinger-Westphal nucleus and synapse in the ciliary ganglion. The second order neurons from the ciliary ganglion will be innervating the iris muscle – so the iris sphincter muscle – associated to pupillary constriction, and the ciliary muscle associated with accommodation of the lens.
The last part related to the oculomotor nerve, we’re going to briefly talk about the clinical significance associated to this nerve. Now, lesions occurring on the oculomotor nerve or its region of origin in the midbrain will be leading to oculomotor nerve palsy. The symptoms can be then connected to the muscles innervated by the third cranial nerve that we’ve seen before that are then affected. For example, paralysis of the levator palpebrae superioris muscle can or would result in ptosis which is a dropping of the eyelid. Now, paralysis of the sphincter pupillae, which is the sphincter of the iris, would then result in mydriasis or pupil dilation. Now, paralysis of the ciliary muscle would lead to loss of accommodation. In addition, paralysis of external muscles of the eye except the superior oblique muscle and the lateral rectus muscle that are then innervated by the trochlear and abducens nerves respectively, this would then result in the down and out position of the affected eye.
We’re moving on and talking about another nerve that you see here highlighted in green from the superior view of the orbit, this one is the trochlear nerve, also known as the fourth cranial nerve. This one is also a motor nerve and innervates only one muscle, the superior oblique muscle, and the superior oblique muscle is arranged as a pulley – as you can also see here on this image. The word for pulley in Latin is then trochlea, hence, then the name given to the nerve supplying this muscle being the trochlear nerve.
Next, we’re going to be talking about the course associated to the trochlear nerve. You can see here on this image now on the screen highlighted in green what is known as the nucleus of the trochlear nerve and here on this image you see then the trochlear nerve exiting the brainstem from a dorsal view. Now, the nucleus of the trochlear nerve is located in the tegmentum of the midbrain at the level of the inferior colliculus. The fibers from this nucleus cross over in the midline and exit from the dorsal aspect of the brainstem – as you see here on this image. And note that is the only cranial nerve to do this and it does so also – notice here just inferior to these structures – the inferior colliculi. Finally, it enters the orbit through the superior orbital fissure and arrives to then the superior oblique muscle where it’s going to innervate it.
On the next slide, we’re going to, of course, talk about clinical significance or a few clinical points that you need to know about the trochlear nerve. Now, injuries to the trochlear nerve will certainly affect the superior oblique muscle and this will cause then the eyeball to drift upward, this due to the fact that the superior oblique muscle cannot drag the eyeball downward anymore. Also, patients experience diplopia (double vision) and note this that due to the decussation of the nerve fibers, a one-sided isolated lesion of the trochlear nucleus leads to symptoms of the contralateral eye.
So, let’s continue and talk about another cranial nerve that you see from this lateral view of the orbit – notice the highlighted nerve here – this is then the trigeminal nerve also known as the fifth cranial nerve which is a rather complicated and complex nerve, I would say. Now, this nerve is a mixed nerve which carries both sensory and motor information and it has three well-defined divisions which are separate nerves, I would say, that we can look into and we will have separate tutorials on Kenhub – so, one of them is the ophthalmic nerve, the maxillary nerve, and the other branch also known as the mandibular nerve. The branches of this nerve provides sensory innervation to the skin of the anterior part of the head, the oral and nasal cavities, the teeth and the meninges. In addition, the mandibular division also carries motor fibers to the muscles used for chewing – fancy name – the muscles of mastication, which we also have a tutorial here at Kenhub, if you would like to learn a bit more about the muscles that help you chew. And if I show you here the superior view of the orbit, you can also see the trigeminal nerve highlighted in green on this image.
Next, I would like to briefly talk about the nuclei of the trigeminal nerve. The trigeminal nerve has one sensory nucleus and one motor nucleus. Now, the sensory nucleus is divided into 3 parts: the mesencephalic nucleus which you see here highlighted in green, the principal sensory nucleus highlighted in green as well on this image – and on these images, note here that we’re looking at the posterior view of the brainstem where we have here all these little structures which represent the different cranial nerve nuclei – the last part is the spinal trigeminal nucleus that you also see here highlighted in green.
Now, medially to the principal nucleus, we see the trigeminal motor nucleus, which you see here on these first 2 images where we highlighted from different perspectives – here from a posterior view but here where we cut the brainstem in half and show you a medial view with the trigeminal motor nucleus highlighted in green. The trigeminal nerve exits from the anterolateral surface of the pons as a large sensory root and a small motor root. The sensory root in the middle cranial fossa expand into this structure that you see here highlighted in green, the trigeminal ganglion. Arising from the anterior border of the trigeminal ganglion are the 3 terminal divisions of the trigeminal nerve, you notice here the 3 divisions – so those that we listed before – the ophthalmic, the maxillary and the mandibular nerves which we’ll look at in more details on the next slides.
So, as I promised, on the next slides, we’re going to be talking in a little bit more detail about the other branches or the branches of the trigeminal nerve that I listed before – the three branches of the trigeminal nerve – starting with this one that you see here highlighted in green, which is the ophthalmic nerve or the ophthalmic branch of the trigeminal nerve. We’re going to talk a little bit about the course associated to the ophthalmic nerve. Now, this is the first branch and, sometimes, we write it as the ophthalmic nerve or CN V1 - so you see this, V1, which shows as the cranial nerve number 5 and the first branch as number 1. Now, it is responsible for the innervation of the skin of the forehead, nose and contents of the orbit. This branch will be exiting the skull through the superior orbital fissure and then reaches the orbit as you can see here.
Next, we’re going to briefly talk about the next branch which you see here highlighted in green, this one is the maxillary nerve. A bit on the course associated to the second branch of the trigeminal nerve. We also name it as the maxillary nerve CN V2 – as you can see here – cranial nerve number 5, the second branch. Now, this one leaves the cranial cavity through the foramen rotundum, which you also see here on this image. The maxillary nerve receives sensory branches from the nasal cavity, the nasopharynx, the skin covering the lower eyelid, the side of the nose, the cheek and the upper eyelid. One important nerve branching off from the maxillary nerve is the superior alveolar nerve which supplies the upper teeth within the oral cavity.
And, if you remember well, the next one will be then the – highlighted here in green – the mandibular nerve. This is the third and final branch of the trigeminal nerve which we also call it or can spell it as the mandibular nerve or CN V3 as you see here on the slide. Now, it leaves the cranial cavity through the foramen ovale and the mandibular nerve is the only branch of the trigeminal nerve that has both sensory and motor fibers. Now, the sensory fibers cover sensation of the lowest part of the face – so a very important point here – the lower jaw as well and the floor of the oral cavity, while the motor fibers will be innervating the muscles of mastication and all the jaw muscles.
Next, a very important topic, we’re going to be discussing the clinical significance associated to the trigeminal nerve. Now, trigeminal neuralgia or TN is one of the pathological presentations involving the trigeminal nerve and it is described as severe pain in your face with a sudden and erratic onset. Now, the pain is chronic and progressive gradually getting worse with time and most cases of trigeminal neuralgia are caused by compression of the trigeminal nerve root usually within a few millimeters of entry into the pons. Trigeminal neuralgia is divided by presumed etiology into classic TN and also secondary TN. Classic TN encompasses both idiopathic TN cases and those related to vascular compression while secondary or symptomatic TN is reserved for patients with trigeminal neuralgia caused by structural lesions other than vascular compression.
The next topic of this tutorial will be this nerve that you see here also from a superior view of the orbit – you see highlighted in green – the abducens nerve. This is the sixth cranial nerve and this nerve is a motor nerve and has one job – the innervation of the lateral rectus muscle. The name abducens nerve comes from the fact that the muscle it innervates abducts the eyeball.
As for the course associated to the abducens nerve, we’re going to show you these two images. Again, on the left side, you see the posterior view of the brainstem while on the right side, you see the basal view of the brain, this time showing you the abducens nerve here highlighted in green or the abducens nerves – remember these are paired nerves. Now, on this image here, you can see the nucleus of the abducens nerve highlighted in green. The nucleus of this nerve is located in the pons related to the seven motor nucleus. Afterwards, the nerve arises from the pontomedullary junction near the midline. Finally, it exits the skull as you can see here on this image. Now, it exits the skull via the superior orbital fissure when it runs towards the muscle that it’s going innervating, the lateral rectus muscle.
Next, we’re going to be talking about the clinical significance associated to this nerve. If the abducens nerve is damaged, this can lead to what is known to as abducens nerve palsy. Eventually, this will also lead to the following symptoms that are then caused by the paresis of the lateral rectus muscle which include then reduced lateral eyeball movement and double vision. Now, the function of the lateral rectus muscle can be easily tested by asking your patient to look laterally or look to the side.
Now, the next nerve we’re going to be talking about – this one highlighted in green – is the facial nerve also known as the seventh cranial nerve. Now, this is a very important one and the facial nerve is a mixed nerve and supplies the muscles of facial expression – so all these muscles that you see here that make you smile, that make you frown, a lot of the expressions that you use on a daily basis. Other functions associated to the facial nerve includes then the taste sensation from the anterior potion of the tongue and oral cavity. It is also involved in the parasympathetic secretomotor function of the salivary, lacrimal, nasal and palatine glands.
On the next images, we’re going to be showing you here the nucleus of the facial nerve highlighted in green – notice here. The nucleus of the facial nerve is located on the caudal portion of the pontine tegmentum and its axons take an unusual course then traveling dorsally and looping around the abducens nucleus. Now, then traveling ventrally to exit the ventral pons medially to the spinal trigeminal nucleus as you see here also the facial nerve exiting here. Finally, it exits the skull through the internal acoustic meatus as you can see here on this image as well. Now, its exact course with the multiple branches will be explained in a more detailed video tutorial about the facial nerve that we’ll do on a different time.
Now, the next topic will be then the clinical significance associated to the facial nerve. Acute facial nerve paralysis, example, Bell’s palsy, as a possible result from a viral infection, is then characterized by unilateral facial weakness. So, if only one facial nerve is affected then the possible symptoms will be unilateral facial weakness. In the chorda tympani, we’re going to see symptoms which include then loss of taste and decreased salivation. In the stapedius muscle, we’re going to be seeing then hyperacusis and, in the petrosal branch – the greater petrosal branch – we’re going to be seeing then tear secretion. Hyperacusis happens because the facial nerve innervates the stapedius muscle that dampens sonic vibrations and acts as a protector for the inner ear. Now, the chorda tympani is responsible for taste and saliva, and the greater petrosal nerve or the greater petrosal branch for the, then, lacrimal glands.
The next structure we’re going to be talking about seen here highlighted in green, this one is the vestibulocochlear nerve, also known as the eighth cranial nerve. This one is a sensory nerve. As the name indicates, the vestibulocochlear nerve contains both vestibular and also cochlear fibers. The cochlear fibers terminate in the cochlea and are responsible for hearing whereas the vestibular fibers terminate in the ampullae of the vestibulum and are responsible for balance of the body. So, as a reminder here, the cochlear fibers will be terminating at the cochlea – or in the cochlea – while the vestibular fibers will be terminating in the ampullae of the vestibulum.
Next, we’re going to be briefly talking about the nuclei of the vestibulocochlear nerve. This nerve has two nuclei: a vestibular one which you see here on the first image on the left and a cochlear one which you can see then on the image on the right side highlighted in green. Now, these are located laterally in the floor of the fourth ventricle. As for the course associated to the vestibulocochlear nerve, this nerve attaches to the lateral surface of the brainstem between the pons and the medulla oblongata which you see here on this image after emerging from the internal acoustic meatus which you also see here on the image here below and then crossing the posterior cranial fossa. As for the clinical significance associated to this nerve, nerve damage at the eighth cranial nerve affects functions of the inner ear causing some or all of the following symptoms: hearing loss, vertigo, loss of equilibrium, nystagmus, tinnitus.
The next nerve we’re going to be talking about here on our list is the glossopharyngeal nerve and, on this image, we’re looking at it from a posterior view of the pharynx – so seeing it from the parapharyngeal space. This is the ninth cranial nerve and this nerve is a mixed nerve which as we have seen previously means that it carries both sensory and motor information. The main function of the glossopharyngeal nerve is sensory supply of the oropharynx and the posterior 1/3 of the tongue. Now, note that the anterior 2/3 of the tongue are supplied by the facial nerve. The glossopharyngeal nerve also provides sensory innervation to the carotid sinus, the carotid body, and the skin of the external acoustic meatus and tympanic membrane.
The other functions of the glossopharyngeal nerve include then motor innervation to the stylopharyngeus muscle as well as conveying parasympathetic fibers to the parotid gland. The glossopharyngeal nerve has three nerve nuclei namely the first one that you see here highlighted in green – the nucleus ambiguus – for motor innervation of the stylopharyngeus muscle. There’s also this one that you see here in the middle highlighted in green, this is the solitary nucleus for then sensory fibers and the other nucleus which is known as the inferior salivatory nucleus or a salivary nucleus for then the parasympathetic fibers.
As for the course associated to the glossopharyngeal nerve, this one arises from several rootlets on the anterolateral surface of the upper medulla oblongata. The rootlets enter the jugular foramen and, within the foramen, they emerge to form the glossopharyngeal nerve. Finally, the branches of the nerve run towards the terminating structures as you can see here also on this image. So, this is just a brief description of the course of the glossopharyngeal nerve.
Next, we’re going to briefly mention the clinical significance or few clinical points associated to this nerve. Now, glossopharyngeal nerve lesions can lead to loss of sensation of the pharynx, also decreased salivation, syncope, and a loss of gag reflex. Also a clinical manifestation is glossopharyngeal neuralgia, which is a condition that features recurrent unilateral sharp pain in the tongue, angle of the jaw, external auditory meatus and the throat. Now, episodes may be triggered by cough, sneeze, swallowing or pressure on the tragus of the ear.
The next cranial nerve we’re going to be highlighting here – a very famous one – the vagus nerve. And, on this image, we’re looking at the anterior view of the neck where we stripped a lot of the structures here especially muscles and highlighted the two nerves that are the focus of the nest topics, the vagus nerves, which are also known as the tenth cranial nerves.
Now, the vagus nerve is the most extensively distributed of all cranial nerves – I can guarantee you that. Its name reflects both its wide distribution and the type of sensation it will be conveying. So, in Latin, vagus means indefinite or wandering. It is also a mixed nerve and the main functions of the vagus nerve include phonation and swallowing through its innervation of the skeletal muscles that control these functions. It also transmits cutaneous sensory fibers from the posterior part of the external auditory meatus and the tympanic membrane and, in addition to all of that, it will innervate a few very, very important structures including the heart, the lungs, the palate, the pharynx, larynx, the trachea, the bronchi, and the gastrointestinal tract, playing then a role in the autonomic nervous system.
So, as you can see – the name says it all – vagus nerve is a very, very important nerve of your body. It also has a few nuclei that we need to briefly talk about – so it originates from 4 nuclei to be specific. One that you see here highlighted in green which is the nucleus ambiguus. We also have here the dorsal nucleus of the vagus nerve. Another point of origin, the sensory nucleus of the trigeminal nerve and, finally, the solitary nucleus. Now, as for the course of the vagus nerve, like the glossopharyngeal nerve, the vagus nerve originates between the olives and the cerebellum, and it exits the skull via the jugular foramen running until the abdomen – at the left colic flexure to be specific – and on its course, it parasympathetically innervates the organs mentioned previously in the other slide that I just listed a very important list of structures that are innervated by this nerve – so, all of these – in order to maintain balance of the autonomic nervous system as you would expect since this is a very important nerve.
And, now we’re looking at it from a lateral view of the thorax – notice here the highlighted nerve – this is the vagus nerve, and I just want to briefly mention a few clinical points. Now, vagus nerve lesion results in palatal and pharyngeal paralysis, laryngeal paralysis, abnormalities of the esophageal motility, gastric acid secretion, gallbladder emptying and heart rate, and further autonomic dysfunction. So, a lot of important things – so a lot of things – can actually go wrong if there is a lesion in this very important structure – this important nerve.
For the next nerve that we’re going to be seeing here from this dorsal view of the trunk just these bones right now and the nerves highlighted in green, this is the accessory nerve or the accessory nerves. These are the eleventh cranial nerves and the accessory nerve is a motor nerve that will be innervating two very important muscles of your neck – the sternocleidomastoid muscle and the trapezius. The accessory nerve is a unique cranial nerve because its roots arise from motor neurons in the upper five segments of the cervical spinal cord and you can see here on these two images the highlighted origins of the accessory nerve.
As for the course associated to the accessory nerve, the fibers of this nerve which come from, let’s say, the spinal roots as mentioned in the previous slide leave the lateral surface of the spinal cord and join together as they ascend and enter the cranial cavity through the foramen magnum. The accessory nerve then continues in the posterior cranial fossa and exits the cranial vault through the jugular foramen – you can see it also exiting here on this image and also how it leaves the lateral surface of the spinal cord here on this image. When it exits the jugular foramen, then it descends to the neck to innervate the two muscles we’ve already mentioned – the sternocleidomastoid and the trapezius. In some descriptions of the accessory nerve refer to a few rootlets arising from the caudal part of the medulla oblongata that join with the vagus nerve. So, just a few words here to add that the cranial part will be joining the vagus nerve while the spinal part innervates the sternocleidomastoid and the trapezius muscles.
Next, a few points associated to the accessory nerve on the clinical significance of this nerve and injury to the accessory nerve affects the two innervated muscles and then presents as – so it presents as – inability to shrug your shoulders when it happens to the trapezius, and to the sternocleidomastoid muscle will then affect the inability of head rotation.
Finally, we’re going to be talking about the twelfth cranial nerve which you see here highlighted on this image, this is the hypoglossal nerve. And we see here on this image, the muscles of the tongue just a section here where you can then see the highlighted nerve, the hypoglossal nerve. And, as I mentioned before, this is the twelfth cranial nerve. This is a motor nerve and, as the name suggests – hypo meaning below and glossus meaning tongue – the hypoglossal nerve innervates the intrinsic and extrinsic muscles of the tongue with the exception of the palatoglossus muscle which is then innervated by the tenth cranial nerve, the vagus nerve. So, the list of muscles that are then innervated by the hypoglossal nerve includes the styloglossus, the hyoglossus, genioglossus, the superior and inferior longitudinal muscles of the tongue, the transverse muscles of the tongue, and the vertical muscles of the tongue.
As for the course associated to this nerve, the fibers of the hypoglossal nerve originate from the hypoglossal nucleus which is located in the medulla close to the midline and you see it here highlighted on this image on the right side. The fibers then arise from between the pyramid and the olive in the pre-olivary sulcus and exits the skull via the hypoglossal canal. It then descends and innervates the muscles of the tongue except the palatoglossus – don’t forget that – which is then innervated by the vagus nerve. And the last part associated to the hypoglossal nerve, the clinical significance of this nerve. Now, hypoglossal nerve palsy leads to peripheral lesion which then leads to deviation of the protruded tongue towards the affected side and a central lesion will then lead to inability to protrude the tongue, dysfunctional tongue movement, dysphagia, and misarticulation.
Before we end this tutorial, I’d like to share with you a mnemonic that may help you to remember which of the cranial nerves are sensory, motor or mixed. If you Google it, you are going to find several mnemonics for this and just choose which one works for you. So, for me, the best one is the following: Some Say Marry Money But My Brother Says Big Brains Matter More. So, S for sensory, M for motor, and B for both. So, S for sensory for the olfactory nerve (CN I), S for optic nerve, M for motor oculomotor nerve, M for motor the trochlear nerve, B for both the trigeminal nerve, M for motor abducens nerve, B for both facial nerve, S for sensory vestibulocochlear nerve, Both the glossopharyngeal nerve, Both also the vagus nerve, Motor for the accessory nerve, and Motor for the hypoglossal nerve. So, I hope this mnemonic really helps you in the next exam for any time you have to remember what cranial nerves do what.
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Now, good luck everyone, and I will see you next time.