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Anatomy, course and branches of the vestibulocochlear nerve.
Hey everyone! This is Nicole from Kenhub, and today, we're going to be talking about the vestibulocochlear nerve. And in this tutorial, there are a few things that we're going to be talking about with respect to the vestibulocochlear nerve and this includes the basic anatomy of the vestibulocochlear nerve followed by its course and its components and its pathway to the CNS – that is, we'll be looking at the nuclei of the brainstem. And once we have covered these areas, we'll conclude this tutorial with some clinical notes relating to the vestibulocochlear nerve.
But first, let's just orientate ourselves and have a look at the image that we're going to be looking at mainly today. And you can see inside this lovely anatomical woman, we want to be looking at this region highlighted in blue here, and if we zoom in a bit, we can see the part of the ear that we're going to be looking at today including an anterior view of the temporal bone in the coronal plan specifically the right temporal bone. And, of course, the temporal bone is the part highlighted in blue. We'll also have the brainstem in this image whose ventral side we're looking at and the superior petrosal sinus up here, but what we're really interested in today are the components of the ear which I've highlighted for you in blue. So let's get cracking and have a look at its anatomy.
So, of course, although we know what the external ear looks like, in this image, we're excluding the external ear which sits on the head and is otherwise known as the auricle, and instead we're looking at the internal more proximal aspect of it. And so, in this image, the external ear is highlighted in green and as you can see, this part of the ear extends to the tympanic membrane which is this thin membrane here highlighted in dark grey. And it's otherwise known as the eardrum. The canal that we see here too is known as the external auditory meatus, and you can see it outlined in dark grey as well. This is, of course, the part of the ear that a clinician examines with an otoscope which you can see in our little animation just here.
Let's now have a look at what lies beyond the tympanic membrane. Now this region extending from the medial surface of the tympanic membrane to the mucosa covering the next bony surface medially is known as the middle ear. And, again, you can see this highlighted in green. And there are five major anatomical entities of importance in this compartment that I want to talk about and we'll be looking at this in a clockwise direction. And the first one of these is the auditory ossicles, which are tiny bones in the inner ear that help with the vibrations of sound. And let's just zoom in a little bit so we can see the middle ear a little bit closer.
And in this image, you can see the auditory ossicles outlined in blue and these tiny bones are bones that transmit sound waves from an air medium in the external ear to a water medium in the inner ear. And although it may not be so obvious, you might be able to see that there are three bones – a malleus which is the largest and most lateral of the bones and this little bone transmits vibrations to the next little bone which is called the incus and as you can see, the incus which is the middle auditory ossicle, sometimes called the anvil because of its shape, and this bone itself connects to the stapes which is the most proximal as well as the smallest ossicle and you can see that the stapes fits into the oval window which we'll talk about next.
So, the oval window is marked by this little blue line on the screen and the oval window is essentially a small hole in the petrous segment of the sphenoid bone and is covered by a thin lamina. And, of course, our stapes, which is here again in blue, attaches to this membrane and transmits pulsations of the three ossicles to provoke water oscillations within the inner ear.
Moving on, let's look at the third structure which is the round window. And like the oval window, this is also a tiny hole covered by a thin membrane which you can see highlighted in blue. However, you can also see that there's no ossicle attached to it. So, the round window's presence is more to ensure that fluid will move within the inner ear which the membrane does by vibrating when its hit as it results in the relieving of pressure created by the vibrations entering the inner ear through the oval window.
Moving downwards, we have the pharyngotympanic tube also known as the Eustachian tube and this tube which is down here serves as the communication between the middle ear and the nasopharynx especially when it's open, for example, when swallowing.
And, finally, we have the chorda tympani nerve which is a branch of the facial nerve. And in our zoom, you can see it running along the base of the middle ear highlighted in yellow which is our color for our nerves when we can use it. And this nerve originates in the taste buds and runs through the middle ear transmitting sensory information to the brain and allowing us to have a sense of taste. So as you might expect, if a patient presents with the loss of taste after ear surgery, this nerve is likely to be damaged, and we'll talk about this nerve in greater detail a bit later in our tutorial, so don't worry.
So, lastly with our compartments of the ear, let's talk about the inner ear, and you may have thought the vestibulocochlear nerve was invented to confuse medical students but its etymology is actually quite interesting. And when people discovered the inner ear in the sixteenth century, it was common to use Latin or Greek for medical terminology and those who discovered the inner ear visualized it as a compartment with two chambers – one like a snail shell which they called the cochlea which is the Greek word for snail and is this little section just here. And let's just have a bit of fun and just put a picture of a snail in there so that we remember what the cochlea looks like. And also there was a small section of the inner ear which reminded them of a small foyer or a vestibule leading to the largest space of the cranial cavity. And in this image, you can see a small foyer which I've inserted in for you behind some steps.
And during that era, such small rooms were common to the entrances of temples and palaces and they were called vestibula from the Latin word vestibulum which means antechamber or entry room. So, hopefully, you will not forget either of those as we should be able to recognize them from how they look.
So now that we've talked about the components of the ear, let's briefly talk about some of the nerves that are associated with this region and, of course, the main nerve we want to be looking at today is the vestibulocochlear nerve which is highlighted in green on this image. And as you might have already guessed, the vestibulocochlear nerve has two components – the vestibular chamber which holds the vestibular nerve which you can see outlined in blue and the cochlear chamber which holds the cochlear nerve which is outlined in blue now too. And these two nerves unite as they are about to exit the inner ear and then to the cranial cavity, and you can see that point circled in blue, and this very point is known as the internal acoustic meatus. So if we move beyond it, we can see the vestibulocochlear nerve which I've dotted out in dark grey. And another way to look at this is by following the vestibulocochlear nerve from the central nervous system to the periphery. So where we would say, the internal acoustic meatus is the point at which this nerve divides into a vestibular and into a cochlear nerve.
An important thing to note about the vestibulocochlear nerve is that it is the eighth cranial nerve which is one of the twelve cranial nerves to emerge from the brain. There's another nerve, however, visible in this image that you might be curious about so we'll have a little chat about that one. And in this image, you can see that we have the facial nerve highlighted in green. And the facial nerve is the seventh cranial nerve, and the facial nerve controls the motion of the muscles of facial expression and conveys taste and sensation from some regions of the tongue. As you can see, the facial nerve also exits the cranial cavity via the internal acoustic meatus which again is our little opening here circled in blue but it bypasses the inner ear traveling towards other roots. We can see it again highlighted in the middle ear so we can only imagine that it takes a rather curved course on its way out but we won't go into greater detail about the course of the facial nerve. If you need to brush up on your facial nerve knowledge, please feel free to go to one of our other tutorials that focus on the facial nerve.
But, of course, if we follow this translucent green highlight on the left here, we can imagine it traveling behind the compartments of the middle and the external ear until it exits from the skull via a foramen behind the styloid process which is this structure just here as pointed out by my arrow.
Now, we would be normally leaving the facial nerve behind us if it were not for a little branch it gives off in the middle ear which troubles many ENT surgeons and is related to taste. And if you remember back to our discussion in the middle ear, you might remember this nerve – the chorda tympani. And this small nerve is a branch of the facial nerve, and however despite the fact that it enters the middle ear and travels directly behind the tympanic membrane, it has no relationship whatever to any element of the auditory or vestibular pathway and no relationship to the vestibulocochlear nerve. Instead, it's part of one of three cranial nerves that are involved in taste.
So up until now, we've covered a basic outline of the anatomy of the vestibulocochlear nerve and clarified some common misconceptions regarding the facial nerve and its branches, and now we'll be moving on to discuss important anatomical elements involved in the innervation of the inner ear starting from the periphery and moving towards the CNS. But for now, let's start with a bottom-to-top approach, so, from the periphery to the CNS.
As you may remember, we mentioned that the vestibulocochlear nerve divides into two terminal branches – the vestibular nerve and the cochlear nerve – and we're going to start with the vestibular nerve for the moment. And as you can see, it's highlighted on our right in green and this nerve conveys information concerning changes in the linear and angular acceleration of the head in any axis or what we may more simply call a sense of balance. The vestibulocochlear nerve innervates several structures collectively known as the vestibular apparatus and play their own significant roles in equilibrioception.
And three are three vestibular apparatus that we're going to talk about today, and these three are the semicircular canals, the utricle and the saccule. So let's begin, of course, with the semicircular canals and their innervation.
The semicircular canals are three bony loops found within the inner ear and they're filled with fluid and they help with a sense of balance. And of course, there are three of these canals and they have three different names. The first being the anterior canal followed by the lateral canal and lastly by the posterior canal. As you may be able to see, each semicircular canal ends in a widening called an ampulla and I've highlighted them for you in a dark gray. And the ampulla is important as it's involved in the sensing of changes in the angular acceleration in a single plane. In easier terms, each of these ampullae provides a separate sense of directional balance. And each ampulla is innervated by an ampullary nerve which we're going to talk about over the next few slides.
Let's begin with the anterior ampullary nerve. And as you can probably guess, this nerve innervates the anterior ampulla and the semicircular canal which is aligned with the sagittal plane and the anterior ampulla is sensitive to changes in angular acceleration vertical to that axis which basically means that it detects motions in the sagittal plane such as when nodding your head so we expect the anterior ampullary nerve to convey information about the rotation of the head in the sagittal plane. And so if we reduce the size of our image a little bit here on the right and bring in our image of the anatomical woman in the left with the sagittal plane highlighted in green, you can see the direction of rotation highlighted in blue.
The lateral semicircular canal is aligned with the transverse plane so its ampulla detects changes in angular acceleration vertical to that axis such as when turning our heads or when spinning around. This ampulla is innervated by the lateral ampullary nerve which you can see highlighted in this image, and it's safe to say that the lateral ampullary nerve carries information about rotation of the head in the transverse plane. So if we bring in our anatomical woman and we'll highlight the transverse plane in green and you can see the rotation along this axis in blue.
Now, finally, let's look at the posterior semicircular canal and the first thing you may have noticed about this image is that the branch innervating the ampulla – that is, the posterior ampullary nerve – originates from a different branch of the vestibular nerve. It doesn’t seem to arise from the same branch as the other two, and this happens because the posterior semicircular canal is located behind the other two canals and it detects changes in the angular acceleration vertical to the coronal plane so basically it is sensitive to motions of the head in that plane such as tilting the head towards the shoulder. And, therefore, we can say that the posterior ampullary nerve carries information about the rotation of the head in the coronal plane. And so, of course, let's come back to our anatomical woman with the coronal plane highlighted in green, we can see our blue arrow showing the direction of the rotation.
So now that we've finished talking about the rotation in various planes, let's talk now about how we can perceive linear acceleration. Let's begin, of course, with the utricle and you can see it highlighted in green on our image. The utricle is a small membranous sac within the inner ear connected to the semicircular canals. And it's also the region that detects changes in linear acceleration in the horizontal plane and that means that it can tell whether we are moving forwards, backwards or sideways. The utricle is of course innervated by the utricular nerve and therefore conveys information about linear acceleration on the horizontal plane. And just to demonstrate, we're going to move our anatomical lady around a little bit so we're going to move it sideways, forwards then backwards and basically all these directions are going to be detected by the utricle.
The saccule is another membranous sac within the region of the inner ear – and it's responsible for the perception of linear acceleration in the vertical plane and can therefore detect motion upwards or downwards such as when we fall due to the effect of gravity. So, of course, from that we can infer that the saccular nerve conveys information regarding linear acceleration in the vertical axis. So let's come back to our image of the anatomical female and observe how she moves around the vertical axis. And of course this movement will be detected by the saccule.
So let's just come back to our image of the vestibular nerve as a whole and as you can see, the smaller branches of the vestibular nerve merge to form two major branches – that is, a superior branch and an inferior branch. And so we're going to have a little talk now about what these two branches supply.
So, of course, we're going to begin with the superior branch of the vestibular nerve, and as you can see, the vestibular nerve is formed by the junction of the anterior ampullary nerve, the lateral ampullary nerve and the utricular nerve which I'm pointing out with my arrow. The inferior branch, on the other hand, is formed by the contribution of two nerves and these are the posterior ampullary nerve and the saccular nerve.
Before we finish our discussion on the course of the vestibular nerve, let's have a bit of a chat about these two prominent tuberosities we can see just before the superior and inferior branches merge. And these tuberosities together make up the vestibular ganglia which contains the cell bodies of the afferent bipolar neurons that innervate the anatomical elements of the vestibule. And as you can see, there's an upper part and a lower part. Of course, let's have a brief chat about both of those.
So, the upper part of the ganglion is formally called the superior part and it contains the cell bodies of bipolar neurons that constitute the superior branch of the vestibular nerve and as we've discussed before, these neurons receive and convey information from the anterior and lateral ampullae and the utricle. The lower part of the ganglion is formally called the inferior part and it contains the cell bodies of bipolar neurons that constitute the inferior branch of the vestibular nerve and as we've discussed before, these neurons receive and convey information from the posterior ampulla and the saccule.
Now that we're finished talking about the vestibular nerve, let's talk about the cochlear nerve and some of its components. The cochlear nerve which you can see highlighted in this image is much simpler in its anatomy than the vestibular nerve and as you can see, it's pretty much distributed along the spirally-shaped cochlea that receives information about sound intensity and frequency. It also has afferent bipolar neurons. And the cochlear nerve also has a ganglion, however, it looks quite different to the vestibular ganglion in the vestibular nerve. The sensory ends of the cochlear nerve are distributed spirally along the length of the organ of the Corti which is in the cochlea and this region is known as the spiral ganglion.
So, so far, we've discussed the anatomy of the inner ear as well as the vestibular and cochlear branches of the vestibulocochlear nerve in great detail but now I want to have a chat about the vestibulocochlear nerve as it enters the cranial cavity and the brainstem and see how these bipolar neurons convey information about either sound or balance into the brainstem. But, first, let's just talk a little bit about these nuclei and their number.
So there are two groups of vestibulocochlear nuclei, the first being the vestibular nuclei and as for them, there are four of them with cells from this complex of the nuclei being detected as high as the pons and as low as the lower medulla and I've highlighted them for you in blue over here on our image. And please note that these nuclei are actually located underneath the posterior side of the medulla. As for the cochlear nuclei, there are only two of them and both of those are located in the medulla oblongata which again you can see highlighted in blue.
And let's now go through our nuclei and, of course, let's begin with the vestibular nuclei and there are four of these located on each side of the brainstem and you can remember the names of these nuclei easily because each has a name of an anatomical direction. The first nucleus we're going to look at is the superior nucleus which is the most superior and lateral nucleus and is located within the pons. Next is the inferior nucleus which is the most inferior nucleus and is located within the medulla. The lateral nucleus is the next one and it's located in the lower pons and note that it lies next to the superior nucleus which I'm pointing out with my blue arrow. And finally, we are seeing the medial vestibular nucleus. We can see that this is the largest of the vestibular nuclei and that it extends from the pons to the medulla, and if we have a look at this cross section, we can see that this nucleus lies medially to the lateral vestibular nucleus.
Let's now, of course, have a look at the cochlear nuclei and of the two cochlear nuclei, one is anterior and the other is posterior. Let's move on to have a look first at the anterior nucleus. So right now, we can see the anterior cochlear nucleus which is located in the medulla and we can see both the depiction of its position in space as well as a cross-section. We can see that this nucleus is located in the posterior medulla. Now, let's have a look at the posterior nucleus which we can see highlighted in green, and the posterior nucleus is also located in the posterior medulla just behind the anterior nucleus.
Now that we finished talking about the vestibulocochlear nerve, let's of course have a bit of a chat at about some clinical notes.
Now, the main thing I want to talk to you about today about the clinical notes regarding the vestibulocochlear nerve are disorders of this particular nerve. So first I want to talk about disorders of the vestibular system. So, conditions of the vestibular system and the vestibular nerve can present as vertigo. And if we define vertigo, we can define vertigo as a symptom describing the perception of space rotating on its own. Now, vertigo can be either central or it can be peripheral. And central vertigo is related to cerebellar and brainstem lesions and is usually accompanied by vertical nystagmus – that is, the eyes of the patient may do repeated movements in the up and down axis should the patient be asked to move the eyes up and down. And peripheral vertigo which is due to a lesion or a condition of the vestibular system and nerves, they present with horizontal nystagmus – so, in our little image, we've drawn some arrows horizontally and that means if the patient is asked to move his eyes horizontally, they may do repeated movements either rightwards or leftwards.
And lesions of the cochlear nerve can be the cause of sensorineural deafness, a condition where sound cannot be transmitted from the inner ear to the central nervous system. Sensorineural deafness can be differentiated from conductive deafness which is deafness due to a mechanical obstruction of the sound waves by the use of a tuning fork. And so now, we're going to bring up our little image of a tuning fork and do a little bit of animation. So should you place a tuning fork on the forehead of a person with normal hearing, they will hear a similar sound in both ears while a person with sensorineural deafness will not hear any sound in their affected ear and a person with conductive deafness will hear a high volume in their affected ear.
And now that we're finally finished talking about the clinical notes, let's just go through everything that we've been talking about today as we always do. So, of course, in this tutorial, we started by talking about the anatomy of the ear beginning with talking about the compartments of the ear. And the ear is made up of the external ear which is highlighted in green just here and it runs from the tympanic membrane to the auricle followed by the middle ear which is a section of the ear that contains five important structures and these are the auditory ossicles which are small bones that help with transmitting vibrations from the outside of the body into the watery inside of the inner ear, the oval window which is attached to the stapes and also helps with this transmission, the round window which relieves pressure in the inner ear by vibrating, the pharyngotympanic tube which facilitates as communication between the middle ear and the nasopharynx, and the chorda tympani which is a small nerve that is associated with taste. We then looked at the inner ear which is made up of the cochlea and the vestibule which we determined was made up of the cochlea which is in the shape of a snail and the vestibule which looks like a small room.
The next thing we talked about within the section of anatomy is the associated nerves that are to do with this topic and, of course, our main nerve is the vestibulocochlear nerve which is highlighted in green on our right followed by the facial nerve which is also known as cranial nerve seven and, like the vestibulocochlear nerve, passes through the internal acoustic meatus, the chorda tympani which as we mentioned is a branch of the facial nerve and is involved with taste.
The next thing we wanted to talk about are the course and components of the vestibulocochlear nerve and the first component we wanted to talk about was the vestibular nerve which is involved with your sense of space. And the vestibular nerve begins if we're going from the CNS to the periphery with the vestibular ganglion which contains afferent bipolar neurons and the vestibular ganglion can be broken down into two parts – an upper and a lower part – but the upper part otherwise known as the superior part is the one that gives rise to the superior branch which in turn gives rise to the anterior ampullary nerve which supplies the anterior semicircular canal, the lateral ampullary nerve which supplies the lateral semicircular canal, and the utricular nerve which supplies the utricle.
The other part of the vestibular ganglion is the inferior part of the ganglion and the inferior par, of course, gives rise to the inferior branch of the vestibular nerve which in turn gives rise to the posterior ampullary nerve which supplies the posterior semicircular canal and the saccular nerve which gives rise to the saccule. The other nerve that we wanted to look at which was a major component of the vestibulocochlear nerve is, of course, the cochlear nerve which is a short nerve that runs spirally within the cochlea and the cochlear nerve also has a spiral ganglion.
The next thing we looked at after looking at course and components of the vestibulocochlear nerve is the nuclei which are located within the brainstem and, of course, these are divided into two parts – the vestibular nuclei which contained four individual nuclei so the superior nucleus which is located in the pons, the inferior nucleus which is located within the medulla, the lateral nucleus which is located in the lower pons and the medial nucleus which is located in the pons and the medulla; and of course we also looked at the cochlear nuclei of which there is an anterior nucleus which lies in the posterior medulla and also a posterior nucleus which also lies in the posterior medulla.
The last thing that we wanted to look at in this tutorial were some clinical notes relating to the vestibulocochlear nerve, and the first one we wanted to talk about lesions of the vestibular nerve which involve vertigo including central vertigo which is related to cerebellar and brainstem lesions and has vertical nystagmus as well as peripheral vertigo which is involved with a lesion of the vestibular system and whose symptoms may present with horizontal nystagmus. And then we also ended with lesions of the cochlear nerve which include sensorineural deafness which is a condition where sound cannot be transmitted from the inner ear to the central nervous system.
And that's all we have for today. Thanks again for watching.
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