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Overview of the structures of the outer ear and auditory tube.
Hello everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial, and today, we're going to be looking at the structures of the outer ear and the auditory tube as well as other important surrounding structures of the middle ear and inner ear as well, like you see here on this image. What you see here is a section of the ear where you can then see the auricle, or as you commonly know as your ear here, and then you can see a few structures that we're going to be talking about on this tutorial. Now, the ear is a vital component for one of 5 senses which is hearing. But aside from that, the parts of the auditory apparatus also function in aiding with balance and spatial orientation.
Now, let's start by looking at the structures of the external ear. The external ear consists of the part attached to the lateral aspect of your head known as the auricle that you see here highlighted in green or that you probably call it all the time, your ear. The auricle is an organ located bilaterally on the human cranium – essentially, when I mean bilateral, I mean on each side of your head. It consists of cartilage which you can see a little bit here – a little bit of cartilage – covered with – as you can also see here in this cut – with skin and arranged in a pattern of various elevations and also depressions. So, this is basically the visible part of the ear that you can see and also touch. And its function is to collect sound acting as a funnel which amplifies and directs the sound to the auditory canal.
The other part of the external layer is the external acoustic meatus which you see here highlighted in green. Now, this is a tube which extends – as you see in this image – from the deepest part of the auricle – right here – to then this membrane here which is known as the tympanic membrane, or famously known as the eardrum. And this is a distance of approximately 2.5 centimeters. Now, the lateral one-third is formed by cartilage – as you can also see here – while the medial two-thirds is essentially a bony tunnel formed by this bone here which is the temporal bone. Throughout its length, the external acoustic meatus is covered with skin some of which contains hair and modified sweat glands, cerumen or as you know as earwax.
I'd like to add here an important clinical note associated to the external acoustic meatus. Now, this structure does not follow a straight course. The observation of the external acoustic meatus and tympanic membrane in clinical examination requires what is known as a special tool known as the otoscope and can be improved by pulling the ear superiorly, posteriorly and slightly laterally. Now, this type of examination is very important in cases of external otitis, or as it is widely known as swimmer's ear. Now, this is a painful condition resulting from infection in the external acoustic meatus and, as the name indicates, it happens often to swimmers.
We're going to continue on this course a bit further medially to then highlight this membrane here which is the eardrum or the tympanic membrane. It forms a boundary between the external acoustic meatus of the outer ear and the middle ear. So, the outer ear here and the middle ear about here. Its function is to then transmit sound waves from the external acoustic meatus to the ossicles inside the middle ear – as you can see here, these ossicles, which we will talk about – and to the oval window converting the vibrations in the ear to vibrations in fluid.
I also like to add here a clinical note associated to the eardrum. One clinical condition that is related to the eardrum is the tympanic membrane perforation. Although this type of perforation has many causes, trauma and infection are still the common – the most common – causes today. Now, most ruptures of the tympanic membrane tend to heal spontaneously but surgical intervention may be necessary if the rupture is large.
Another structure you can see here and before we move on to look at the structures of the middle ear, I'd like to highlight this structure that we talked about before – this is a bone – this is the temporal bone. This bone houses all the parts of the ear – so houses all three parts of the ear – the external ear, the middle ear and also the inner ear. And just for a bit of visual here, this is the external ear, the middle ear and then we have here the inner ear. So, again, this is the external ear, middle ear and then the inner ear going from the sides all the way medially to the inner ear.
The temporal bone is a large bone of the skull that is bilaterally symmetrical and forms the base of the cranial vault. Now, this is just a little bit of information about the bone that you probably have seen on other tutorials here at Kenhub but I would like to add that this bone ascends to participate in the formation of the lateral walls of your skull and is comprised of a squamous part, tympanic part, the styloid process, and a petrous part. The tympanic part of the temporal bone contributes to the anterior, posterior and inferior walls of the external acoustic meatus. The middle ear and the inner ear as well are located inside the petrous part of the temporal bone.
Now, let's have a look at the middle ear. Now, the middle ear is an air-filled mucous-lined space in the temporal bone between the eardrum which you see here – or the tympanic membrane – laterally and also the wall of the inner ear medially. It consists of two parts – one that you see here highlighted in green which is the tympanic cavity which is a hollow recess and the epitympanic recess which is basically the dome of the tympanic cavity. Now, the tympanic cavity is separated from the external acoustic meatus by the tympanic membrane as we talked about and you see a little bit here of the external acoustic meatus. It is located in the temporal bone and it contains three important bones that we're going to talk about – the incus, the malleus and the stapes. These bones that you can find – these three bones you find them here. With these bones, the vibrations from sound waves are then transmitted from the external environment to your inner ear.
The next structure we're going to be seeing here is known as the epitympanic recess. And as we saw in the previous slide, is basically the dome of the tympanic cavity. This hollow recess is located on the superior aspect of the middle ear above the upper margin of the tympanic membrane.
The next structure we see here highlighted in green is one of the three ossicles that I mentioned previously and that is the malleus. The malleus is also known as the hammer and is a small bone of the middle ear and the first of the three ossicles. It lies between the tympanic membrane and the incus – this other bone here that we're going to be talking about soon. The malleus bounces upon the tympanic membrane and functions to then transmit sound vibrations from the tympanic membrane to the incus.
Now, the incus that you see now highlighted in green is the second of the ossicles of the middle ear. This is an anvil-shaped bone which connects the malleus to the other one, the stapes – this is the stapes which we will talk about next. Now, the incus forms an articulation with the malleus, receives vibrations from the malleus, and transmits them to the stapes which happens to be the next bone that we're going to be highlighting here – the stapes – and this is the third ossicle found in the middle ear which receives vibrations from the incus and transmits them to the oval window to which it is connected by the annular ligament. The oval window is the –what we call – the doorway to the inner ear. The stapes is the smallest of the bones of the middle ear and is stirrup-shaped – hence its name – and it is a vital component in the conduction of sound from your middle ear to your inner ear.
On the next image on this highlight, we can see the base of the stapes which rests on the oval window and is sometimes to as referred to as the footplate. Another part of the stapes is this seen here highlighted in green which is known as the crura. The crura of the stapes refers to the anterior and posterior limbs of the stapes that connect the head of the stapes which then articulates with the incus with the base of the stapes. So, you can see here the head of the stapes then connected to the incus and how the two crura then connect the head with the base of the stapes.
So, we have seen three ossicles here – the malleus, the incus and the stapes. They form an osseous chain across the middle ear and transmit the vibrations from the tympanic membrane to the oval window, and throughout this to then the inner ear. Just below to the oval window, we see another opening known as the round window. The round window is an opening at the end of the scala tympani located below and behind the oval window. Now, this opening is closed by the secondary tympanic membrane and its function is to compensate the changes in the cochlear pressure.
Now, before we move on to the inner ear, let's look at a couple of structures that are not part of the middle ear that are related to it. The first of these is this one that you see here highlighted in green, the tegmen tympani. This is a thin plate of bone that forms the roof of the tympanic cavity and lies lateral to the arcuate eminence of the petrous part of the temporal bone. The stapes and incus are attached to the atticus of the tegmen tympani and this structure separates the cranial cavity from the tympanic cavity. Because, if you remember, this is the temporal bone and the cranial cavity should be here – so above all these structures here.
Another structure that is related to the middle ear is this one here highlighted in green known as the Eustachian tube, sometimes, simply referred to as the auditory tube. The Eustachian tube, also known as the pharyngotympanic tube, is basically a narrow canal that is located between the anterior wall of the middle ear and the lateral wall of the nasopharynx. This comprise of both bone and cartilage and is about four centimeters in length. The Eustachian tube is closed under normal circumstances, however, in the cases where pressure needs to be equalized between the middle ear and the atmosphere – for example, during take-off in a plane when you fly or for example when you scuba dive – the Eustachian tube can open to then allow a small amount of air through. By doing so, this equalizes the pressure and prevents damage to your middle ear. The Eustachian tube also functions to drain mucuos from the middle ear.
We just saw the Eustachian tube connects the middle ear with the nasopharynx that you see here highlighted in green. So, this tube here is connecting the middle ear found right about here and then a bit of the nasopharynx – a cut of the nasopharynx – that you see here on this image.
Even though the main focus of this tutorial is the outer ear and the auditory tube, we still talk about a few structures that we can see here in the middle ear as well as some structures that we're going to be discussing now around here which is known as the internal or the inner ear. It's always good to talk about all these structures so you can understand what is happening here and how they are connected to one another, and they are clearly seen here from this image and perspective.
The internal ear consists of a series of bony cavities – the bony labyrinth – and then a few membranous ducts and sacs known as the membranous labyrinth within these cavities. All these structures are in the petrous part of the temporal bone so they are all contained within, more or less, this area here found between the middle ear laterally and the internal acoustic meatus medially. The bony labyrinth consists of the vestibule, three semicircular canals and the cochlea – structures that we're going to be highlighting later on on this tutorial. Now, these bony cavities are lined with periosteum and contain a clear fluid known as perilymph.
So, as I mentioned, on the next slides, we're going to be seeing these structures. The first one here is the vestibule of the ear. This is the most central part of the bony labyrinth and contains the oval window in its lateral wall. It encloses parts of the bouncing apparatus – so the inner ear – known as the utricle and the saccule which cannot be seen here in this illustration. It communicates anteriorly with the cochlea and posteriorly with the semicircular canals.
As I just mentioned, projecting in a posterosuperior direction from the vestibule are three semicircular canals – the anterior semicircular canal, there is also a posterior and lateral one – but here on this image, we see then highlighted in green the anterior semicircular canal. This one is oriented vertically and lies slightly perpendicular to the petrous part of the temporal bone. It contains perilymph and encloses a membranous semicircular duct as to the other two semicircular canals. It functions to detect rotations of the head in the sagittal plane. Now, the anterior semicircular canal also shares a common crus with the posterior semicircular canal which opens into the upper and medial part of the vestibule.
Next, we're going to be seeing here highlighted what is known to be as the bony ampulla – or plural bony ampullae – which are dilations found at the base of each of the three semicircular canals. Each of the three semicircular canals forms two-thirds of a circle connected at both ends of the vestibule and with one end dilated to form the ampulla that you are seeing here highlighted in green. There is an anterior, a posterior and a lateral bony ampulla – so three ampullae. Each one corresponding to one of the semicircular canals. The bony ampullae also contain the crista ampullaris which is responsible for providing the sense of spatial orientation. On this image here where I show a bit more of the labyrinth, you can see one ampulla highlighted, this one is the lateral one.
Projecting in an anterior direction from the vestibule is this one – this structure that you see here highlighted in green – the cochlea.
The cochlea is a spiral-shaped cavity that makes up the auditory portion of the inner ear. This spiral-shaped cavity is a hollow, conical chamber of bone making up about two-halves to two, three-quarter turns around its axis. As you can see here in this illustration, the cochlea looks like a snail shell and its function is to propagate sound waves from the base to the apex. It also contains specialized structures including – now on this image, you can see a bit of a close-up here highlight of the cochlea. So, it contains specialized structures known as the scala vestibuli, the scala tympani and the scala media also known as the cochlear duct – so, these are three chambers – also, the Reissner's membrane and the basilar membrane which separate the three chambers, the organ of Corti which is a sensory epithelium found on the basilar membrane, some hair cells as well which are specialized sensory cells in the organ of Corti, and helicotrema which we will discuss next which is seen now here highlighted in green, this is the helicotrema.
This is where the scala vestibuli and the scala tympani meet at the apex of the cochlea. Hair cells found in the area of the helicotrema are best suited for detection of low frequency sounds. You can also see here this highlighted structure – a bit of zoom in here – for the, where you can see then clearly the helicotrema.
Now that we have discussed some of the structures of the external, middle and inner ears, let's look at nerves that supply this area. And firstly, this one that you see here highlighted in green is known as the facial nerve, also known as cranial nerve number seven. This one emerges from your brainstem and the motor and sensory parts pass together through the posterior cranial fossa and entering the petrous part of the temporal bone through the internal acoustic meatus. You see here the part of the nerve on this image which passes through then the internal acoustic meatus before exiting it and entering the facial canal and it's coursed towards the stylomastoid foramen. We can also see the facial nerve here on this image where we focus on the labyrinth – so the facial nerve seen here highlighted in green.
The next nerve we're going to be highlighting now on this image is known as the vestibulocochlear nerve. This is also known as the eighth cranial nerve. This is a sensory nerve and it contains both vestibular and cochlear fibers. And the cochlear fibers, they terminate at the cochlea – in the cochlea to be more specific – and are responsible for hearing.
On this image here, we see the inferior view of the brain where you can then see the vestibulocochlear nerve highlighted in green and all the other cranial nerves as well but this time in yellow. And as I mentioned before, the vestibulocochlear nerve also has then vestibular fibers which terminate in the ampullae of the vestibulum and are responsible for balance in your body.
As we just saw, the vestibulocochlear nerve is comprised of two nerves. The cochlear nerve that you see here highlighted in green carries then auditory sensory information from the cochlea to the brain directly – and you can also see the cochlear nerve highlighted in green on this image of the labyrinth.
The next part of the vestibulocochlear nerve that you also see here more, in a bit more focused and highlighted in green – notice this little piece here highlighted in green behind, a bit behind the facial nerve – this is then the vestibular nerve. It carries spatial? (20:26) information from the semilunar canals to the brain. It transmits sensory information from the vestibular hair cells located in the utricle, saccule and the semilunar canals via the vestibular ganglion. Now, individuals that suffered vestibular nerve damage can experience acute attacks of vertigo.
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