Video: Middle ear

Ladies and gentlemen, this is your captain speaking. We're about to start our descent into Kenhubopolis. We'll be landing in 15 minutes. Thank you for flying Kenhub Airlines.

If you happen to find yourself jetting off from time to time, I'm sure you experienced your ears popping while descending to your destination. But have you ever wondered what exactly in your ear is popping? This sensation is caused by the changes in air pressure affecting a part of your ear, known as the middle ear, and an associated structure, known as the auditory tube.

Let's find out more now as we explore the anatomy, boundaries, and function of the middle ear.

The middle ear is located in the petrous part of the temporal bone and extends from the tympanic membrane of the external ear, also known as the eardrum, to the lateral wall of the internal ear. It comprises two main parts, which are the tympanic cavity as well as the auditory tube.

The tympanic cavity is a somewhat irregularly-shaped, air-filled space which is lined with a mucous membrane and houses the small bones of the middle ear known as the auditory ossicles. It comprises two main parts, the lower of which is the tympanic cavity proper which is continuous anteriorly with the auditory tube, and posteriorly with another cavity of the temporal bone, known as the mastoid antrum. The upper part of the tympanic cavity above the level of the tympanic membrane is known as epitympanic recess, or epitympanum.

Let's explore the boundaries of the tympanic cavity now in further detail.

Overall, it is shaped like a narrow box with concave sides and is bounded by six walls. The roof is largely formed by a thin plate of squamous and petrous parts of the temporal bone called the tegmen tympani and is therefore known as the tegmental wall of the tympanic cavity. It separates the tympanic cavity from the middle cranial fossa. The floor of the tympanic cavity is formed by a thin layer of bone that separates the middle ear from the jugular fossa, which marks the beginning of the internal jugular vein, therefore, this wall is known as the jugular wall.

Laterally, the tympanic cavity is bounded mainly by the tympanic membrane, so this wall is known as the membranous wall. Medially, the tympanic cavity is bounded by what's known as the bony labyrinth of the temporal bone which houses the internal ear, so this wall is known as the labyrinthine wall. It features a prominence known as the promontory of the tympanic cavity which is formed by the base of the cochlea.

The mucous membrane of the promontory is covered by a plexus of nerves known as the tympanic plexus, which we'll look at later.

The medial wall of the tympanic cavity also bears two ‘windows’ to the internal ear. The first is known as the vestibular, or oval window, which is closed in life by the base of this bone here, known as the stapes, and the other is known as the cochlear, or round window, which is closed by what's known as the secondary tympanic membrane.

Continuing on with the posterior wall of the tympanic cavity, you'll remember that the tympanic cavity is continuous posteriorly with the mastoid antrum, so this wall is known as the mastoid wall. An opening in the superior part of this wall, known as the aditus to the mastoid antrum, allows these spaces to communicate with each other.

The facial nerve descends posterior to the mastoid wall in the facial canal. As it passes, it gives off a branch known as the chorda tympani which enters the tympanic cavity via the mastoid wall. The mastoid wall also gives passage to the tendon of the stapedius muscle, which we'll look at a little later in this tutorial.

Finally, let's look at the anterior wall of the tympanic cavity, which is known as the carotid wall. It consists of two parts. The inferior part is formed by the wall of the carotid canal, separating the tympanic cavity from the internal carotid artery, while the superiormost part is formed by the anterior part of the epitympanic recess.

Between these structures is the opening of a bony passageway, which is known as the musculotubal canal. And you can see here that it's actually composed of two channels, or semicanals -- the upper semicanal for the tensor tympani muscle and the lower semicanal for the auditory tube, also known as the Eustachian tube. They're separated by this bony shelf known as the septum of the musculotubal canal.

Now that we've had a closer look at the walls of the tympanic cavity, let's look at this chain of three small bones here, known as the auditory ossicles. These are responsible for transmitting sound vibrations from the tympanic membrane to the internal ear.

The largest of those three bones is the malleus, which literally means hammer. It has three main parts: The head of the malleus is the rounded superior part of the bone that lies within the epitympanic recess. Inferior to the head is a small constricted portion known as the neck. And finally, like all hammers, we have this long elongated portion here, which is the handle of the malleus, also known as the manubrium, which is embedded in the tympanic membrane.

The handle of the malleus has two processes: a lateral, or short process, which attaches to the superior part of the tympanic membrane; and an anterior, or long process, which you can see here. The malleus is suspended by three mallear ligaments: the anterior, superior, and lateral ligaments of the malleus.

The incus is found between the malleus and stapes and is shaped like an anvil. It too consists of three main parts: The large body is located within the epitympanic recess and articulates with the head of the malleus to form the incudomallear joint. A short limb extends posteriorly, while the long limb extends inferiorly from the body. It terminates with a medially-oriented projection known as the lenticular process of the incus.

The incus is suspended by two ligaments, which are the superior and posterior ligaments of the incus.

The stapes is the smallest and most medial member of the auditory ossicles and is perhaps the easiest to identify as it looks like the stirrup of a saddle. Like the other ossicles, it too has three parts: The head articulates with the lenticular process of the incus to form the incudostapedial joint. Extending from the head are two limbs, or crura; one anterior and one posterior, which are both connected to the final part of the stapes -- its base.

Also known as the footplate, the base of the stapes fits right into the vestibular window of the labyrinthine wall of the middle ear, where it is held in place by an annular ligament, together forming a joint known as the tympanostapedial joint, also known as the stapediovestibular joint. This articulation facilitates the transmission of sound vibrations in the middle ear to the fluid in the vestibule of the internal ear.

Let's move on now to look at a number of muscles that are associated with these auditory ossicles.

First is the tensor tympani muscle which is a short muscle which extends from the superior semicanal of the musculotubal canal which we looked at earlier. It originates from the superior surface of the cartilaginous part of the auditory tube, the greater wing of the sphenoid bone, and the petrous part of the temporal bone.

The tensor tympani inserts into the handle of the malleus and pulls the handle of the malleus medially which, as the name suggests, tenses the tympanic membrane reducing the amplitude of sound waves absorbed by it. This is a protective mechanism that prevents damage to the internal ear when exposed to very loud sounds, like when you're at a concert and hearing very loud drums playing.

The tensor tympani muscle is innervated by the nerve to medial pterygoid muscle, a branch of the mandibular nerve.

The second muscle associated with the middle ear is the stapedius. It originates from within the pyramidal eminence, which is a small elevation on the mastoid wall of the middle ear. Its tendon enters the tympanic cavity through its apex and passes forward to attach to the posterior surface of the neck of the stapes.

The stapedius pulls the stapes posteriorly, tightening the annular ligament which holds the stapes in place. This dampens sound vibrations by preventing excessive movement of the stapes, protecting the internal ear from overstimulation from loud sounds.

The stapedius is innervated by the nerve to stapedius muscle, a branch of the facial nerve.

The final structure relating to the middle ear which I want to look at now is this structure here, which is the auditory tube, also known as the Eustachian tube. This is a part-bony, part-cartilaginous structure, which extends anteriorly, inferiorly, and medially from its tympanic opening in the carotid or anterior wall of the tympanic cavity all the way through to its pharyngeal opening in the nasopharynx.

The bony part, which forms the posterolateral third of the auditory tube, is about 1.5 centimeters long and is contained within the inferior semicanal of the musculotubal canal. This part is the narrowest part of the auditory canal.

The cartilaginous part comprises the anteromedial part of the auditory tube and is almost twice the length of the bony part. It is formed by a curled triangular plate of fibrocartilage which is embedded in its wall and also forms an elevation around the pharyngeal opening of the auditory tube, also known as torus tubarius.

The auditory tube is normally collapsed, but when there are changes in atmospheric pressure, the auditory tube serves to equalize pressure in the middle ear. For example, when a plane takes off and reaches a higher altitude, the atmospheric pressure drops. This creates a pulling force on the tympanic membrane, which expands the volume of the middle ear. Air can then be drawn through the auditory tube to equalize the pressure in the middle ear.

When the plane descends, the atmospheric pressure increases, pushing the tympanic membrane medially, therefore compressing air in the middle ear. This pressure can be relieved by the opening of the auditory tube allowing the middle ear to equalize with the nasopharynx.

The three muscles primarily responsible for the opening of the auditory tube are the tensor and levator veli palatini muscles and the salpingopharyngeus muscle.

And that covers the parts and the morphology of the middle ear. Who knew such a small region could have so much to it!

Before we wrap up, let's take a quick look at the neurovasculature of the middle ear, beginning with the arterial supply.

The arterial supply to the middle ear is complex and depends primarily on several branches of the external carotid artery. In total, there are five main branches to consider here.

Let's begin with the anterior tympanic artery, which is a direct branch of the maxillary artery. It supplies the anterior and medial walls of the tympanic cavity as well as the malleus and incus. Next, we have the superior tympanic artery, which is a branch of the middle meningeal artery. It provides arterial supply primarily to the roof of the tympanic cavity.

Continuing clockwise, we have the posterior tympanic artery, a branch of the stylomastoid artery, which is derived from the posterior auricular artery. It supplies the posterior wall of the tympanic cavity as well as the stapes.

Following this, we have the inferior tympanic artery, which is a branch of the ascending pharyngeal artery. It primarily supplies the floor of the tympanic cavity. And finally, we have the caroticotympanic artery which, unlike all of the other branches, arises from the internal carotid artery. It also supplies the anterior and medial walls of the tympanic cavity.

The venous drainage is similarly extensive and is facilitated via several tympanic veins that drain to the retromandibular vein, superior petrosal sinus, and pterygoid venous plexus.

The middle ear is innervated by the tympanic plexus that innervates the mucous membrane lining the walls and contents of the middle ear as well, as the mastoid antrum and bony part of the auditory tube. This plexus is primarily formed by the tympanic nerve, a branch of the glossopharyngeal nerve, and from sympathetic branches from the internal carotid plexus.

This brings us to the end of this anatomy tutorial. So today we looked at the anatomy of the middle ear and explored its boundaries, blood supply, and innervation.

See you next time.