Anatomy and physiology of the ear

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Introduction0:00–0:38

Our ears help us hear and balance ourselves in space, and they have three parts. The first part is the outer ear which is the part you see and hang earrings on, called the pinna, as well as the ear canal.
The second part is the middle ear, which is a tiny chamber that houses even tinier ear bones—the malleus, incus, and stapes.
The third part is the inner ear, which contains the cochlea, a special structure that converts sound waves into electrical impulses for the brain, as well as the semicircular canals which help with balance.

External ear0:38–1:31

Let’s start with the external ear. The pinna, also called the auricle, is made up of cartilage that gives our ears their various shapes and sizes, and it also has a fleshy bit at the bottom- called the ear lobe, or lobule.
The pinna directs sound waves towards the opening of the ear canal. The ear canal, or the external acoustic meatus, is a short, curved tube that burrows through the temporal bone for about 1 inch - or 2 and a half centimeters - and ends at the tympanic membrane.
On the inside, the ear canal is covered by skin, along with hair follicles and ceruminous glands - which secrete cerumen, or the sticky, yellow-ish, earwax.
Cerumen helps prevents foreign objects or tiny insects from getting in and damaging the tympanic membrane. That’s a creepy thought.

Tympanic membrane1:31–1:54

The tympanic membrane is also called the eardrum, and it’s a thin, translucent membrane that separates the external ear from the middle ear.
It’s shaped a bit like a cone, protruding slightly into the middle ear. When sound waves reach the eardrum, it vibrates and transmits those vibrations to the tiny bones in the middle ear.

Middle ear1:54–3:47

Now, the middle ear is an air-filled cavity inside the temporal bone, shaped like tiny chamber with 4 walls, a floor and a roof.
The eardrum makes up the lateral wall of this cavity - and opposite from it there is the medial, or internal wall, that separates the middle ear from the inner ear.
The internal wall has two windows: an oval window above, and a round window below. The two other walls of the middle ear are the posterior wall - towards the back of our head - and the anterior wall - towards the front.
The posterior wall has an opening called the mastoid antrum, and it connects the middle ear with the mastoid cavity within the temporal bone.
The anterior wall has an opening for the eustachian tube, which connects the middle ear to the nasopharynx. The eustachian tube has three main functions—equalizing pressure across the tympanic membrane, protecting the middle ear from reflux of fluids going up from the nasopharynx, and clearing out middle ear secretions.
The roof of the middle ear is dome-shaped, and it’s called the epitympanic recess. Finally, the floor of the middle ear is a thin layer of bone that sits right above our friendly basement neighbor - the jugular vein.
Now, inside the tiny middle ear chamber, there are three tiny bones that are arranged from the eardrum to the oval window: the malleus, incus, and stapes - named after their resemblance to a hammer, an anvil, and stirrups, respectively.
The “handle” of the malleus rests on the eardrum, and the base of the stapes is on the oval window - so when the eardrum vibrates, the vibrations are transmitted from the malleus to the incus, then to the stapes, and finally to the oval window, which transfers the vibrations over to the inner ear.
The inner ear, sometimes called the labyrinth, is a marvelous bit of anatomical engineering. On the outside, the inner ear has a tough bony shell - the bony labyrinth; and inside the bony labyrinth, there is the membranous labyrinth.

Inner ear3:47–5:19

Now, both of these sections are filled with fluid - the bony labyrinth contains a fluid called perilymph, while the membranous labyrinth contains endolymph.
The bony and membranous labyrinth make up the structure of all three parts of the inner ear. The first is the vestibule, which is like a hallway that leads up to two other parts or rooms - the cochlea, towards the front of our head, that deals with hearing, and a second room containing the three semicircular canals, towards the back, which play a role in balance.
The movement of perilymph and endolymph within the labyrinth forms the basis for both hearing and balance. The cochlea is shaped like a snail’s shell, and inside it, and in cross-section there are three parts: from top to bottom, there’s the scala vestibuli, which is connected to the middle ear through the oval window, and contains perilymph.
Then there’s the cochlear duct, which is filled with endolymph, and houses the organ of Corti. The organ of Corti is the mastermind of our hearing sense, and it contains our hearing receptors, or hair cells.
Finally, there’s the scala tympani, which is connected to the middle ear through the round window, and it also contains perilymph.

Sound transmission5:19–6:09

So let’s say your kitten starts to meow for food at 6am - even though her bowl is still half full. When she meows, the sound vibrations travel through the external ear and the ossicles in the middle ear.
When the foot of the stapes beats against the oval window - it transfers the vibrations over to the perilymph inside the scala vestibuli, forcing the fluid into motion.
This motion transmits to the organ of Corti inside the cochlear duct. The hearing receptors convert the vibrations into an electrical impulse.
And the electrical impulse is sent to the brain via the auditory branch of the eight cranial nerve, and the perilymph inside the scala tympani is also set in motion.
This makes the round window bulge back out towards the middle ear, relieving the pressure. So now your brain knows the cat is meowing - you want to get out of bed and feed it.

Vestibular apparatus6:09–7:58

But, even the tiniest movement, like getting your head off the pillow, engages the second part of your inner ear - the vestibular apparatus.
Now the vestibular apparatus - has two parts - the first part, the three semicircular canals, and the second part, which includes both the utricle and saccule - both of which deal with different aspects of balance.
The three semicircular canals are shaped like three letter U’s oriented in the three directions of space, with each of them forming a 90 degree angle with the other two - kinda like the corner of box, where the 3 sides meet.
So there’s an anterior, a posterior, and a lateral semicircular canal, and inside each one is a membranous semicircular duct - which contains endolymph, and opens in the utricle.
At one end of these canals, there is an enlarged portion called the ampulla, that detects changes in our head rotation - so our dynamic equilibrium.
Inside the ampulla, there are balance receptors - also called hair cells. When we get out of bed to feed the cat, the endolymph inside the canals moves and the hair cells fire off an electrical signal.
That signal’s picked up by the vestibular branch of the eight cranial nerve, and it carries the signal all the way up to the brain.
Now, finally, we have the utricle and the saccule, which are also filled with endolymph. Both of them have a region called the macula, which contain sensory cells called balance receptors.
Balance receptors detect changes in our static equilibrium - so changes in our head position in relation to horizontal or vertical acceleration, like when we feel pushed towards the back of the seat in a speeding car, or when we go up or down an elevator.
Alright, as a quick recap… The ear is the organ responsible for our sense of hearing and balance. Sound waves come in through the outer ear and make the eardrum vibrate, and those vibrations get amplified through the tiny bones of the middle ear, and reach the inner ear.

Review7:58–8:35

In the inner ear, the motion of perilymph and endolymph converts sound waves to electrical impulses which are sent to the brain.
The inner ear also houses the vestibular apparatus which is made up of the three semicircular canals, which perceive rotational movement, and also of the utricle and saccule, which detect our head position in relation to gravity.