Anatomy of the inner ear

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Introduction 0:00–0:23

How is it you can listen to your favorite song, close your eyes, dance, and not fall on your face? Well, there's a little thing called the inner ear that contains the vestibulo cochlear organ, which gives you the ability to perceive sounds and maintain your balance.
The inner ear is found in the petrus part of the temporal bone, between the middle ear laterally and the internal acoustic meatus medially.

Position 0:23–0:46

It is a small and important area which houses the irregularly shaped vestibulocochlear organ. Which kind of looks like a snail shell attached to a few bony rings.
Now, the inner ear contains the bony labyrinth and the membranous labyrinth. The bony labyrinth is connected to the middle ear by two windows.

Bony & membranous labyrinths 0:46–1:51

The oval window is found on the lateral wall of the vestibule and is covered by the base of the stapes. While the round window is found at the base of the cochlea and is covered by the secondary tympanic membrane.
The bony labyrinth within the otic capsule is filled with paralymph and is made of a series of cavities, which are the vestibule, the semicircular canals, and the cochlea.
Suspended within the bony labyrinth, there's the membranous labyrinth, which is basically a series of sacs and ducts filled with endo lymph.
The membranous labyrinth is organized into the utericle and saccule within the vestibule, the three semicircular ducts and their membranous ampillae, and the cochlear duct within the cochlea.
Let's take a look at the structures responsible for balance, which are the semicircular canals on the one hand, and the utericle and saccule on the other hand.

Semicircular canals 1:51–2:48

There are 3 semicircular canals an anterior, posterior, and lateral canal oriented in the 3 different planes of space. Each canal contains a dilated end called the ampulla that contains an area of sensory epithelium called the amppillary crest.
This is lined with tiny hair cells that pick up information about rotational movements of the head in the plane of the duct within which it is contained.
The hair cells of the amppillary crest are innervated by the vestibular branch of the vestibulocochlear nerve, and the cell bodies of these neurons are found in the vestibular ganglion.
Now the utericle and the saccule both contain areas of sensory epithelium called maculae, which contain hair cells that detect information about linear acceleration.

Utricle and saccule 2:48–3:15

The hair cells in the utericle detect movements in the horizontal plane, while those in the saccule detect movement in the vertical plane.
The macular hair cells are also innervated by the vestibular branch of the vestibulocochlear nerve. Now that was a lot of info.

Quiz 3:15–3:28

For a quick break, feel free to pause the video and remember the parts of the inner ear that play a role in balance. Now let's switch gears and look at the structures responsible for hearing, starting with the cochlea.

Cochlea 3:28–4:26

The cochlea is a bony tube that spirals on itself, resembling the shape of a snail shell, and contains three fluid-filled cavities called the scala vestibuli, the cochlear duct, and the scalar tympani.
Now, the cochlear duct is between the scalar vestibula and the scala tympani, and its roof is formed by the vestibular membrane, while the floor is formed by the basilar membrane.
The organ of hearing, called the spiral organ or the organ of corti, is fixed to the basilar membrane and contains hair cells.
The tips of these hair cells insert into the tectorial membrane, which is a gelatinous membrane that overlies the spiral organ.
Now, how do these structures help us hear? Well, when you are listening to a song, sound waves travel to the tympanic membrane and cause it to vibrate.

Sound conduction 4:26–5:38

These vibrations are transferred to the auditory ossicles in the middle ear. The base of the stapes in the oval window vibrates and creates waves of hydraulic pressure in the paralymph, which move within the scalar vestibuli.
These waves then reach the heliotrema, which is the point where the scalar vestibula and scala tympani are connected, and then continue on in the scalar tympani.
They travel within the scalar tympani to reach the round window where the pressure waves are dampened by the secondary tympanic membrane.
While traveling within the cochlea, the pressure waves push on the basilar membrane and cause it to move, stimulating the hair cells of the organ of corti.
This creates action potentials that are sent along the cochlear branch of the vestibulocochlear nerve, which has cell bodies in the spiral ganglion.
The cochlear branch and the vestibular branch come together to form the vestibulocochlear nerve. The nerve then travels through the internal acoustic meatus, where it is accompanied by the facial nerve and the labyrinthine artery.

Nerves 5:38–6:08

The vestibulocochlear nerve exits the internal acoustic meatus into the posterior cranial fossa and enters the brain stem.
Just a quick break before the recap. Can you name all the elements in this image?

Quiz 6:08–6:20

All right, as a quick recap, the inner ear is found in the petrus part of the temporal bone and is made of the bony labyrinth, which contains cavities filled with paralymph, and the membranous labyrinth, which is made of sacks and ducts suspended in the bony labyrinth.

Review 6:20–8:24

The parts of the bony labyrinth are the semicircular canals, the vestibule, and the cochlea. The membranous labyrinth is organized into the cochlear duct, the utericle and saccule, and the three semicircular ducts.
Both the utericle and saccule contain areas of sensory epithelium called the maculae, which detect linear acceleration. Each ampulla of a semicircular duct contains an area of sensory epithelium called the amppillary crest and detects rotational movements of the head.
Both of these cells are innervated by the vestibular branch of the vestibulocochlear nerve. The cochlea is a bony tube that spirals on itself, containing three fluid-filled cavities called the scale vestibuli, the cochlear duct.
And the scale timpani. The roof of the cochlear duct is formed by the vestibular membrane.
While the floor is formed by the basilar membrane. The organ of hearing, called the organ of corti, is fixed to the basilar membrane and is covered by the tectorial membrane.
The base of the stapes vibrates, causing waves in the paralymph in the scalar vestibula, which continue within the scala tympani at the heliotrema to reach the round window.
Pressure waves push on the basilar membrane, causing it to move and stimulating the hair cells of the organ of corti. Which are innervated by the cochlear branch of the vestibulo cochlear nerve.