Inner Ear

00:01 Now we've reached the deepest part of the ear, the inner ear.

00:05 Which we can think of as having a bony labyrinth and then an inner membranous labyrinth.

00:12 We have an oval window, which interacts with the stapes from the middle ear cavity.

00:19 A round window.

00:21 A spiral shaped cochlea, cochlea actually means snail so it kind of looks like a snail.

00:28 We have these three semicircular canals, an anterior, posterior and lateral canal.

00:36 And then in between more centrally, we have this thing called the vestibule.

00:41 The membranous labyrinth is membranous tissue that lies within these bony components.

00:49 Inside the cochlea, we have the cochlear duct.

00:53 In terms of the vestibular apparatus, we have the saccule and the utricle.

00:58 And then we have these widenings of the semicircular canals called the ampulla for each one.

01:05 We also have a utricosaccular duct and something called an endolymphatic sac and duct that carry something called endolymph.

01:17 Let's first look at the cochlea.

01:20 The cochlea has an outer osseous labyrinth with a space inside, that's going to be our spiral canal of the cochlea.

01:30 There's also going to be a tiny bony projection within this canal.

01:34 That's going to be the osseous spiral lamina.

01:39 The axis of this spiral is something called the modiolus.

01:45 At the center of this, receiving all of these tiny inputs is going to be the cochlear nerve and that's going to go back into the brain and perceive sound.

01:57 Here's a cross section through one of those spiral canals.

02:01 We see that that spiral lamina connects to the surrounding bone via the basilar membrane.

02:09 Then there's another thin membrane called the vestibular or Reissner's membrane, and these two are connected peripherally by a spiral ligament.

02:21 This encloses a space called the cochlear duct or the scalar media.

02:28 On the other side of the vestibular membrane is the scala vestibuli.

02:34 And on the other side of the basilar membrane is the scala tympani.

02:40 Within the cochlear duct side, on the basilar membrane, we have something called the spiral organ of corti and this is the actual organ of hearing.

02:51 This cochlear duct or scala media is filled with a fluid called endolymph.

02:58 The scala vestibuli in tympani, on the other hand, are filled with something called perilymph.

03:05 And it's going to be vibration of this fluid that gets translated by the spiral organ of corti.

03:13 That is passed through a spiral or ganglion into the cochlear nerve.

03:18 That will be how we perceive sound.

03:21 And to zoom out for just a second, we're going to have the vestibular nerve carrying out vestibular information, joining the cochlear nerve to form the vestibulocochlear nerve or cranial nerve VIII, which is going to travel with the facial nerve through the internal acoustic meatus.

03:42 So how is hearing happening? Well, sound is going to be transmitted through the external ear to hit the eardrum to vibrate those ossicles ultimately causing vibration on the oval window.

03:59 And that oval window will cause fluid to vibrate.

04:04 And that vibration will be picked up by little cells on the organ of corti and transmitted as information back to the cochlear nerve.

04:14 That round window is going to basically compensate for pressure changes within the fluid.

04:21 So as the oval window pushes in, a round window will bulge out, so that things don't burst.

04:31 So this leads us to understanding certain types of hearing loss.

04:35 So if we were to have damage to the auditory nerves themselves, or the cochlea, where we have the spiral organ of corti, that will be something called sensory neural deafness, meaning whether or not sound reaches this area, it can't be perceived.

04:54 Which is in contrast to conductive deafness, which is a problem with getting sound waves to the sensory neural apparatus in the first place.

05:05 And that can be external in the ear canal, such as some type of blockage of earwax, it could be a problem with the tympanic membrane, such as a perforation of the eardrum, or it could be a filling defect where there's otitis media or inflammation, filling up the middle ear cavity.

05:27 Now let's take a look at the other portion of the inner ear, the portion responsible for equilibrium.

05:34 What's happening here on a very small scale, in areas like the utricle and the saccule, is that we have this fluid containing otoliths or very tiny stones that can knock over certain types of hair cells.

05:51 So these are cells that have stereocilia, or cilia that don't beat but are really sensing movement.

05:58 And what happens is as the head moves, we have linear acceleration, causing the otoliths to tip over these hair cells.

06:08 And in the case of the utricle, this tells us that our head is moving or accelerating in a linear direction.

06:17 The saccule does something very similar but in a vertical direction, such as sensing that you're moving in an elevator.

06:26 In the semicircular canals, we have something similar happening but for rotational acceleration.

06:33 At the ampulla of the semicircular canals, we have these gelatinous structures called the cupula and more hair cells with rotational acceleration.

06:45 It will tip over these hair cells providing information about rotation, and because the semicircular canals are in the X, Y, Z axis, we can tell which axis the rotation is happening as well as if it's happening in a combination of different axis.