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Anatomy of Inner ear – The Vestibular Apparatus

The diseases of vestibular apparatus (aka inner ear) sounds so confusing and difficult the first time we hear it because of the lack of understanding of the inner ear anatomy. So let’s understand the inner ear anatomy and trust me on this – You will understand diseases of inner ear better after reading this.

Let’s understand certain things before we go deeper in this topic

Take some 10-15 seconds and look at this image and try to get an orientation about the anatomy of inner ear; its medial and lateral sides, its relations with adjacent structures and all.

Cut section of ear

Let’s first learn about the inner structure –

Membranous labyrinth.

Imagine a water balloon. It has water inside and it has no external connections. The water stays inside no matter what. Membranous labyrinth is that water balloon and the fluid inside is called Endolymph.

This spherical water balloon now changes its shape into a more complex structure with endolymph freely floating inside.

We already mentioned that membranous labyrinth has no external connections, which means that endolymph has to be secreted and absorbed from within the structure. Endolymph is secreted by scala media and is absorbed by endolymphatic sac (both – parts of membranous labyrinth).

Here is an image of Membranous labyrinth alone. (This image is a two dimensional representation of membranous labyrinth for easy labeling and better understanding. Kindly look back at the first image to get a three dimensional orientation of inner ear)

Membranous labyrinth alone

Lets dig deeper.

Membranous labyrinth has the following parts

Functions of Membranous Labyrinth

(Diseases involving apex of cochlea will lead to defective perception of low frequency sound and those involving base of cochlea will lead to defective perception of high frequency sounds.)

Lets have a quick recap. Here is the same image of membranous labyrinth without labeling. Try to identify all the parts and cross check with the labeled image.

Bony Labryrinth

Inner ear image – bony + membranous labyrinth

Note this one fact – Oval window is opening into bony labyrinth at the region of vestibule – which is the region around Utricle and sacule (regions responsible for linear balance). This point is crucial in understanding certain clinical things which we will soon discuss.

Other features of Bony labyrinth

Few more points about cochlea

Cochlea is the part of bony labyrinth around scala media.

Lets understand this a bit more.

Imagine a ball point pen. The body of the pen is the bony cochlea. The refill tube inside it is the scala media. The ink inside the refill tube is endolymph and the air between body of the pen and the refill tube is perilymph. Try to visualize this.

Now coil this ball point pen 2.75 turns – You have cochlea

There are some membranous separations inside cochlea, which is better depicted with a diagram below.

Cut section image of cochlea

Clinical aspects

Sound waves are transmitted from middle ear to inner ear via the oval window.

The vibrations are picked up by the scala media and organ of corti inside it, which is sensed by hair cells and impulse is generated which is then perceived as sound.

We discussed earlier that oval window opens on vestibule, which is located around sacule and utricle.

In normal cases, the vibrations transmitted by oval window are not strong enough to dislodge or disrupt Maculae, the sensory end organs in Utricle and Sacule and create vertigo.

However due to some reason, if vibrations from oval window dislodge Maculae, the person will develop vertigo associated with sound perception.

This is seen in:

So that’s it! I hope the anatomy of inner ear is clear to you now.

Stay tuned for more detailed notes

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