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

  • Inner ear is also called as labyrinth
  • It has two parts; an inner membranous labyrinth and an outer bony labyrinth
  • Membranous labyrinth is a closed structure filled with a fluid, which has no connections to any external structures. (Read this point again and try to understand clearly)
  • Bony labyrinth has connections to middle ear and cranium

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

  • 3 loop like structures called Semicircular canals (Posterior, Superior and Lateral)
  • Utricle – to which all 3 Semicircular canals open via 5 openings (2 for lateral SCC, 1 each for Superior and Posterior and a common opening of Posterior and Superior SCC)
  • Sacule – a sac like structure which has a duct which joins the duct from Utricle and reaches endolymphatic sac.
  • Endolymphatic sac – Endolymphatic sac is located intradurally (between the two layers of dura)
  • Scala Media – A coiled loop like structure at the bottom part of membranous labyrinth. Here we can see that there is a coiled portion and a non coiled portion. Coiled portion is called apex and non coiled portion is called base.

Functions of Membranous Labyrinth

  • Scala media part is responsible for hearing and Semicircular canals, sacule and utricle are responsible for maintaining balance. This means there has to be sensory end organs in inner ear for hearing as well as balance. (Important point.)
  • Sensory end organ of hearing is Organ of Corti. It is a modification inside scala media seen along and throughout the lengths of it.
  • Apex of organ of corti (tip of the coiled portion) is responsible for perception of low frequency sounds and base of organ of corti (linear part of scala media) is responsible for perception of high frequency sounds.

(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.)

  • Sensory end organ of maintaining balance is Cristae and Maculae
    • Cristae is present on the Semicircular canals; there are 3 Cristae one on each SCC
    • Maculae is present on Utricle and Sacule: there are 2 Maculae, one each for Sacule and Utricle.
    • Cristae is responsible for maintaining rotational acceleration (maintaining balance when we turn our head sideways or around)
    • Maculae is responsible for linear acceleration(maintaining balance when we walk straight)

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

  • Bony labyrinth is just a bony cast or covering or coating around the whole membranous labyrinth having the same shape as that of membranous labyrinth, with a fluid inside.
  • This fluid – Perilymph, is present between bony and membranous labyrinth (perilymphatic space).
  • Most important point to note here is that bony labyrinth, unlike membranous labyrinth has external connections. (Read the following points carefully and don’t read further unless you are thorough with the concept)
    • Bony labyrinth is connected laterally to middle ear via Round window and Oval window. Round window is covered by a membrane called Secondary Tympanic Membrane and oval window is covered by the footplate of Stapes.
    • Bony Labyrinth is connected medially to cranium via Cochlear aqueduct (Perilymph is a derivative of CSF which reaches the perilymphatic space via cochlear aqueduct). It is this route through which Labyrynthitis can progress to meningitis and vice versa.

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

  • Bony part around scala media is called Cochlea.
  • Bony part of Superior SCC bulge towards Base of Skull
  • Bony part of Lateral SCC bulges towards Middle ear
  • Bony part of Posterior SCC bulge towards Mastoid
  • Bony part around Utricle and sacule is called Vestibule
  • Bony portion superior to Scala media which communicates with vestibule is called Scala vestibuli
  • Bony portion inferior to scala media is called scala tympani.

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:

  • Menieres disease: Sacule and Utricle is enlarged externally towards bony labyrinth and gets closer to the oval window opening. So vibrations from oval window gets easily transmitted to uricle and sacule and maculae is dislodged leading to vertigo
  • Congenital Syphillis: There is hypermobility of Footplate of stapes, which leads to excessive vibrations being transmitted to inner ear, which is strong enough to dislodge the Maculae, leading to vertigo.

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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