LIGN 113 - Middle and Inner Ear Anatomy

# The Anatomy of the Middle and Inner Ear

Will Styler - LIGN 113

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### Today's Plan

- The Middle Ear

- The Vestibular System

- The Cochlea

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## The Middle Ear

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### The Outer Ear ends at the TM

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### The Middle Ear

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### The Eustachian Tubes

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### The Eustachian Tubes

- a.k.a 'Auditory Tube', 'Pharyngotympanic tube'

- Serve to regulate pressure inside the middle ear

- Allowing release of vaccuum and pressure in both directions

- Not always open!

- Opened by the tensor and levator veli palatini

- Otherwise, you'd hear your nasopharynx more!

- Contains a bony and cartilaginous portion

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### The Ossicular Chain

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### Comprised of three bones

- Malleus ('Hammer')

- Incus ('Anvil')

- Stapes ('Stirrup')

- Suspended by ligaments and muscles

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### The Ossicular Chain connects the TM to the cochlea

- TM is attached to the malleus

- Malleus to incus to stapes

- via little tiny joints

- Stapes to the Oval window

- Ligaments serve to support the ossicles within the middle ear

- ... and allow them to pivot

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### The Ossicles serve four important roles

- They promote the acoustic reflex

- They amplify the acoustic signal

- They match the impedance between (external) air and (cochlear) fluid

- They protect the cochlea

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### The Acoustic Reflex

- Tightening the *stapedius* and *tensor tympani* muscles dampens the vibration of the ossicles

- This happens in (delayed) response to loud noises and our own speech

- More on this later!

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### Enabling the acoustic reflex

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### Signal Amplification and Impedance Matching

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### Impedance Matching and Amplification

- Shrinking from TM to stapes increases force and improves *transduction*

- Movement of bones matches the impedance

- We wouldn't want the TM to be on the side of the cochlea

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## Protecting the Cochlea

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### The Ossicular chain can be injured

- Usually from head injury, TM perforation, or pressure

- Massive 'conductive' hearing loss

- Usually at the incus-stapes joint

- Sometimes at the incus-malleus joint

- An intact stapes usually means better outcomes

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### This is a feature, not a bug!

- Better to injure the ossicles than cochlea!

- They can heal and repair, often

- [Prosthetic ossicles](http://www.gracemedical.com/products/ossicular-prostheses/) are a thing!

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### The Chorda Tympani Nerve

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### The Chorda Tympani Nerve

- Carries tongue sensation, among other things

- One more reason you want your ear drums intact, thanks

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### That's the middle ear!

- The ossicular chain

- The musculature involved in the acoustic reflex

- The Eustachian tubes

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### The 'Inner Ear'

- Comprised of the bony cavities within the temporal bone

- The 'bony labyrinth'

- Contains the cochlea and the vestibular system

- These structures are tiny

- [The Cochlea averages about 9mm long and 5mm high](https://link.springer.com/article/10.1007/s00330-005-2750-9)

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### The Vestibular System

- Dedicated to balance and orientation

- Includes both semicircular canals

- Filled with fluid and a method of detecting movement of that fluid

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### More on this later!

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## The Cochlea

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### The Cochlea is a coiled tube

- About 2 3/4 turns

- Uncoiled it'd be ~35mm

- That's 1.378" in silly units

- We talk about the 'base' and 'apex' of the cochlea

- The base is the widest part, the apex is the narrowest

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### The interior of the tube is complex

- There are three 'cavities' within the tube

- The Scala Vestibuli

- The Scala Media (also 'cochlear duct')

- The Scala Tympani

- The 'tympani' here refers to the round window

- Not the tympanic membrane

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### These cavities are filled with fluid

- 'Perilymph' in the Scalas Vestibuli and Tympani

- 'Peri' like periphery, outside

- 'Endolymph' in the Scala Media

- 'Endo' like 'endonym', inside
	
- **The fact that there are two different fluids is super important later on**

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### Connections within the cochlea

- The Scala Tympani and Scala Vestibuli meet at the *helicotrema*

- The Scala Media terminates at the helicotrema

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### The Scala Tympani and Vestibuli are continuous

- You could swim from the oval window to the round window

- Push off the oval window, up the Vestibuli, to the Helicotrema, down the Tympani, down to the round window

- The Scala media terminates at the helicotrema

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### The Round and Oval Windows are both needed

- The round window pushes out when the oval window (stapes) pushes in

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### Cochlea-internal landmarks

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### Reissner's Membrane (also called the 'Vestibular Membrane')

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### The Basilar Membrane

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### Basilar Membrane (think 'base of the organ of corti')

- This is the surface which sound is able to deflect

- Stiffer at the base, more flexible at the apex

- Different areas respond to different frequencies
	
	- This is called 'tonotopic organization'

- Pressure waves cause basilar membrane displacement

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### Lots more on tonotopic organization later!

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### So, the basilar membrane vibrates in different places depending on frequency of input

- ... but how do we pick that up?

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### The Organ of Corti!

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### Wrapping Up

- The Middle Ear gets sound from the TM to the Oval Window

- The inner ear is a cavity within the temporal bone

- It contains the vestibular loops and the cochlea

- Cochlea is a spiraling wonderland of different cavities

- The basilar membrane moves with sound

- The Vibrations are picked up by the Organ of Corti!

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### Next time

- We're gonna corti like it's 1999

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<huge>Thank you!</huge>