The Anatomy of the Middle and Inner Ear

Will Styler - LIGN 113

Today’s Plan

• The Middle Ear

• The Vestibular System

• The Cochlea

The Middle Ear

The Outer Ear ends at the TM

The Middle Ear

The Eustachian Tubes

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

The Ossicular Chain

Comprised of three bones

• Malleus (‘Hammer’)

• Incus (‘Anvil’)

• Stapes (‘Stirrup’)

• Suspended by ligaments and muscles

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

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

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!

Enabling the acoustic reflex

Signal Amplification and Impedance Matching

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

Protecting the Cochlea

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

This is a feature, not a bug!

• Better to injure the ossicles than cochlea!

• They can heal and repair, often

• Prosthetic ossicles are a thing!

The Chorda Tympani Nerve

The Chorda Tympani Nerve

• Carries tongue sensation, among other things

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

That’s the middle ear!

• The ossicular chain

• The musculature involved in the acoustic reflex

• The Eustachian tubes

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

The Vestibular System

• Dedicated to balance and orientation

• Includes both semicircular canals

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

More on this later!

The Cochlea

The Cochlea is a coiled tube

• About 2 3/4 turns

• Uncoiled it’d be ~35mm

  • That’s 1.378” in ’Murica

• We talk about the ‘base’ and ‘apex’ of the cochlea

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

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

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

Connections within the cochlea

• The Scala Tympani and Scala Vestibuli meet at the helicotrema

• The Scala Media terminates at the helicotrema

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

The Round and Oval Windows are both needed

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

Cochlea-internal landmarks

Reissner’s Membrane (also called the ‘Vestibular Membrane’)

The Basilar Membrane

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

Lots more on tonotopic organization later!

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

• … but how do we pick that up?

The Organ of Corti!

Key takeaways

• 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!

Next time

• We’re gonna corti like it’s 1999

Thank you!