12 - Auditory System 1: structure and function of the Middle and Inner Ear

Question 1

Define sound?

Answer 1

Longitudinal waves passing through a medium.

When a tuning fork moves towards air molecules, it’s called compression.

When the tuning fork moves away from the air molecules, the space left between them is called rarefaction.

Question 2

What is the function of the pinna?

Answer 2

Channels and filters sound to the external canal.

The external canal has resonance properties and amplifies frequencies ~3000 Hz.

Question 3

What is acoustic gain and why is it important?

Answer 3

Amplification you get from the external ear. With the pinna and the canal, there’s a huge acoustic boost that allows you to hear high frequency sounds which is important for speech.

Question 4

What is located in the inner ear?

Answer 4

The eardrum (tympanic membrane) makes up the border of the inner ear.

the malleus attaches to the TM, then the incus attaches to the malleus, and the stapes attaches to the incus at one end and the oval window at the other end.

Question 5

What are the characteristics of the TM?

Answer 5

3 layer translucent membrane about 1 sq. cm vibratory surface.

Cone shaped, which is why when we look at it we can see a reflection of light.

Question 6

What is the function of the middle ear?

Answer 6

Transmit sound pressure waves from air to fluid, but need to overcome the impedance midmatch between media because things can pass more easily through air than fluid.

Question 7

How does the middle ear do impedance matching?

Answer 7

Area ratio, lever action, and bucking of the TM.

This allows for a total gain of the middle ear of ~31 dB.

Question 8

How does the middle ear use area to overcome impedance?

Answer 8

The TM has a large area, while the oval window has a small area.

This difference allows a greater force to be transmitted to the oval window, resulting in a gain of ~23 dB.

Question 9

How does the middle ear use lever action to overcome impedance?

Answer 9

Ossicular chain: the length of the malleus is longer than the process of the incus.

This allows a greater force to be generated and creates a gain of ~2dB.

Question 10

How does the bucking of the TM help us combat impedance?

Answer 10

It allows it to transmit almost twice as much force to the malleus, resulting in a gain of ~6 dB.

Question 11

What are the three chambers found in the segments of the cochlea?

Answer 11

Scala vestibuli, the scala media, and the scala tympani.

The scala media houses the important structures such as the organ of corti.

Question 12

The _____ membrane separates the scala vestibuli from the scala media, while the ______ membrane separates the scala media from the scala tympani.

Answer 12

Reissner’s membrane between the vestibuli and media.

Basilar membrane between media and tympani.

Question 13

Where is the organ of corti located?

Answer 13

It sits on the basilar membrane.

Question 14

Where and what is the stria vascularis?

Answer 14

The blood supply for the cochlea that provides nourishment of structures and ions to maintain the charge of the scala media.

Located on the outer wall of the scala media.

Question 15

Describe the structure of the basilar membrane and its function?

Answer 15

The basilar membrane produces a continuum of encoding frequencies via a traveling wave.

It gets wider and less stiff as you to towards the apex - this part near the apex is for low pitches.

It’s tight and short near the base and results in a higher pitch.

Question 16

Describe the tonotopic organization of the cochlea?

Answer 16

Different frequencies maximally displace the basilar membrane at different points.

High frequencies at base and low frequencies at apex.

Question 17

Describe the hair cells near the tectorial membrane on the basilar membrane?

Answer 17

Hair cells on either side of the .jdahsb.dkjashdlasjdn

Question 18

Describe the shear force that occurs when sound-induced vibration occurs at the basilar membrane.

Answer 18

In the downward phase of the basilar membrane: shear force is inward.

Upward phase: shear force is outward and pushed stereocilia laterally.

Question 19

What is the mechanism by which the cochlea increases the basilar membrane deflection?

Answer 19

There’s an active mechanism other than just the mechanical up and down of the basilar membrane.

This is caused by hair cell motility.

Question 20

Outer hair cells have ____ properties. How does this work?

Answer 20

Motile.

Dynamic mts and microfilaments alone the membrane allow for contractile behaviors.

Question 21

What is the function of the inner hair cells?

Answer 21

Acoustic (mechanical) enegery to electrochemical energy- ie mechanical to neural transduction.

Afferent fibers are 95% innervated by inner hair cells (5% outer), and have 20 contacts per inner hair cell.

Question 22

What is the composition of the endolymph in the scala media? What about the perilymph of the scala vestibuli and scala tympani?

Answer 22

Endolymph is high in K+ and is at about 80 mV.

Scala tympani and vestibuli perilymph is low in K+ and have a charge of 0 mV.

Question 23

What is the charge of inner hair cells?

Answer 23

-45 mV.

Question 24

What occurs when stereocilia are deflected?

Answer 24

Tiplinks are pores on top of stereocilia that open and close.

When they open, K+ flow from the endolymph into the depolarizes the hair cell.

Hair cells release nts.

Question 25

Which deflection phase of the basilar membrane results in depolarization?

Answer 25

The upward phase results in deflection of stereocilia laterally and tip links open, resulting in K+ flowing in to depolarize the cell. Vesicles dock and release nts.

During the downward phase channels close and the hair cells are hyperpolarized.

Question 26

What is the normal intensity of sound that healthy people can hear on an audiogram?

Answer 26

20 dB or softer.

Question 27

What are the types of hearing loss?

Answer 27

Conductive or sensorineural.

Sensorineural can be congenital, due to noise exposure/trauma, medications, or age (presbycusis).

Question 28

What are ways that hearing loss can be treated?

Answer 28

Amplification devices that sit behind the ear, in-ear devices, in-canal devices, completely in canal, and invisible in the canal.

These can be modified to give people the frequencies that they need.

Question 29

What is done when people have ear damage that’s too severe for them to be treated with amplification devices?

Answer 29

Cochlear implant.

These have electrodes with contacts based on where they are in the cochlea and they mimic the frequency range of the normal cochlea.