Hearing

Question 1

Main function of the External Ear

Answer 1

To gather sound energy and focus it onto tympanic membrane.

Question 2

Structural components of the External Ear

Answer 2

External Auditory Meatus (EAM)
The Pinna
The Concha

Question 3

Role of the Pinna and the Concha

Answer 3

Involved in elevation detection as selectively filter diff. frequencies to provide cues on elevation

Question 4

How is the structure of the Pinna related to its function?

Answer 4

The vertically asymmetrical convolutions of the pinna allow the external ear to transmit higher frequency components from elevated sources rather than those at ear level.

Question 5

Structure and Function of the External Auditory Meatus

Answer 5

• Open-ended tube of length 2-3 cm
• Specific length allows it to selectively boost sound pressure 30-100x at 2-5kH via passive resonance effects
• Makes humans esp. sensitive to this frequency range.

Question 6

Action of tympanic membrane in response to collision with sound waves specifically directed by external ear

Answer 6

As the sound wave exists as a cycle of increasing and decreasing air pressure, the membrane vibrates and moves inwards and outwards matching the timing of the wave.

Question 7

Main function of the middle ear

Answer 7

To ensure the efficient transmission of sound from the air into the fluid-filled chambers of the inner ear through amplification.

Question 8

What are the three inner ear bones each called and referred to as collectively?

Answer 8

The ossicles: the malleus, incus and stapes

Question 9

How do the ossicles respond to the movement of the tympanic membrane?

Answer 9

The vibrations of the tympanic membrane displaces the malleus, which in turn causes the incus and the stapes to move also.
Stapes acts like a piston, alternately pushing against and retracting from the oval window.

Question 10

Define impedance

Answer 10

A medium’s resistance to movement.

Question 11

What happens when sound travels from a low to high impedance?

Answer 11

Much of the acoustic energy is reflected and thus not transmitted effectively.

Question 12

Define impedance matching

Answer 12

A process that matches low-impedance airborne sounds to the higher-impedance fluid of the inner ear.
Achieved by boosting the pressure at the tympanic membrane by 200 fold by the time it reaches the inner ear.

Question 13

How does the pressure boosting necessary for impedance matching occur?

Answer 13

Force impinging on large-diameter TYP membrane is focused onto much smaller window of oval window. This buckling motion combined with lever action of the ossicles increases the pressure of the sound waves by the time they reach the inner ear.

Question 14

Name and describe the middle ear muscles

Answer 14

The tensor tympani that connects to the neck of the malleus, and the stapedius that connects to the neck of the stapes

Question 15

Role of the middle ear muscles

Answer 15

Protect the inner ear from loud noises and own vocalisation.

Question 16

How do the middle ear musscles carry out their function?

Answer 16

Contract automatically in response to loud noises (100ms after) or self-generated vocalisations (before they occur). Counteracts the mvmt of the ossicles THF reducing amount of sound energy transmitted to cochlea.

Question 17

Main function of the cochlea

Answer 17

Site at which energy from sonically generated sound waves is amplified and transformed into neural impulses. Also acts as mechanical frequency analyser, decomposing complex acoustic signals into component frequencies.

Question 18

Length of cochlea coiled vs uncoiled

Answer 18

Coiled: ~10mm wide
Uncoiled: ~35mm long

Question 19

Oval and round windows

Answer 19

Located at the basal end of the tube

Question 20

What are the fluid-filled chambres on each side of the cochlear partition called?

Answer 20

The scala vestibuli

The scala tympani

Question 21

What is the name of the chamber running within the cochlear partition?

Answer 21

Scala media

Question 22

Describe the cochlear partition

Answer 22

A flexible structure that bisects the tube and supports the basilar and tympanic membranes. Is narrower and stiffer near the base & vibrates maximally for high frequency sounds near the base and at the apex for lower frequency sounds.

Question 23

Define the helicotrema

Answer 23

The opening that joins the scala vestibuli to the scala tympani, allowing their fluid to mix.

Question 24

What fluid is contained within the scala vestibuli?

Answer 24

Perilymph

Question 25

What fluid is contained within the scala tympani

Answer 25

Perilymph

Question 26

What fluid is contained within the scala media?

Answer 26

Endolymph

Question 27

Which window directly leads to the scala tympani?

Answer 27

Round window

Question 28

Which window directly leads to the scala vestibuli?

Answer 28

Oval window

Question 29

How does the cochlear respond to the movement of the oval window?

Answer 29

Causes the incompressible fluids in the cochlea to vibrate and displaces them

Question 30

How does the inner ear apparatus respond to the (inward) movement of the oval window?

Answer 30

The (inward) movement of the window displaces the perilymph inside the scala vestibuli.
As the fluid is incompressible, the vibrating fluid stretches the cochlea partition (BM) deflecting it downwards.
This in turn increases the pressure in the scala tympani which displaces the tympani perilymph.
This movement of the fluid pushes the round window out.
THF, each cycle of sound stimulus evokes a complete cycle of up and down movement of the basilar membrane.

Question 31

Physical properties of the basilar membrane and the resulting effects on its movement.

Answer 31

Thinner at the base and grows wider as it reaches the apex.
Stiff at the base but decreases in stiffness as it reaches the apex.
The non-uniform stiffness and width of the basilar membrane means that for each tone of sound, different parts of the membrane do not oscillate in phase - some sections will be moving up whilst others are moving down etc.

Question 32

Define a travelling wave

Answer 32

A unique moving waveform whose point of maximal displacement traces out a specific set of locations.

Question 33

Define the organ of corti

Answer 33

The receptor organ of the inner ear. A ridge of epithelium extending along the basilar membrane that contains roughly 16,000 hair cells and is innervated by 30,000 afferent fibres. Single row of inner hair cells (3,500) and 3 rows of outer hair cells (12,000). Supported by phalangeal cells.

Question 34

How are auditory stimuli with different frequencies represented by the movement of the basilar membrane?

Answer 34

Different auditory stimuli that have different frequencies will cause a peak in the wave at different locations along the basilar membrane.
High frequency sounds will peak nearer the base.
Lower frequency sounds will peak nearer the apex.
Basilar membrane is THF tuned to different frequencies at each point along its length.

Question 35

Describe the tectorial membrane

Answer 35

Covers the organ of corti. Is anchored at its base by interdental cells and its distal edge forms a fragile connection with the organ of corti.

Question 36

How does the movement of the travelling wave affect the movement of the basilar and tectorial membranes and what structural features allow this to occur?

Answer 36

The movement of the wave displaces the basilar membrane causing it to move up and down. As it runs parallel to the organ of corti and the tectorial membrane, these structures move as well. However, as the basilar and tectorial membrane are anchored at different points, they pivot about different positions of insertion. This causes a shearing motion between the two membranes.

Question 37

Describe the shearing motion observed between the basilar and tectorial membranes and how this affects the hair cells

Answer 37

Occurs when the membrane on top (tectorial) moves more than that on the bottom (basilar). This force occurs btw the hair cell stereocilia embedded in the tectorial membrane and the hair cells themselves embedded in the basilar membrane. It is this shearing force that transduces mechanical energy into electrical energy.