Lecture 18; Sound 2

Question 1

How do we not hear all sound?

Answer 1

The outer and middle ear filter out sounds that dont need to be heard, essentially to focus on speech

Excludes a lot of other sounds and tries to focus on what needs to be heard

Question 2

How does the pina work?

Answer 2

It reflects and deflects sound, filtering it, enabling sound to be heard in space

Question 3

What is the role of the outer ear?

Answer 3

• Important for collecting sounds
• Protects middle ear
• Resonances increases sound pressure at the TM (esp sound Hz of speech)
• Sound localisation
• Skin lining contains serum glands (wax) for self cleaning

Question 4

What is special about the ear canal epithelium?

Answer 4

It migrates

Question 5

Describe ear canal epithelium migration;

Answer 5

“Epithelial Conveyor Belt”

• Skin growth/ production starts at the enter of the ear drum and moves along canal
• Similar speed to fingernail growth

Question 6

What produces ear wax and what is its function?

Answer 6

• Produced my sebaceous and sweat glands
• Very important, has antimicrobial properties(antibodies) and helps clean the ear, as it carried out along with cells and debris
• Lubrication
• Humidifies
• Repels water
• With hairs it traps dust and debris

Question 7

Describe the pressure gain of the outer ear;

Answer 7

• The outer ear is a resonator and there is a pressure gain b/w start and finish of ear canal
• 1/4 wave resonator with peak gain ~ 2-3khz

~20db gain

Question 8

Describe the location of the eustachian tube;

Answer 8

Drains middle ear into nasopharynx

Question 9

Describe how the eustachian tube functions;

Answer 9

• The middle ear contains mucosal layer and produces a lot of mucous which is drained down through the eustachian tube which has a ciliated lining to help with this.
• Maintains pressure of across the ear drum (as the TM moves)

Question 10

Describe the TM strucutre:

Answer 10

Three layers;

• Epithelial layer, continuous with outer ear canal
• Fibrous layer, concentric or circular rings of collagen, or radial
• Mucosal membrane layer, continous with middle ear

Question 11

Whats special about the TM?

Answer 11

It can self repair

• Designed not to lose much energy

Question 12

What is the TM connected to?

Answer 12

The long arm of the malleous/ ossicular chain

Question 13

What are the bones of the ossicular tree?

Answer 13

Malleous
Incus
Stapes

Suspended by ligaments
They’re articulating joints

Question 14

Why is there an ossicular tree? Why not just a membrane?

Answer 14

• Bone conduction; (very good) vibration of the skull transfers energy to the cochlear sensory cells directly
• Less sensitive (40-50db) than air conduction, but very important for monitoring voices

Question 15

Why do we have a middle ear and not just air conduction?

Answer 15

• The inner ear has a lot of impendance and therefore the middle ear needs to transfer a lot of energy through the oval window, thus if it was just air conduction (not ossicles) you would suffer a hearing loss of 30-40db as it requires little energy for sound to move through air. (also why the outer ear acts as a resonator)

Question 16

Whats the function of the middle ear?

Answer 16

• Filters sound so that only hz (voices) you want to detect are heard
• Middle ear acts as a transformer to overcome resistance of inner ear fluid.

Question 17

How does the middle ear act as a transformer?

Answer 17

1. Area difference between TM and stapes (oval window) (F = P.A, so if A decreases P inc) (20 fold inc)
2. Lever ratio of malleous(longer);incus(shorter) 1.5 increased
3. Lever ratio of TM (the ear drum is connicle), large movement of TM, small more forceful oval window)

Question 18

How much of the energy does the middle ear transfer?

Answer 18

90%, so 1-1.5db loss instead of 40db loss

Question 19

Describe energy loss and sound filtering;

Answer 19

Transfer of energy across frequencies is not equivalent but greatest over speech frequencies

Gets rid of low and high hz

Question 20

How does the middle ear filter sound

Answer 20

To do with the relationship between mass and stiffness;

Stiffness;

• Ossicular ligaments
• Eardrum
• Air in middle ear space

Mass
- Ossicular chain

Question 21

What are the muscles of the inner ear?

Answer 21

Tensor tympani

Stapedius

Question 22

What is the tensor tympani?

Answer 22

• Attached to membrane of malleous
• innervated by trigeminal nerve
• tenses ear drum (attenuation relfex)

Question 23

What is the stapedius muscle?

Answer 23

• Attatched to stapes head
• Smallest straited muscle in bod
• Innervated by facial nerve
• Reflex initiated by loud sounds, limits excess movement by stapes (attenuation relfexes)

Also important for focusing on sounds in oud environment

Question 24

Where is the inner ear located?

Answer 24

Deep within the temporal bone

Question 25

What are the parts of the inner ear?

Answer 25

Vestibular system

Cochlear (coiled tube)

Question 26

Where is the oval window/ stapes insertion?

Answer 26

The oval window is in the vestibular system

… so how can we hear?

Question 27

What are the two fluids systems of the inner ear that are shared by the cochlear and vestibular system?

Answer 27

• Perilymphatic fluid
• Endolympatic fluid (has sac and duct)

Both connect with CSF to maintain fluid concentrations

Question 28

Describe perilymph;

Answer 28

Found in the big ducts (scala tympani, scala vestibuli) of the cochlear, similar to that of normal ECF

Na = 150mm
K = 5-10mm

Question 29

Describe the tubes of the cochlear

Answer 29

Scala vestibule
- Scala media ( organ of corti in here)
Scala tympani

Question 30

Describe endolymph;

Answer 30

Na 10mm
K 140mm

Scala media

Essential for endocochlear potential - generated by the stria vascularis(Potential ~80mV)

Question 31

What maintains the fluid concentrations of the cochlear?

Answer 31

Stria vascularis generates this fluid concentrations

Question 32

What seperates scala media from scala vestibule?

Answer 32

Reissners membrnae

Question 33

Describe the organ of corti;

Answer 33

Tectorali membrane

IHC 1 row
-Piller cells (separates sensory cells)
OHC 3-5 rows
- Dieter cells

Basilar membrane (fibrous collagen)

Question 34

Describe sensory cell innervation

Answer 34

IHC (lots of afferent innervation, some efferent) (predominant input)

OHC (lots of efferent innervation, some afferent) (frequency tuning)

Question 35

Describe sensory cell projections;

Answer 35

plates of stereocilia,

Movement of these lead to transduction

Question 36

Describes how the inner ear allows fluid movement?

Answer 36

The round window allows movement of the fluid, pressure relief valve, so the stapes and oval window can may and generate movement of the tectorali membrane.