Session 8 - The Inner Ear

Question 1

Outline the nature of sound (type of wave, speed and how it is measured).

Answer 1

Sound is a Compressive Wave, which travels at 343m/sec in air and at over 1,500m/sec in water.
Sound has two properties:
o Frequency – Hertz (Hz)
o Volume – Decibels (dB)

Question 2

What frequencies are detected by the ear?

Answer 2

Sound is a Compressive Wave,

Detected by cochlear at ranges between 20 and 20,000 Hz.

Question 3

How can level of funciton of the tympanic membrane be assesed?

Answer 3

o Level of function can be measured via tympanometry, which involves insertion of probe into external acoustic meatus and generation of sound at 226Hz. If more reflected back from tympanic membrane than normal range suggests then there may be some hardening of tympanic membrane.

Question 4

Give two parts of tympanic membrane

Answer 4

Pars flaccida

Pars tensa

Question 5

What are three funcitions of the eustachian tube?

Answer 5

o Equalisation of air pressure in middle ear
o Mucus drainage from middle ear
o Closing insulates middle ear from loud sounds

Question 6

What three muscles hold open the eustachian tube?

Answer 6

Salpingopharyngeus
Levator veli palantini
Tensor veli palantini

Question 7

What happens if mastoiditis breaks through superior wall?

Answer 7

breaking into middle cranial fossa, affecting the meninges and temporal lobe of the brain, causing meningitis or abscess.

Question 8

What happens if posterior cranial fossa is the site of mastoid breakthrough?

Answer 8

breaking into middle cranial fossa, affecting the meninges and temporal lobe of the brain, causing meningitis or abscess.

Question 9

Name the five contents of the inner ear

Answer 9

Vestibule
Semi-circular ducts and canals
Cochlea
Cochlear duct
Organ of corti

Question 10

What is the vestibule and what does it detect?

Answer 10

 Small bony chamber, containing the Utricle and Saccule, which are sensitive to rotational acceleration and the static pull of gravity

Question 11

What is the semi-circular duct and canal, and what does it detect?

Answer 11

 Communicate with the vestibule

 Contain receptors that respond to Rotational Acceleration in three different planes

Question 12

What is the chochlea?

Answer 12

 Shell shaped portion of the bony labyrinth containing the Cochlear Duct

Question 13

What is the cochlear duct?

Answer 13

 Accommodates the spiral Organ of Corti

Question 14

What is the organ of corti?

Answer 14

 Contains the receptors of the auditory apparatus

Question 15

Outline the structure of the cochlea duct

Answer 15

• Scala vestibule
• ——-vestibular membrane
• ——-basilar membrane
• Scala tympani

Question 16

What is the scala media filled with?

Answer 16

Endolymph

Question 17

What separates scala media from the basilar membrane?

Answer 17

Vestibular membrane

Question 18

What is found on top of the basilar membrane?

Answer 18

Spiral organ of corti

Question 19

What is the principal sensory receptor of the spiral organ of corti?

Answer 19

Inner hair cells

Question 20

How many nerves attached to each inner hair cell

Answer 20

20

Question 21

What do outer hair cels odo

Answer 21

Serve as amplifiers

Question 22

Outline the mechanotransduction of sound

Answer 22

o Vibrations of tympanic membrane are transmitted via ossicles to oval window
o Vibrations of the stapes are converted to pressure waves in the Scala Vestibuli (upper compartment)
o The pressure waves are transmitted through the Vestibular Membrane to reach the Basilar Membrane
o In response to local resonance causing the bending of their Stereocilia, Hair cell K+ channels open
o Influx of K+ ions from endolymph
 Endolymph = 140mM[K+]O compared to 5mM normally
o Depolarisation of Hair Cells
o Opening of Voltage Gated Ca2+ channels
o Influx of Ca2+, triggering neurotransmitter release onto afferent nerves of the Spiral Ganglia

Question 23

How is endolymph different from normal ECF?

Answer 23

140mM of K+ compared to 5

Question 24

How does the inner ear distinguish between loud and quiet sounds?

Answer 24

o High frequency pressure waves, caused by high pitched sounds cause the short fibres of the basilar membrane at the base of the cochlea to resonate
o Low frequency pressure waves, caused by low pitched sounds cause the long fibres of the basilar membrane at the apex of the cochlea to resonate

Question 25

How is volume moderated in hearing?

Answer 25

A louder sound produces more action potentials and in a larger number of axons.

Question 26

Define the central auditory pathway to the brainstem and cerebral cortex

Answer 26

Cochlea -> Spiral ganglia -> Cochlear nucleus -> (decussation) -> Superior olivary complex -> Inferior colliculus -> Medial geniculate nucleus -> Primary auditory cortex

Question 27

Name 7 causes of hearing loss

Answer 27

``` Age Loud noises Congenital defects Infections Otoxic drugs Trauma Meniere's disease ```

Question 28

Give three congenital defects associated with hearing impairment (inheritance pattern & defect type)

Answer 28

 DFN – X-linked – Hair cell defect  DFNA – Autosomal dominant – Tectorial membrane proteins  DFNB – Autosomal recessive – Gap junction proteins

Question 29

What infection causes hearing impairmetn

Answer 29

Rubella

Question 30

Name two drugs which damage hearing

Answer 30

Aminoglycosides | Furosemide

Question 31

Give four ways in which hearing can be assesed

Answer 31

``` o Visual inspection  Otoscope o Audiographs  Plot sensitivity against frequency o Otoacoustic emission o Auditory Brainstem Response (ABR) ```

Question 32

Give four causes of conductive hearing loss

Answer 32

 Blockage  Ruptured tympanic membrane  Fluid Accumulation (Otitis Media)  Otosclerosis (progressive ossicles immobilisation)

Question 33

Give three causes of sensory hearing loss

Answer 33

 Hair cell destruction (Physical, noise related)  Hair cell death (Ototoxicity)  Menieres disease (too much endolymph)

Question 34

Give four causes of neural hearing loss

Answer 34

 Spiral Ganglion Damage (E.g. Acoustic neuroma)  Tinnitus (Phantom, ringing sound)  Auditory Neuropathy (associated with neonatal jaundice)  Monayrak deafness destroys ability to localise a sound

Question 35

What is your diagnosis in a pateint wil loss of balance, facial palsy and the loss of corneal reflex

Answer 35

acoustic neuroma (benign tumour of schwanna cells) at the cerebellopontine angle, due its dual compression of CN VIII and CN VII.