4c. The Peripheral Auditory System

Question 1

External Auditory Meatus

- Effect on Sound

Answer 1

Increases amplitude (dB) of sounds with 2-5kHz frequency, which is the range of frequencies that coincides with human speech

Question 2

Length of Outer Ear Canal Equation

Answer 2

F = c/4L

Question 3

Middle Ear

- 3 Functions

Answer 3

• Impendance matching between the air and cochlear fluids
• Protection from loud sounds, especially self-vocalisations
• Anti-masking of high frequency sounds by low frequency sounds

Question 4

Impendance Matching

-2 Methods

Answer 4

• Relative areas of tympanic and oval membrane, where the tympanic is much larger
• Ossicle lever ratio, where the malleus is longer than the incus

Question 5

Impendance Matching

- Effect

Answer 5

30dB increase in sound pressure from airborne sound to oval window

Question 6

Anti-Masking

Answer 6

Antimasking of high frequency sounds by low frequency sounds, particularly at high sound levels, acting as a high-pass filter

Achieved by middle ear muscles

Question 7

Middle Ear Muscles

- Delay

Answer 7

Contract 100ms after sharp loud sounds, so cannot protect from them

Contract before self-made vocalisations

Question 8

Middle Ear Muscles

- Function

Answer 8

• Attenuate low frequency more than high frequency, acting as a high pass filter
• Protect from loud sounds and self-vocalisations

Question 9

Middle Ear Muscles

- 2

Answer 9

• Tensor tympani

- Stapedius

Question 10

Middle Ear Muscle Reflex

- Activation

Answer 10

Sounds 80-90dB above hearing threshold

Question 11

Middle Ear Muscle Reflex

- Pathway

Answer 11

1. Cochlea

CNVIII

2. Ventral cochlear nucleus

Trapezoid body

3. Both superior olivary complexes (brainstem)
4. Facial nerve nuclei
   - Ipsilateral projects to both
   - Contralateral only to its own

CNVII

5. Stapedius muscle contraction

Question 12

Hearing Loss

- 2 Types

Answer 12

• Conductive

- Sensorineural

Question 13

Sensorineural Hearing Loss

- Definition

Answer 13

Cochlea or auditory pathway is damaged

Question 14

Conductive Hearing Loss

- Definition

Answer 14

Pathology that prevents conduction of sound to the cochlea

Question 15

Conductive Hearing Loss

- Causes

Answer 15

• Otitis
• Wax build up
• Tumours
• Tympanic rupture
• Nerve defects

Question 16

Rinne Test

Answer 16

Differentiates between conductive and sensorineural hearing loss

Placing a vibrating tuning form on the mastoid process and the entrance of the external auditory meatus.

Sensorineural = air conduction is better than bone conduction

Conductive = bone conduction is better than air conduction

Question 17

Basilar Membrane

- Function

Answer 17

Decompose sounds into their component frequencies

Question 18

Auditory Transduction

- Speed

Answer 18

Direct mechanical system with no 2nd messenger cascade = fast

Question 19

Basilar Membrane

- Functional Properties

Answer 19

Creates a tonotopic map along its length

Thick, wide, stiff base vibrates at high frequencies

Thin, narrow, less stiff base vibrates at low frequencies

Question 20

Auditory Transduction

- Stereocilia Movement

Answer 20

Sheering towards the tallest stereo cilia (kinocilium) opens mechanically gated ion channels
- Depolarisation

Sheering away from the tallest stereo cilia (kinocilium) closes mechanically gated ion channels
= Hyperpolarisation

Question 21

Auditory Transduction

- Transducer Current

Answer 21

K+ influx

Driving force is:

• Electrical, due to gradient of 130mV
• Concentration, as K+ is abundant in endolymph

Question 22

Endolymph Composition

Answer 22

Low Na+
Low Ca2+
High K+ maintained by stria vascularis

Question 23

Endocochlear Potential

Answer 23

+80mV

Question 24

Perilymph Composition

Answer 24

High Na+
Low K+

Similar to CSF

Question 25

Auditory Transduction | - Outer Hair Cells

Answer 25

Depolarisation decreases OHC length, feeding back on the basilar membrane augmenting its motion

Question 26

Outer Hair Cells | - 3 Roles

Answer 26

- Increase sensitivity - Frequency selectivity through lateral inhibition - Oto-acoustic emissions

Question 27

Inner Hair Cells | - 2 Roles

Answer 27

- Decline in phase locking | - Adaptation at the synapse

Question 28

Frequency Resolution

Answer 28

The ability to detect 1 frequency in a multifrequency complex stimulus, when all components are presented simultaneously. Determined at the basilar membrane.

Question 29

Efferent Innervation of the Cochlea

Answer 29

- Lateral pathway | - Medial Pathway

Question 30

Efferent Innervation of the Cochlea | - Lateral Pathway

Answer 30

Neurones with their cell body in the lateral olivocochlear nuclei synapse with dendrites of afferent type I ganglion cells that innervate inner hair cells

Question 31

Efferent Innervation of the Cochlea | - Lateral Pathway Role

Answer 31

Very little is known about the role of these neurones

Question 32

Efferent Innervation of the Cochlea | - Medial Pathway

Answer 32

Neurones with their cell body medial to the lateral olivocochlear nuclei, in the peri-olivary region of the superior olive synapse directly with outer hair cells

Question 33

Efferent Innervation of the Cochlea | - Medial Pathway Roles

Answer 33

- Protection from loud sounds | - Anti-masking

Question 34

Otoacoustic Emissions

Answer 34

Sounds generated by the movement of the sensory hair cells in the cochlea Strongest piece of evidence suggesting that the OHCs are mediating an active process.

Question 35

Otoacoustic Emissions | - Use

Answer 35

Measurement is used to diagnose neonatal deafness, in whom it is difficult to establish the absence of hearing.

Question 36

Otoacoustic Emission | - Loss

Answer 36

Lost in patients with significant sensorineural hearing loss

Question 37

Prebycusis | - Description

Answer 37

Threshold for hearing increases with age. Hearing at high frequencies also decreases

Question 38

Prebycusis | - Cause

Answer 38

Lateral wall degeneration causing loss of endocochlear potential

Question 39

Prebycusis | - Test

Answer 39

Selectively lowering the endocochlear potential in 1 ear by application fo furosemide to the round window, causing a loss in threshold which is greater at high frequencies than low frequencies