Physiology of hearing and balance

Question 1

What is sound?

Answer 1

A pressure wave caused by oscillating molecules, set in motion by vibration

Question 2

What determines the pitch of a sound?

Answer 2

Its frequency

Question 3

What determines the volume of a sound?

Answer 3

Its amplitude

Question 4

What frequency of sound can humans perceive?

Answer 4

20Hz - 20,000 Hz
(A vibration less than the width of a H2 molecule can be heard by a human)

Question 5

What is the function of the middle ear in hearing?

Answer 5

The middle ear acts as an amplifier mechanism at a ratio of around 18:1 from the tympanic membrane to the oval window

This gives a theoretical gain of around 38dB

Question 6

How many times does the cochlear turn?

Answer 6

The cochlear turns around 2.5 times around the central modiolus

Question 7

What structures contains the hair cells within the ear?

Answer 7

Organ or corti

Question 8

What are the 2 types of hair cells within the organ of corti?

Answer 8

Inner cells - Receive sound
Outer cells - Modulate sound

Question 9

Where in the cochlear are high-pitched sounds detected?

Answer 9

Base of the cochlear (near oval window)

Question 10

Where in the cochlear are low-pitched sounds detected?

Answer 10

At the end of the cochlear

Question 11

Describe the whole process of hearing

Answer 11

1. Sound enters through the ear canal
2. This vibrates the tympanic membrane
3. This causes vibration of the ossicles
4. This vibrates the oval window
5. This stimulates movement of the scala vestibuli
6. This causes movement of hairs
7. This stimulates depolarisation of the cochlear nerve
8. This vibration passes into the scala tympani at the helicotrema
9. These vibrations are dulled by the round window
10. Cochlear nerve joins the vestibular nerve and moves into the cochlear nucleus
11. This moves up the brainstem via the superior olivary complex, lateral lemniscus and inferior collicus
12. This terminates in the primary auditory cortex in the posterior superior temporal gyrus

Question 12

At what week of development can a foetus hear?

Answer 12

18 weeks

Question 13

At what week of development can a foetus respond to sound and voice?

Answer 13

26 weeks

Question 14

What are the 4 main balance system inputs?

Answer 14

• Visual - Eyes
• Proprioceptive - Joint perception
• Cardiovascular - Blood pressure
• Vestibular - Head movement

Question 15

What are the 2 outputs of the central pathways of the balance system?

Answer 15

• Vestibulo-spinal tract - Gives tone to back and muscles to maintain stability
• Vestibulo-ocular reflex - Ability for the eyes to stay focussed on one point despite moving

Question 16

What re the 3 main structures of the vestibular system?

Answer 16

Utricle
Saccule
3 semicircular canals

Question 17

What nerve innervates the lateral and anterior canals and the utricle?

Answer 17

Superior vestibular nerve

Question 18

What nerve innervates the posterior canal and saccule?

Answer 18

Inferior vestibular nerve

Question 19

What are the 2 hair cells that make up the hairs of the inner ear?

Answer 19

• Kinocilium - Longest hair cells
• Sterocilia - Shorter hair cells

Question 20

How do hair cells detect head movement?

Answer 20

They have a resting action potential
Deflection towards the kinocilium increases APs
Deflection away from the kinocilium decreases APs
The difference is perceived as changes in activity by the inner ear

Question 21

How do the utricle and saccule detect linear acceleration?

Answer 21

They contain sterocilia projecting into a gelatinous matrix with otoconia providing weight due to gravity
This allows them to perceive position and linear motion, as well as head tilting

Question 22

How does the vestibule-ocular reflex occur?

Answer 22

As we turn our head from left to right, the perilymph stays still but the bone moves, causing the perception of deflection of the cupula

This triggers the vestibulo-ocular reflex, causing the muscles around the eye to remain straight while the head turns

Question 23

What are the central pathways of the vestibulo-occular reflex?

Answer 23

• Vestibulospinal tract - Motor output to the neck, back and leg muscles, controlling posture
• Medial longitudonal fasciculus and ocular muscles - Motor output to the eyes, gaze stabilisation
• Medial lemniscus and thalamus to cerebrum for awareness

Question 24

Which ear will be affected in right nystagmus and why?

Answer 24

Right vestibule
Right vestibular damage means that there will be a higher firing rate in the left ear, so the brain detects left movement and the eye turns right

Question 25

What causes unidirectional nystagmus?

Answer 25

Peripheral pathology within the vestibular apparatus

Question 26

What causes bidirectional nystagmus?

Answer 26

Central pathology