Physiology of Hearing and Balance

Question 1

Functions of the external, middle and inner ear?

Answer 1

External - sound collector (shaped to receive sound)
Middle - mechanical force amplified
Inner - sound transducer/analyser

Question 2

Function of the ossicular chain?

Answer 2

Malleus, incus and stapes act as a piston/lever arm with

Question 3

Describe impedance matching air to liquid

Answer 3

Tympanic membrane : oval window ratio is 18:1, i.e: the tympanic membrane is much larger

This acts to amplify sound in the middle ear and leads to a pressure gain

Question 4

What is the eustachian tube?

Answer 4

Ventilation pathway for middle ear mucosa; it has bony and cartilaginous portions

Also important for equalising ears

Question 5

Opening and closure of eustachian tube?

Answer 5

Resting state of cartilaginous tube is closed

It is opened by the tensor veli palatini and levator palatine muscles

Question 6

Consequences of dysfunction of the eustachian tube?

Answer 6

Leads to middle ear negative pressure, as gas is taken up by mucosal structures

Question 7

What are the 2 openings of the oval and round window?

Answer 7

2 openings of the cochlea to the middle ear; there is in-phase movement of the windows

They permit transmission of pressure wave to the oval window and perilymph; this allows vibration of the basilar membrane

Question 8

Structure of the cochlea?

Answer 8

Curved spiral lamina with 2 1/2 turns around the central modiolus

Question 9

Sections of the cochlea?

Answer 9

Scala media (endolymph) - in between scala tympani and scala vestibuli (perilymph)

Question 10

Function of the different sections of the cochlea?

Answer 10

There is an ion gradient between the scala media and tympani; this powers the sensory cells

Question 11

Functions of hair cells?

Answer 11

Transduction - conversion of mechanical “bending force” into an electrical impulse:
• Depolarisation of the organ of Cortis occurs when deflected towards the longest
• Hyperpolarisation occurs when deflected away

Question 12

Arrangement of stereocilia?

Answer 12

Height order, with tip links connecting them together

Question 13

How is the basilar membrane organised?

Answer 13

When unrolled, it has tonotopic organisation; the thinner apex responds to low frequency sounds and the thicker base responds to high frequency sounds

Question 14

Describe the central pathways

Answer 14

E. Coli (on both sides):
• 8th cranial nerve 
• Cochlear nucleus
• (superior) Olivary complex
• Laternal lemniscus
• Inferior colliculus 

This culminates in activity at the superior temporal gyrus, at Broadman areas 41/42 (inv. with hearing)

Question 15

Which systems are inv. with balance?

Answer 15

Cardiac and central NS; also, ears, eyes and proprioception

Question 16

Structure of the inner ear?

Answer 16

Complex structure termed the labyrinth; it is has a membranous structure within the outer bony structure

Question 17

What are the 5 key vestibular end-organs?

Answer 17

• Maculae of the urticle and saccule - termed the otolith organs

Ampullae of lateral, posterior and superior semi-circular canals

Question 18

Structures of the maculae of the utricle and saccule?

Answer 18

Have stereocilia projecting upwards into a gelatinous matrix with otoconia (small crystals of calcium carbonate)

These lend weight and, due to gravity, the brain perceives movement when the head is tilted/in an elevator (i.e: with linear motion)

Question 19

How is movement perceived?

Answer 19

Stereocilia are orientated in all directions so all movements are perceived by depolarisation/hyperpolarisation

When firing stops, become aware that movement has stopped

Question 20

Purpose of the 360 orientation of the otolith organs?

Answer 20

So all planes of motion are detected

Question 21

Arrangement of the semi-circular canals?

Answer 21

Orientated at 90 degrees to each other, so paired, equal and opposite

Question 22

Difference between type I and II hair cells?

Answer 22

No known functional difference

Question 23

Structure of a semi-circular canal?

Answer 23

Cupula sits in the ampulla of a semi-circular canal

Question 24

Movement of the endolymph?

Answer 24

As it moves, the gel barrier is dragged with it in 1 direction and hair cells are dragged with it; this causes hyper/depolarisation

Inertia of liquid causes dizziness initially but then the fluid catches up and there is no longer any dizziness

Question 25

What is the vestibulo-ocular reflex?

Answer 25

Activation of the vestibular system causes eye movement in the direction opposite to the head movement

Question 26

Describe how spontaneous nystagmus occurs

Answer 26

Movement of the eyes without a cognitive, visual or vestibular stimulus; the person loses function on one side and so the eye moves slowly towards the side of decreased firing, and then “jerks” back towards the center

This occurs because the brain thinks the person is turning their head and then realises they are not

Question 27

Consequences of abnormal stimulation of posterior semi-circular canal?

Answer 27

BPPV (Benign Paroxysmal Positional Vertigo) - characterised by torsional nystagmus

Question 28

Describe torsional nystagmus

Answer 28

Rhythmic oscillation of the eyes, characterized by a slow rotational drift followed by a fast correction back

It occurs when looking straight ahead (the primary position)

Question 29

Consequences of vestibular schwannoma?

Answer 29

Benign but affects auditory pathways; it causes slow, incremental changes with imbalance but, as the changes are so slow, the brain adapts and their is not vertigo

Question 30

Which central pathway is important for keeping a person upright?

Answer 30

Vestibulospinal tract

Question 31

Which central pathway is important for the vestibulo-ocular reflex?

Answer 31

Medial longitudinal fasciculus