Physiology of hearing and balance

Question 1

Anatomy of

a) External ear
b) middle ear
c) Inner ear

Answer 1

a) External or outer ear, consisting of:

Pinna or auricle. This is the outside part of the ear.

External auditory canal or tube. This is the tube that connects the outer ear to the inside or middle ear.

Tympanic membrane (eardrum). The tympanic membrane divides the external ear from the middle ear.

b) Middle ear (tympanic cavity), consisting of:

Ossicles. Three small bones that are connected and transmit the sound waves to the inner ear. The bones are called:

Malleus

Incus

Stapes

Eustachian tube. A canal that links the middle ear with the back of the nose. The eustachian tube helps to equalize the pressure in the middle ear. Equalized pressure is needed for the proper transfer of sound waves. The eustachian tube is lined with mucous, just like the inside of the nose and throat.

C) Inner ear, consisting of:

Cochlea. This contains the nerves for hearing.

Vestibule. This contains receptors for balance.

Semicircular canals. This contains receptors for balance.

Question 2

3 Functions of the middle ear ?

3

Answer 2

1) Critical Damping (so that you are protected from loud sounds , stapes attaches into oval window = stops huge amount of sound)
2) Amplification ( oval window is small compared to membrane = increases pressure , vibrations , which is important so it can move liquid in the inner ear )
3) Impedance matching ( vibrations increase to move liquid )

Question 3

Explain the perception of sound, tone (frequency) and loudness in the cochlea

Answer 3

3 chambers:
Scala vestibuli are linked at the end of the cochlea Scala Tympani (High in Na+) = perilymph
Scala Media (High is K+) = endolymph

Pump in stria Vascularis to maintain

• Stapes cause vibration
• Basillar membrane vibrates
• Organ of Corti (mechanical –> chemical)
• Hair cells in cochlea = rows of stereocilia getting smaller
• Hair cells wave backward and forwards causing potassium trap door to open hence AP in 8th nerve.

High frequency can move the tougher fibres closer to the oval window , and low frequency can move the fibres on the other side .

Question 4

how do we code for pitch and frequency ?

2

Answer 4

1) High frequency sounds can maximally displace hair cells near the oval window (these cells are narrow and stiff)
2) Low frequency sounds can only displace the other side - the cells are wide and floppy , like a low string

Question 5

What is loudness ?

Answer 5

number of Ap’s per second

Question 6

where is sound processed in the brain ?

4

Answer 6

1) superior temporal lobe, embedded in sylvian fissure
2) Primary area = columnar map of basilar membrane

Secondary areas:

• Wernickes area = makes sense of stuff , language
• Broca’s area= motor expression of sense ( lesion in this area can cause people not to produce speech)

Question 7

vestibular apparatus ( parts 3)

Answer 7

3 semi circular canals which make rotation in 3 planes (angular acceleration )
Vestibule : linear Acceleration
Utricle = linear acceleration in horizontal plane
Saccule = linear acceleration in vertical plank

Question 8

Anatomy of the semi circular canals, how do the semi circular canals work ?

Answer 8

Semi circular canals are at right angles to one another
1)ampulla at the base with Crista ampullaris
2) Gelatinous mass = CUPULA contains hair cells
3) Sterocilia
4)Fixed Kinocillium
( think of accelerating car, when it moves forward you pull back into your chair)
- inertia of the endolymph causes it to lag behind, exerting pressure that deflects the cupula in the opposite direction
-This deflection stimulates the hair cells by bending their stereocilia in the opposite direction.
- towards kinocilium = increases firing rate (more AP)
-Away from Kinocilium = decreases APs

Question 9

how do the Utricle and Saccule work (vestibule)

Answer 9

Linear acceleration
Urtricle = horizontal
Saccule = vertical
1) As we tilt our heads or move forward
2) The otoliths ( pebble like substances )move in opposite direction, moving the otolithic membrane and hence the hair tufts
3) movement towards the Kinocilium = increased AP and away from Kinocilium = Decreases AP
4) these changes in firing UPs show the linear acceleration or deceleration

Question 10

what parts of the brain control balance ?

Answer 10

1) Cerebellum ( balance )
2) Spinal cord
3) Equilibrium cortex in sylvan fissure