Physiology of Hearing and Balance

Question 1

What are the structures and function of the outer ear?

Answer 1

Pinna, ear canal, tympanic membrane

Directs sounds to tympanic membrane

Question 2

What are the structures and function of the middle ear?

Answer 2

ossicular lever system
- malleus
- incus
- stapes
Transmits vibrations from tympanic membrane to inner ear

Question 3

What are the structures and function of the inner year?

Answer 3

• Cochlea (hearing)
• Vestibule + semi-circular canals
  Conversion of mechanical vibrations to electrical signals

Question 4

Which bone and muscles are part of the middle ear?

Answer 4

Bones: malleus, incus, stapes
Muscles: tensor timpani, stapedius

Question 5

What are the 3 jobs of the middle ear?

Answer 5

• Tympanic membrane must be a resonator but also critically damped (reflex) through connection to bones and muscles
• Amplification - large area (tympanic membrane) vibrating a small area (oval window) to create a greater pressure
• Impedance matching - vibrations between air and liquid in the cochlea

Question 6

Describe the structure of the cochlea.

Answer 6

Scala vestibuli in contact with scala tympani through helicotrema

• Vestibular (Reissner’s) membrane
• Scala media + organ of Corti (converts mechanical vibrations to nerve signals in cochlear nerve)
• Basilar membrane around scala tympani - inner hair cells (transducers) and outer hair cells (modulators) with stereocilia - connected with rods of Corti - vertical motion converted to horizontal

Movements of oval window are transmitted to scala vestibuli and scala media. Mechanical vibration moves the basilar membrane.

Question 7

Describe the structure and function of the hair cells in the cochlea.

Answer 7

• Rows of stereocilia getting progressively smaller
• Filaments linking one hair cell to the next
• Open ion channels, allowing K+ to enter cells, causing depolarisation and increases AP firing in 8th nerve

Question 8

How is the frequency of a sound coded?

Answer 8

The basilar membrane is narrow and stiff near the oval window and wide and floppy at the other end. Each portion vibrates maximally for a particular frequency of sound.

• High freq sounds max. displace hair cells near the oval window
• Low freq sounds max. displace hair cells at other end

Question 9

What unit is frequency and loudness measured in?

Answer 9

Freq - Hz

Loudness - dB

Question 10

How is loudness detected by the ear?

Answer 10

Pressure generated against tympanic membrane

Question 11

What are the cues to sound direction?

Answer 11

Timing difference (below 3000 Hz - as length of low freq sound waves are longer than the head and wrap around it, equal loudness)

• the peak of a sound wave strikes ear facing the source before striking the other ear
• timing diff least for sounds coming from in front and most when coming from particular side

Question 12

How is directionality of sound determined above 3000 Hz?

Answer 12

Loudness difference

Question 13

Where is the auditory cortex found? What are the different areas found there?

Answer 13

Superior temporal lobe, embedded in sylvian fissure

• the primary area is a columnar map of basilar membrane (high frequency posterior, low frequencies anterior)
• secondary area for language processing (Wernicke’s area for making sense of stuff and Broca’s area for the motor expression of sense)

Question 14

Which part of the ear is responsible for balance?

Answer 14

Inner ear - vestibule (linear acceleration) and 3 semi-circular canals (rotational acceleration in 3 planes)

Question 15

Name the parts of the vestibule.

Answer 15

Utricle - horizontal acceleration

Saccule - vertical

Question 16

Name the parts of the semi-circular canals.

Answer 16

• Ampulla at base
• Crista ampullaris
• Hair cells in gelatinous mass (cupula)
• ## nerve fibres

Question 17

How are the hair cells and SC canals involved in balance?

Answer 17

• When hairs are undisturbed, vestibular afferents have baseline firing rate
• When head moves, inertia of crystals (otoliths) bends hair cells in opposite direction (like accelerating a car)
• start of rotation (towards kinocilium) firing rate increase in 8th nerve afferents
• end of rotation (away) firing rate decrease due to hyperpol
• Each of the 6 canals is best activated by a different direction of head rotation
• K+ influx sets up AP

Question 18

What is the utricle and saccule?

Answer 18

The otolithic organs - both make up the vestibule
Involved in linear acceleration
- utricle - horizontal (moving forward or backward, L or R) - cilia move in opposite direction
- saccule - vertical (looking up and down) - head flexion APs decrease, head extension APs increase *

*Otoliths have weight, when equilibrium is disturbed, they pull otolithic membrane in direction of gravity, which moves hair tufts (cilia), sends signals in neurons

Question 19

What makes up the vestibular apparatus

Answer 19

utricle, saccule, 3 SC canals

Question 20

Which parts of CNS are involved in balance?

Answer 20

Signals sent to cerebellum - balance, predictive function

• spinal cord involved in balance and equilibrium
• reticular formation - arousal, sensitising entire motor pool
• equilibrium cortex in sylvian fissure, conscious sensation and VOR

Question 21

What is nystagmus?

Answer 21

Involuntary, rhythmic side-to-side, up and down, or circular motion of the eyes that occur with a variety of conditions
e.g. brain injury, severe drunkeness

Question 22

Describe the vestibular ocular reflex (VOR).

Answer 22

The function is to stabilise retinal image during rotation of the head.

• requires maintenance of stable eye position in space during any head movement
• when the head rotates with a certain speed and direction, the eyes rotate with same speed in opposite direction

Question 23

Why does dizziness and motion sickness occur?

Answer 23

Conflicting signals between vestibular and visual systems

• When we spin around for a long time, elasticity of cupula gradually restores it to its upright position
• The drive to VOR stops (falsely telling the brain it is stationary)