9. AUDITORY & VESTIBULAR SYSTEM

Question 1

Describe the structure of a hair cell

Answer 1

• Hair cells can act as water motion, gravity or sound detectors
• Stereocilia are found on top of the hair cell
• An extracellular matrix surrounds the hair cell & protects it known as the tectorial membrane

Question 2

What do hair bundles consist of?

Answer 2

• Actin

- Stiff rods

Question 3

What’s the extracellular matrix for hair cells?

Answer 3

• tectorial membrane

Question 4

What’s the extracellular matrix for the macula?

Answer 4

• otoconial membrane is found in the macula of the vestibular system

Question 5

How are sterocilia arranged?

Answer 5

• Stereocilia are arranged in bundles

Question 6

What two structures link the sterocilia?

Answer 6

1. Connectors - link the stereocilia laterally

2. Tip links - found at the top of the stereocilia

Question 7

What are the three types of connectors?

Answer 7

1. Top connectors
2. Shaft connectors
3. Ankle links

Question 8

How does current flow through the stereocilia in response to motion?

Answer 8

• When the hair bundle moves in one direction, there’s pull or stretch at the tip links
• This distorts the tip links mechanically
• tip links are associated with ion channels, so distortion of the stereocilia causes ion channels to open or close
• The movement of the hair bundle determines if depolarisation or hyperpolarization occurs
• This allows current to flow through the stereocilia

Question 9

In what direction does the hair bundle move for depolarisation?

Answer 9

• Towards the tallest stereocilia

Question 10

In what direction does the hair bundle move for hyperpolarisation?

Answer 10

• Away from the tallest stereocilia

Question 11

Describe the process by which depolarisation & neurotransmitter release occurs in the tip links

Answer 11

1. Tip links are associated with ion channels, so distortion of the tip links causes ion channel opening
2. The stereocilia are surrounded by endolymph which is high in K+
3. The K+ influx causes depolarisation
4. VGCC open due to depolarisation
5. Ca2+ results in neurotransmitter release
6. Current flows proportionately

Question 12

What cells detect water motion in hair cells?

Answer 12

• Neuromasts or Mechanoreceptors can detect the flow of water including the velocity & direction

Question 13

What is the cupula?

Answer 13

• A gelatinous extracellular matrix surrounding the hair cells to prevent direct contact with water

Question 14

How does the cupula help to detect water motion?

Answer 14

• As water flows in the lateral line system (fish), the cupula will move in the opposite direction of water flow to protect the hair cells

Question 15

What does the vestibular system detect?

Answer 15

1. Motion
2. Head position
3. Spatial orientation

Question 16

What are the three regions of the ear?

Answer 16

1. OUTER EAR
2. MIDDLE EAR
3. INNER EAR

Question 17

What two structures make the inner ear?

Answer 17

1. Semi-circular canals - vestibular system

2. Cochlea - auditory system

Question 18

What are the three semi-circular canals?

Answer 18

1. Posterior semi-circular canal
2. Anterior semi-circular canal
3. Horizontal posterior canal

Question 19

What type of motion does the posterior semi-circular canal detect?

Answer 19

• Roll - shoulder to shoulder movement

Question 20

What type of motion does the anterior semi-circular canal detect?

Answer 20

• Pitch - forwards & backwards movement

Question 21

What type of motion does the horizontal semi-circular canal detect?

Answer 21

• Yaw - side to side movement

Question 22

What are the cilia of the semi-circular canals attached to?

Answer 22

• Gelatinous cupula

Question 23

Describe motion detection in the semi-circular canals

Answer 23

• The semi-circular canals are filled with endolymph, the endolymph moves in the opposite direction to head rotation
• Head rotation causes fluid motion in the semi-circular canals
• However, the fluid motion in the semi-circular canals lags due to inertia. This pulls the cupula in the opposite direction to head movement
• The cilia are displaced causing depolarisation

Question 24

What are the otolith organs?

Answer 24

• The otolith organs are part of the vestibular system
• The otolith organs detect linear acceleration (forwards & backwards and gravitational forces)
• Utricle & Saccule or Utricular macular & Saccular macular
• Detect different directions due to arrangement of hair cells

Question 25

What does the utricular macular detect?

Answer 25

- Right & left | - Horizontal

Question 26

What does the saccular macular detect?

Answer 26

- Forwards & backwards

Question 27

Describe motion detection in the otolith organs

Answer 27

- The hair cells of the otolith organs are surrounded by a layer of otoconia crystals - Upon linear acceleration (movement), the otoconia crystals are displaced, deflecting the cilia - The movement of the otoconia crystals in response to motion, results in deflection of the stereocilia beneath - The stereocilia are located in the otolithic membrane

Question 28

What is rarefaction & compression?

Answer 28

- Compression = high pressure, molecules close together | - Rarefaction = low pressure, molecules spread out

Question 29

What determines the wavelength & frequency of a sound wave?

Answer 29

- The rate at which rarefaction & compression occurs determines the wavelength - The rate at which the pressure cycles between rarefaction & compression determines frequency

Question 30

What is the pathway of sound waves to the auditory cortex?

Answer 30

1. Changes in air pressure detected as sound eaves 2. Causes motion in the cochlea 3. Electrical activity conduction to cochelar nucleus 4. Cochlear nucleus -> Olivary complex -> lateral lemniscus -> inferior colliculus -> medial geniculate body -> auditory complex

Question 31

What is the pinna & what does it do?

Answer 31

- The pinna is found in the outer ear - It gathers sound from the environment and passes it on to the eardrum - Made entirely of cartilage & covered with skin

Question 32

What is microtia?

Answer 32

- Malformation of the external part of the ear (pinna) | - Varies in severity (Grade I - 4)

Question 33

What is Grade 1 microtia?

Answer 33

- Less than complete development of external ear | - identifiable structures and a small but present ear canal

Question 34

What is Grade 2 microtia?

Answer 34

- Partially developed ear | - closed stenotic external ear canal producing conductive hearing loss

Question 35

What is Grade 3 microtia?

Answer 35

- Absence of the external ear - small peanut like vestige structure - absence of external ear canal and drum

Question 36

What is Grade 4 microtia?

Answer 36

- absence of total ear/ anotia

Question 37

What is otitis media/glue ear?

Answer 37

- Collection of fluid in the middle ear which impedes the movement of the ossicles - Loss of amplification as eardrum is pushing against the fluid

Question 38

What structures make up the middle ear?

Answer 38

- Tympanic membrane - Ossicles - Oval window

Question 39

What is the tympanic membrane?

Answer 39

- The tympanic membrane is the eardrum, it is connected to the ossicles - Arriva of a sound wave causes vibration of the tympmanic membrane, separating the middle ear from the inner ear

Question 40

What are the ossicles?

Answer 40

- Ossicles = trio of tiny bones on the other side of the tympanic membrane - Vibration is passed on to ossicles form tympanic membrane which then conducts it to oval window of the cochlear - Smallest bones of the body

Question 41

What is the cochlea?

Answer 41

- Cochlea is found in the inner ear - It consists of fluid filled canals that run parallel to each other. - The cochlea is divided by a flexible basilar membrane

Question 42

What are the three chambers of the cochlea?

Answer 42

1. Scala vestibuli 2. Scala media 3. Scala tympani

Question 43

Where is the basilar membrane & it's function?

Answer 43

- It's between the scala tympanis and the scala media of the cochlear - Basilar membrane filters sound according to frequency

Question 44

What does the stapes do?

Answer 44

- Stapes causes compression of fluid within cochlea

Question 45

What is the organ of corti?

Answer 45

- Movement of the basilar membrane is converted into electrical activity in the organ of corti - Organ of corti lies on top of cochlear membrane

Question 46

Describe how the movement of the basilar membrane is converted into electrical activity?

Answer 46

- Inner & outer hair cells are attached to the organ of corti - Motion of the basilar membrane causes displacement of the stereocilia (hair cells) on the Organ of Corti - Displacement of the cilia leads to depolarisation which causes the outer hair cell to touch the tectorial membrane

Question 47

What's the function of an inner hair cell?

Answer 47

Inner hair cells have afferent synapses which take information away from the hair, towards the CNS - Movement of the tectorial membrane causes inner cell to move which causes depolarisation - There's an influx of K+ from the endolymph - A smaller influx results in a lower inner hair cell output

Question 48

What's the function of an outer hair cell?

Answer 48

- Outer hair cells have efferent synapses meaning they receive information - They are involved in cochlear amplification

Question 49

Describe the process of cochlear amplification?

Answer 49

1. Outer hair cells are motile, upon displacement there's depolarisation 2. The influx of positive ions causes the outer hair cell to contract & shrink, contracting the tectorial membrane 3. Basilar membrane is pulled inwards towards the basilar which causes inner hair cell to move 4. K+ influx results in inner hair cell output to CNS