DD - hearing and balance

Question 1

What is the significance of the decrease in SA from the ear drum to the oval window?

Answer 1

Allows for pressure amplification, as more energy is needed to move liquid than air.

Question 2

What membrane is the organ of corti located on?

Answer 2

Basilar membrane

Question 3

What membrane do the outer and inner hair cells project into?

Answer 3

Tectorial membrane

Question 4

What is the relative amount of potassium in endolymph?

Answer 4

K+ = 80mV

Question 5

What is the relative amount of potassium in perilymph?

Answer 5

K+ = 0 mV

Question 6

What is the main function of inner hair cells? What are it’s innervation and ion channels?

Answer 6

Sound encoding.
Mainly afferent innervation. 10-20 per cell.
Inward calcium channels.

Question 7

What is the main function of outer hair cells? What are it’s innervation and ion channels?

Answer 7

Amplification and tuning.
Little afferent innervation
Outward potassium channels.

Question 8

What is prestin?

Answer 8

Motor protein found in the lateral membranes of the outer hair cells. Allows these hair cells to contract and shorten.

Question 9

What range of movements can be detected by the stereocilia?

Answer 9

0.3nm-20nm

Question 10

How does bundle stimulation cause ion channels to open?

Answer 10

Stereocilia project from the same hair cell and into endolymph. This shearing movement causes displacement of the stereocilia, which puts tension on the tip links, which opens the potassium channels.
The electrical driving force is from the ionic differences of endolymph -> perilymph
Influx of potassium causes calcium channels to open, so there is an influx of calcium, which leads to depolarisation.

Question 11

What is the importance of phase locking?

Answer 11

Sound waves pass down the basilar membrane at the same frequency that they arrive at the ear.

Question 12

What is a tonotopic map?

Answer 12

Spectral analysis of sounds, which is mapped to the auditory cortex.

Question 13

How does tonotopy vary across the cochlear duct?

Answer 13

At the base, the cochlear is narrow and stiff. So higher frequencies are required to displace the stereocilia.

At the apex, the cochlear is wide and floppy. So lower frequencies are required to displace the stereocilia.

This positional information along the cochlear coil gives frequency information to the brain.

Question 14

How is amplitude encoded?

Answer 14

Different populations of afferent nerves.
High spontaneous rate fibres -> easily excited and soon saturated
Medium spontaneous rate fibres
Low spontaneous rate fibres -> Excited at high volumes and can detect changes to sound at high levels.

Question 15

How does the outer hair cell amplify a signal?

Answer 15

Depolarisation due to displacement of the stereocilia in the tectorial membrane.
The influx of calcium activates prestin.
Prestin causes rigidly held hair cells to shorten and this allows for movements of the basilar membrane to be amplified.
This also allows for increased inner hair cell activation, so more coding of the sound.

Question 16

What is the neuronal pathway from the cochlea to the brain, when a sound is detected?

Answer 16

Spiral ganglion
Ventral cochlear nucleus
Superior olive
Inferior colliculus 
Medial geniculate nucleus 
Auditory cortex

Question 17

At which point in the neuronal pathway do the neurones bifurcate to allow for information from both ears to be represented on either side of the CNS?

Answer 17

Superior olive.

Binaural neurones are important in sound localisation.

Question 18

How is sound localised?

Answer 18

Comparison of timing and intensity

Question 19

How is ‘horizontal’ sound localised?

Answer 19

Requires 2 ears and binaural processing

Question 20

How is a ‘low frequency’ sound localised?

Answer 20

Compares the time and phase of the sound as it enters each ear.
Utilises interaural delay.

Question 21

How is a ‘high frequency’ sound localised?

Answer 21

Interaural intensity difference

Question 22

What are the 2 categories that cause deafness?

Answer 22

Conducive -> physical damage

Sensorineural -> sensory damage

Question 23

What are the differences in vestibular hair cell transduction?

Answer 23

Kinocilia
+ve response is towards the tallest kinocilium
High resting rate of nerve firing

Question 24

What is the name of the otolith sensory epithelium?

Answer 24

Macula

Question 25

Which structures detect horizontal and vertical movement within the vestibule? What is the direction of depolarisation in each of these?

Answer 25

Horizontal -> utricle. Depolarisation is towards the striola.
Vertical -> saccule. Depolarisation is away from the striola.

Question 26

What is the striola?

Answer 26

Where the orientation of the hair cells change.

Question 27

What is the importance of static balance?

Answer 27

Maintains posture when an animal is still

Question 28

What is the importance of dynamic balance?

Answer 28

Maintains posture during movement. Requires vestibular reflexes.

Question 29

What is the function of the vestibulo-ocular reflex?

Answer 29

Turning of the head results in compensatory eye movements.

Fluid movement in the canals spreads the information to ocular muscles to allow them to move in the opposite direction.