38 - Hearing

Question 1

Basic manner in which sound is percieved
1
2
3

Answer 1

1) Tympanic membrane moves ossicles.
2) Ossicles move oval window
3) Cochlear transforms physical motion of the oval window into a neural response.

Question 2

Function of ossicles

Answer 2

Normally, when sound passes form air to fluid, most is deflected.
Match the impedance of the air to impedance of fluid in the inner ear.

Question 3

Three chambers of the cochlea

Answer 3

Scala vestibuli
Scala media
Scala tympani

Question 4

Basilar membrane
1
2
3
4
5
6

Answer 4

Lies between scala tympani and scala media.
Responds to sounds.
Wider at apex than base
Stiffer at base
Base responds to high frequencies
Apex responds to low frequencies

Question 5

Organ containing hair cells

Answer 5

Organ of Corti

Question 6

Organ of Corti
1
2
3

Answer 6

Structure containing basilar membrane, tectorial membrane

1) Auditory receptors are hair cells:
2) Sandwiched between basilar membrane and reticular lamina.
3) Inner and outer hair cells.

Question 7

Hair cells

Answer 7

Each has about 100 stereocilia.

Bending of stereocilia causes neuronal signalling

Question 8

To what do inner and outer hair cells attach?

Answer 8

Basilar membrane.

Stereocilia insert into tectorial membrane

Question 9

How do cochlear hair bundles lead to neuronal impulses?
1
2
3
4

Answer 9

1) Potassium channels are partially open.
2) Deflection of hair bundle towards tallest stereocilium causes opening of channels, and depolarization.
3) Increased K+ causes depolarization and opening of voltage-gated Ca2+ channels, which leads to release of neurotransmitter glutamate.
4) Deflection of hair cells in any direction alters membrane potential: towards tallest stereocilia leads to depolarisation (opening of cation channels). Towards smallest stereocilia leads to hyperpolarisation (closure of cation channels).

Question 10

K+ gradient in cochlear

Answer 10

High K+ in endolymph in scala media

Low K+ in scala tympani perilymph

Question 11

Two types of hair cells

Answer 11

1) Inner hair cells:
* 95% of nerves that project to brainstem nuclei
2) Outer hair cells
* Efferent inputs from the superior olivary complex.

Question 12

Role of outer hair cells
1
2
3
4

Answer 12

1) Change length during low intensity stimuli.
2) Change in length accentuates movement of basilar membrane - IE amplification of signal that is received by inner hair cells
3) Depolarization causes cell contraction
4) Hyperpolarization causes cell elongation.

Question 13

Effect of loss of outer hair cells

Answer 13

Basilar membrane movement is ~100 times less

Question 14

Auditory pathway
1
2
3
4
5

Answer 14

1) Ear
- Hair cells (within cochlear
- CNVIII (auditory-vestobuar nerve)
- Spiral ganglion
2) Brainstem
- Cochlear nucleus (medulla)
- Superior olive
- Lateral lemniscus
3) Midbrain
- Inferior colliculus
4) Thalamus
- Medial geniculate nucleus
5) Temporal lobe
- Auditory cortex

Question 15

Projections from cochlear nuclei

Answer 15

Auditory nerve from cochlea projects to neurons in the cochlear nuclei in rostral medulla.
These project directly to inferior colliculus (on both sides), or to neurons in both superior olives, which then project to inferior colliculi.

Question 16

Relay nuclei in brainstem which mediate sound locatlisation

Answer 16

Anteroventral cochlear nucleus projection to superior olivary complex

Question 17

Superior olivary complex
1
2
3

Answer 17

Involved in localisation of sound
Consists of:
1) Lateral superior olive 
2) Medial superior olive
3) Trapezoid body.

MSO: Localisation of sound by measuring time delay.
LSO: localisation of sound by sensing intensity differences

Question 18

Medial superior olive role

Answer 18

Localisation of sound by measuring time delay

Question 19

Lateral superior olive role

Answer 19

Localisation of sound by sensing intensity differences

Question 20

How is sound thought to be localised in the horizontal plane?

Answer 20

Duplex theory:
Difference in the time for sound to arrive at each ear (more for low-frequency).
Difference in intensity between ears (more for high-frequency).

Question 21

Auditory cortex location

Answer 21

Below lateral sulcus.

Question 22

Organisation of auditory cortex
1
2
3
4

Answer 22

1) Neurons are sharply tuned for sound frequency.
2) Columnar organization: cells in same column are tuned to same frequency.
3) Alternating regions of input from both ears
4) Excitatory input from one ear and inhibitory input from the other ear.

Question 23

Lateralisation of speech sounds

Answer 23

Left hemisphere

Question 24

Lateralisation of environmental sounds

Answer 24

Both hemispheres

Question 25

Lateralisation of music

Answer 25

Right hemisphere