Lecture 4

Question 1

Features of sound (4)
Human audible spectrum

Answer 1

Features: amplitude, frequency, waveform, phase
Threshold: 20Hz - 20 kHz
-> SPEECH around 3kHZ **

Question 2

Mechanoelectric transduction & tonotopy

Answer 2

MT: Sound into neural activity
Tonotopy: cochlear frequency representation

Question 3

Outer ear (3 structures, functions)

Answer 3

Sound source elevation cues:
- Pinna
- Cochlear

Auditory meatus
-> gathers sound energy for tonotopic membrane
-> boosts sound around 3 kHz

Question 4

Middle ear (3 structures & function)

Answer 4

Structures: Malleus, incus, staples (OSSICLES)

Function: boots pressure from eardrum 200x
Mechanical processes:
> Lever action of ossicles
>Focus force of large tympanic membrane
(to small diameter oval window)

Question 5

What regulates sound transmission efficiency in the middle ear

Answer 5

Tensor tympani & stapedius muscles

Bonus: innervated by CN V & VIII

Question 6

Inner ear main functions

Answer 6

Transform sonic pressure into neural impulses
Mechanical frequency analyzer

Question 7

Cochlea structure & tonotopy

Answer 7

(see 4.7)

Question 8

Organ of Corti
(where, # stereocilia, inner vs out hair cells)

Answer 8

Where: between basilar & tectorial membranes
30 - couple hundred stereocilia
Inner hair cells: afferent
Outer hair cells: efferent
Connected by tip-links

Question 9

Hair bundle displacement

Answer 9

Towards tallest stereocilia = STRETCH TLs
-> K+ enters → depolarization → Ca+ enter → NT release

Other direction = COMPRESS TLs
-> channels close → hyperpolarization

Question 10

Hai bundle receptor potential (high vs low ƒ ? gating>)

Answer 10

Biphasic (some channels open @ rest)
GRADED potential
-> track displacement!

Low ƒ: hair cells can track sinusoidal motion
High ƒ: hair cells don’t faithfully track

Mechanical gating

Question 11

Labeled line coding

Answer 11

Tonotopic organization of hair cells corresponds to ƒ

Question 12

Auditory fibers & stereocilia

Answer 12

Every auditiory fiber innervates
ONE hair cell → specific

Question 13

Tuning curve & characteristic ƒ

Answer 13

Tuning curve: min. level of sound to increase a fiber’s rate is most “attuned” to (lwoest tuning curve)

Question 14

Phase locking

Answer 14

< 3kHz → fibers only fire in rising phases
Used for sound localization (temporal info)

Question 15

Endolymph

Answer 15

Scala media
Rich in K+
Poor in Na+
+80 mV
Drives INFLUX of K+ into stereocilia
-> depolarization then opening of K+ & Ca2+ channels

Question 16

Perilymph

Answer 16

Scala tympani/vestibuli
Poor in K+
Rich in Na+
0 mV
Drive K+ EFFLUX & repolarization

• Perilymph/endolymph difference: ENDOCOCHLEAR potential

Question 17

Cochlea to brainstem ascending pathway

Answer 17

Innervates 3 cochlear nucleus parts
-> anteroventral, posteroventral, dorsal
CN VIII in cochlear ganglion

*tonotopic organization maintained

Question 18

Parts of cochlear nucleus (3)

Answer 18

Superior olivary complex (pons)
Manaural pathway
Inferior colliculus (midbrain)

Question 19

Interaural time difference (ITD)

Answer 19

Binaural inputs to MSO (from AVCN)
Bipolar dendrites:
> lateral gets ipsilateral input
> medial fets contralateral input

Maps LOCATION - via diff. neurons firing
ONLY SOUNDS < 3kHz ! Need phase locking

(see 4.19)

Question 20

Interaural intensity difference (IID)

Answer 20

Due to “shadow” of lower intensity at far ear
INPUTS to LSO
1) -> direct EPSP from ipsilateral input
2) -> inhibition from MNTB, contralateral input (weaker than (1))

(see 4.20)

Question 21

Localization: horizontal vs vertical

Answer 21

Horizontal: ITD (low ƒ) & IID (high ƒ)
Vertical:
> pinna spectral filtering
> processed in dorsal cochlear nucleus

Question 22

Inferior colliculus

Answer 22

Integrates LSO/MSO + lemniscal complexes + cochlear nucleus directly

Conveys timing, intensity, frequency

Question 23

Auditory thalamus

Answer 23

All ascending auditory info must pass through medial geniculate complex of the thalamus

Question 24

Auditory cortex

Answer 24

Ventral MGC → primary AC
Dorsal MGC → secondary AC
Primary AC: precise tonotopic map (EE & EI)
Secondary AC: belt & parabelt: diffused MGC input

Bonus: Wernicke’s area continuous of 2nd AC