Neuro 10: Sound conduction and transduction

Question 1

Define sound,

Answer 1

Sound = pressure wave in air

Question 2

Define frequency

Answer 2

cycles/ sec (Hz), perceived as pitch

Question 3

Amplitude

Answer 3

intensity, perceived as loudness

Question 4

Decibel range

Answer 4

log scale of loudness (i.e. amplitude)

Question 5

What structures include the outer ear

Answer 5

Pinna (auricle) + external auditory meatus

Question 6

Function of the outer ear

Answer 6

Collects and conducts sound waves towards tympanic membrane

Question 7

What are the boundaries of the middle ear

Answer 7

Air-filled chamber in bone, lying between tympanic membrane laterally, and oval and round windows medially

Question 8

What structures are in the inner ear

Answer 8

Cochlea + organs of balance

Question 9

Function of the inner ear

Answer 9

Hair cells transduce mechanical energy of sound into electrical signal in cochlear nerve

Question 10

What is contained within the air space in the middle ear

Answer 10

The 3 bones of the ear: the maleus, incus and stapes

Question 11

Which structure contiues on from the tympanic cavity (i.e middle ear)

Answer 11

The eustachian tube which links to the nasal cavity (=AUDITORY TUBE)

Question 12

What is the function of the middle ear (and ear in general)

Answer 12

AMPLIFY the sound (conduction through middle ear amplifies sound by 30dB)

Question 13

How is amplification achieved

Answer 13

lever system of articulated ossicles and ratio of area of tympanic membrane to oval window

Question 14

Protective mechanism

Answer 14

1. Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles.. these muscles stabilise the tympanic membrane and the stapes respectively
2. Auditory tube allows equilibration of air pressure on either side of tympanic membrane

Question 15

Define conductive hearing loss

Answer 15

Sound prevented from reaching cochlea

Question 16

5 types of conductive hearing loss

Answer 16

• wax
• otitis media
• otosclerosis of ossicles -perforated tympanic membrane
• congenital malformations

Question 17

When rolled out, what is the top compartment of the chochlea called

Answer 17

scala vestibuli

Question 18

When rolled out, what is the middle compartment of the chochlea called

Answer 18

Scala media (bound by membrane)

Question 19

When rolled out, what is the lower compartment of the chochlea called

Answer 19

scala tympani

Question 20

State which fluid is present in the scala vestibuli, scala media and scala tympani

Answer 20

vestibuli and tympani is perilymph, media is endolymph

Question 21

Whcih membrane separates the scala vestibuli and scala media

Answer 21

vestibular (=Reissner’s) membrane

Question 22

Whcih membrane separates the scala tympani and scala media

Answer 22

basilar membrane

Question 23

Where does the stapes bone articulate to the cochlea

Answer 23

at the oval window (it pushes it in and out, which sends vibration through the scala vestibuli then the scala tympani, so the round window pops out)

Question 24

What is the round window

Answer 24

The area of cochlea which can pop out when stapes pushes in.. is is the fluid in scala tympani pushing out into the round window (see video)

Question 25

Where are cochlear hair cells attached to

Answer 25

BASILAR MEMBRANE

Question 26

What comprises the organ of corti

Answer 26

Hair cells Tectorial membrane (which overlies the hair cells... hair cells lie between tectorial membrane and basillar membrane) Spiral ganglion Stria vascularis

Question 27

Where is the spiral ganglion located

Answer 27

ebedded int he modiolus..... innervates hair cells

Question 28

Function of stria vascularis

Answer 28

secretes endolymph

Question 29

Chemical properties of endolymph

Answer 29

secretes endolymph (high in K+, low in Na+)

Question 30

What is the ultrastructural property of the hair cell

Answer 30

Have many stereocilia per cell

Question 31

State 2 types of hair cell

Answer 31

inner hair cell – about 3,500 cells arranged in single row, densely innervated by about 10 sensory axons/cell outer hair cell – about 20,000 cells arranged in 3 rows sparsely innervated: one axon inervates several cells

Question 32

Which hair cell provides information to the brain

Answer 32

Inner hair cells

Question 33

Role of outer cells

Answer 33

Efferent nerve causes cell to change shape, which in turn amplifies the response to sound of adjacent inner hair cells at the centre of the vibration... ELECTROMOTILITY (mechanical response which drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and provide mechanical feedback amplification)

Question 34

TRANSDUCTION: | what does the basilar membrane do in response to sound

Answer 34

Vibrates

Question 35

TRANSDUCTION: | what happens in upward movement of the basilar membrane

Answer 35

Upward movement displaces stereocilia away from modiolus → K+ channels open → K+ enters from endolymph → hair cell depolarises

Question 36

TRANSDUCTION: | what happens in downward movement of the basilar membrane

Answer 36

Downward movement displaces stereocilia towards modiolus → K+ channels close → hair cell hyperpolarises

Question 37

Why is stria vascularis important in sound transduction

Answer 37

Because movement of K+ into cell depends on maintenance of endolymph at +80 mV by stria vascularis

Question 38

TRANSDUCTION: Effect of depolarisation in the body of the hair cell

Answer 38

Opens Ca++ channels in body of hair cell

Question 39

TRANSDUCTION How is a nerve activated attached to the inner hair cell

Answer 39

Depolarisation leads to Ca2+ release inside cell, this causes glutamate to be released from base --> depolarises axon of spiral ganglion cell → action potential

Question 40

Where in the chochlear are different frequencies picked up

Answer 40

Basilar membrane acts as frequency analyser High frequencies vibrate basilar membrane nearer to base Low frequencies vibrate membrane nearer to apex TONOTOPIC ORGANISATION

Question 41

At what range can humans hear

Answer 41

20 Hz – 20 kHz | most sensitive 1-3 kHz

Question 42

Define tonotopy

Answer 42

spatial organisation of response to frequency is preserved throughout pathway

Question 43

Outline the auditory pathway from cochlea to cortex

Answer 43

Spiral ganglion --> ipsilateral ventral and dorsal cochlear nucleus (still tonotopic organisation in this nucleus) --> From cochlear nucleus, fibres project to ipsilateral AND contralteral superior olivary nuclei --> So then from superior olive to inferior colliculus (now containing mix from both ears... still inbrainstem --> To medial geniculate body in the thalamus --> Auditory cortex in the cerebral hemisphere http://pages.ucsd.edu/~johnson/COGS17/17Mnemonics.html

Question 44

Where else do fibres project apart from this central pathway

Answer 44

Collateral pathways to reticular formation and cerebellum

Question 45

Function of lateral inhibition in the ascending pathway

Answer 45

ascending pathway enhances resolution of similar frequencies

Question 46

Function of inferior colliculi

Answer 46

reflexes eg. startle, head turn

Question 47

T/F.. there is no descending pathway in the auditory pathway

Answer 47

FDescending pathways provide feedback at all levels

Question 48

How is primary auditory cortex organised

Answer 48

subdivided according to frequency response

Question 49

What do cells in the PAC respond to

Answer 49

pecific features of sound e.g. on/off, duration, repetition, intensity + some more complex sound patterns e.g. rising/falling frequencies, animal vocalisations

Question 50

Function of secondary cortex

Answer 50

Secondary cortex – neurones respond to more complex sound patterns

Question 51

Causes of sensorineural deafness (sensory)

Answer 51

``` presbyacusis Exposure to loud noise Ménière’s disease Toxicity e.g. some antibiotics Hereditary disorders ```

Question 52

Causes of sensorineural deafness (neural)

Answer 52

Acoustic neuroma | Viral infection

Question 53

Causes of sensorineural deafness (central)

Answer 53

Demyelination in MS | Injury to central auditory pathway (unlikely to cause serious deafness unless both auditory cortices affected)

Question 54

What is the auditory reflex

Answer 54

involuntary muscle contraction that occurs in the middle ear in response to high-intensity sound stimuli or when the person starts to vocalize. The stapedius and tensor tympani muscles of the ossicles contract.[1] The stapedius stiffens the ossicular chain by pulling the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the tympanic membrane when it pulls the malleus (hammer) in toward the middle ear. Stapedius=facial tensor tympani= V3

Question 55

What is sound intensity and how is that measured

Answer 55

the power carried by sound waves per unit area (joules/m2) Decibel scale is used, which is a log scale of loudness

Question 56

What is the impedance

Answer 56

The reluctance of a system to receive energy from a source. Ossicles have to match the impedance of the sound wave and reduce energy loss when transferring the sound from the ear bones to the cochlea.

Question 57

Outline the impedance of the cochlea

Answer 57

The impedance of the basilar membrane varies along its length, meaning that so does the local resonant frequency

Question 58

What is the resonant frequency

Answer 58

The frequency at which the impedance of the system is minimal is called the resonant frequency.

Question 59

What is mechanotransduction

Answer 59

This is referring to the point at whch vibrations cause deflection of the hair cells against the tectorial membrane, leading to depolarisation (if the cells are deflected AWAY from the modiolus, leading to K+ influx)