2. Special senses: hearing

Question 1

What is sound

Answer 1

Areas of compression and refraction in the air, pressure change movement not change the air molecules.

Question 2

Longer the wavelength means….

Answer 2

Deeper the pitch of the sound

Question 3

Amplitude of sound waves relates to…

Answer 3

Volume.

High amplitude = High volume

Question 4

Infra and ultra sound?

Answer 4

Frequencies above (ultra) and below (infra) the normal range of frequencies in speech.

Question 5

Where is the physical discomfort level?

Answer 5

100 dB of sound pressure

Question 6

Where is the pain threshold level?

Answer 6

140 dB of sound pressure

Question 7

General anatomy of ear

Answer 7

Outer ear: Pinna

Tympanic membrane: Made of connective tissue, attached to malleus, cone shaped

Middle ear:

• Air filled
• Ossicle bones: Malleus, incus, stapes.
• Small muscles
• Auditory/eustrachian/ pharyngotympanic tube (all the same)

Inner ear:

• Coclea
• Semicircular canals
• Vestibulocochlear nerve
• Vestibule (fluid filled)
• Round window for pressure equalisation
• Fluid. Endolymph and perilymph

Question 8

Why is there amplification between the tympanic membrane and ossicles

Answer 8

Allows amplification with tympanic membrane due to transmission from air medium to fluid medium in cochlea

Question 9

Muscles of middle eat

Answer 9

Tensor tympani= Can turn down sound sensitivity when you don’t want to be listening e.g. eating

Stapedius

Question 10

Role of round window

Answer 10

For pressure equalisation as oval window is sealing by stapes

Question 11

What are the 2 types of fluid found in the inner ear?

Answer 11

1. Endolymph
   - In scala media; semicircular canals; vestibule
   - Produced by Stria vascularis
2. Perilymph
   - In scala vestibuli and Scala tympani)

Question 12

What connects the basilar and tectorial membrane in vestibule?

Answer 12

Outer hair cells, so any movement/vibration of cells stimulates the membranes

Question 13

Conversion of sound wave into neural correlate via force transduction

Answer 13

FIRST TRANSDUCTION Sound waves strike the tympanic membrane and become vibrations
The sound wave energy is transferred to the three bones of the middle ear which vibrate

SECOND TRANSDUCTION
The stapes is attached ot the membrane of the oval window. Vibrations of the oval window create fluid waves within the cochlea

THIRD TRANSDUCTION
The fluid waves push the flexible membranes of the cochlear duct. Hair cells bend and release neurotransmitter

FIFTH TRANSDUCTION
Neurotransmitter release onto sensory neurones created action potentials that travel through the cochlear nerve to the brain.

Question 14

How do we detect the sound as pitch?

Hint: Role/change of basilar fibres

Answer 14

Basilar fibres structure changes from short and stiff, to long and floppy along the length of the cochlea.
Role? This means they have resonant frequencies that are graded along the cochlea with high frequency at the base and low at the apex.

When the resonant frequency is activated, it absorbs all the kinetic energy of the wave and effectively stops it at that point.
Other frequencies carry on however

Question 15

Signal detection at the organ of Corti:
______ deflection of the basilar membrane moves the inner and ____ hairs laterally with respect to the tectorial membrane
95% of the cochlea nerve ending terminate on the ____ hair cells even though there are many less of them.
Mechanical activation + neuronal signals from the brainstem to the outer hair cells are thought to ____ and ____ them.

Answer 15

Signal detection at the organ of Corti:
Upward deflection of the basilar membrane moves the inner and outer hairs laterally with respect to the tectorial membrane
95% of the cochlea nerve ending terminate on the inner hair cells even though there are many less of them.
Mechanical activation as well as neuronal signals from the brainstem to the outer hair cells are thought to shorten and stiffen them.

Question 16

What is the purpose of the shortening/stifferening the other hair cells at Organ of Corti (via mechanical activation + neuronal signal from brains stem)

Answer 16

This can tune the cochlea by amplifying select frequencies

Question 17

Proportion of innervation to inner and outer hair cells>

Answer 17

95% of cochlea nerve –> Inner hair cells

5% –> outer

Question 18

What is the process of cochlea tuning?

Purpose?

Answer 18

1. Sound waves displace the basilar membrane
2. Inner hair cells become depolarised and send signals to the cochlea nerve then to the CNS
3. Out hair cells are stimulated by basilar membrane to depolarise, and the cells contract

Purpose: Enhance the auditory signal at the centre of the standing wave and inhibit on either wide

Question 19

Why is cochlea tuning NOT DUE TO mechanical displacement?

Answer 19

Cannot be due to mechanical displacement as this doesn’t provide the sharpness of pitch discrimination recorded

Question 20

Role of olivocochlea neuronal control in cochlea tuning?

Answer 20

Cochlea tuning also under active olivocochlea neuronal control. Fibres along this path release ACh onto the outer hair cells causing them to depolarise.
This dampens hearing in areas of pitch which are of no use
e.g. listening to someone at a party

Question 21

Signal transduction at the hair cell

Answer 21

Displacement of steriocillia in one direction opens K channels, and closes them in the other.
“tip links” are fibres which link the tops of these hair cells so that they move in relation to each other.
The opening of K+ channels causing depolarision, opening Ca2+ voltage gated channels. This allows vesicles to fuse and NT release.

Question 22

Different number of rows of inner and outer hair cells

Answer 22

1 for inner

3 for outer

Question 23

What are stereocilia?

Answer 23

Inner and outer hair cells

Question 24

Displacement of stereociliar, difference in direction?

Answer 24

Towards tallest = Stimulation (depolarisation)

Away from tallest = Inhibition (hyperpolarisation)

Question 25

What baseline activity of stereocilia?

Answer 25

Channels are constitutively open a little, so there is baseline activity which can be enhanced or diminished.

Question 26

Result of K+ entry at stereocilia?

Answer 26

K+ enters at the steriocillia, which causes a receptor generator potential, which opens voltage gated Ca channels leading to NT release onto the appropriate nerve

Question 27

Outer hair cells act as an amplifier for the vibrations at the organ of corti Evidence?

Answer 27

1.Kanamycin (*an antibiotic) Preferentially kills outer hair cells in a specific point along the cochlea results in a specific frequency hearing loss at that point. 2. Knockout of Prestin Prestin= the cell membrane outer hair cell motility protein Prestin knockout --> loss of 40-69decibels of your hearing at that frequency So the outer hair cell amplifier provides a 40-60decibel gain in sensitivity.

Question 28

What is Otoacoustic emission

Answer 28

When sounds comes out of ear

Question 29

How does the signal get to the auditory cortex in the brain?

Answer 29

1. The upper medulla contains a dorsal and a ventral cochleal nucleus All first order fibres synapse here. 2. Here the signal splits up, with some travelling ipsilaterally, but most contralaterally up to the respective inferior colliculus where most of the fibres synapse. 3. Pathways then all project to the MEDIAL GENICULATE NUCLEUS of the THALAMUS where the fibres synapse and join the auditory radiation to the auditory cortex 4. Medial geniculate nucleus projects to primary auditory cortex. 5. Secondary projections from the primary, and some from the thalamic association areas then go to auditory association cortex

Question 30

Pathway to the auditory cortex: 1. The upper medulla contains a dorsal and a ventral ______ nucleus All first order fibres synapse here. 2. Here the signal splits up, with some travelling ipsilaterally, but most _______ up to the respective _____ colliculus where most of the fibres synapse. 3. Pathways then all project to the _______ GENICULATE NUCLEUS of the THALAMUS where the fibres synapse and join the auditory radiation to the auditory cortex. 4. Medial geniculate nucleus projects to primary auditory cortex. 5. Secondary projections from the primary, and some from the thalamic association areas then go to auditory association cortex

Answer 30

Pathway to the auditory cortex: 1. The upper medulla contains a dorsal and a ventral cochleal nucleus All first order fibres synapse here. 2. Here the signal splits up, with some travelling ipsilaterally, but most contralaterally up to the respective inferior colliculus where most of the fibres synapse. 3. Pathways then all project to the MEDIAL GENICULATE NUCLEUS of the THALAMUS where the fibres synapse and join the auditory radiation to the auditory cortex 4. Medial geniculate nucleus projects to primary auditory cortex. 5. Secondary projections from the primary, and some from the thalamic association areas then go to auditory association cortex

Question 31

Special things to note about passage of sound info to auditory cortex? - Which hemisphere for each ear? - Pathways for arousal? - Location sense or not?

Answer 31

* Note* 1. signals from both ears are transmitted to both hemispheres of the brain 2. Collaterals from the pathway project into the reticulum of the brainstem and the vermis of the cerebellum causing arousal responses to noise 3. The termination of nerves from the cochlea has a topographical relationship to the cochlea and this frequency map is preserved all the way to the cortex

Question 32

Where are low frequency sound waves projected to?

Answer 32

Sound is passaged tonographically to the auditory cortical area (both primary and association), with lower frequencies to the anterior in most maps.

Question 33

What are the 3 ways in which sound direction is decided>

Answer 33

1. Volume. 2. Sound shadow: Sound from one side hits the head, which then generates a sound shadow on the other side in which the volume is less. Comparison of signal intensities from both ears determines the ear closest to the sound 3. Sound lag: Sound from a particular direction enters one ear before the other and so there is a slight delay between the sound arriving ipsilaterally at the auditory cortex, and that arriving contralaterally.

Question 34

Which is better at determining horizontal direction sound lag or shadow?

Answer 34

Sound lag is better at determining horizontal direction than sound shadow Neither method detects front to back or above to below directionality This is achieved by the folds in the pinna which changes the characteristics of sound coming from above compared to below etc

Question 35

Which frequencies do sound lag and shadow work best at?

Answer 35

``` Sound lag (which works at lower frequencies) Sound shadow (which is good for high frequencies) ```

Question 36

What determines front/back and high/low hearing?

Answer 36

Neither sound lag or shadow method detects front to back or above to below directionality This is achieved by the folds in the pinna which changes the characteristics of sound coming from above compared to below etc

Question 37

Two many hearing pathologies?

Answer 37

Outer and middle ear deafness (conduction deafness) | Sensoneural deafness

Question 38

Outer and middle ear deafness (conduction deafness), causes?

Answer 38

usually caused by a blockage in the outer ear, or an infection in either the outer or inner ear. Note angle of Eustachian tube in infants causes predisposition to middle ear infections. Can be caused by ossification of the small bones in the middle ear or by rupture of the tympanic membrane

Question 39

What is sensoneural deafness?

Answer 39

Deafness caused by either a breakdown of the cochlea and associated mechanisms, or by damage to the auditory nerve and or auditory cortex

Question 40

Result of loss of association auditory cortex?

Answer 40

Loss of association cortex leads to loss of meaning of sounds such as those seen in Wernickes lesions, but not to loss of differentiation of tone or frequency

Question 41

Bilateral loss of auditory primary cortexes?

Answer 41

Loss of both primary cortex areas dramatically reduces sensitivity to sound

Question 42

Why does unilateral auditory loss not result in complete hearing loss?

Answer 42

Loss of one side has much less effects as the auditory pathway runs bilaterally from the cochlea nucleus

Question 43

Structure of the cochlea?

Answer 43

``` The cochlea is made up of three canals wrapped around a bony axis, the modiolus. These canals are: 1. the scala tympani 2. the scala vestibuli 3. the scala media (or cochlear duct) ``` The scalae tympani and vestibule are filled with PERILYMPH and are linked by a small opening at the apex of the cochlea called the helicotrema. The triangular scala media, situated between the scalae vestibuli and tympani is filled with ENDOLYMPH

Question 44

Between the scala media and the scala tympani is a structure called the _____ ___ ______ The neural elements are the _______ ganglion neurons and the auditory nerve in the modiolar plane.

Answer 44

Between the scala media and the scala tympani is a structure called the organ of Corti. The neural elements are the spiral ganglion neurons and the auditory nerve in the modiolar plane.

Question 45

What is cochlear tonography?

Answer 45

When sound pressure is transmitted to the fluids of the inner ear by the stapes, the pressure wave deforms the basilar membrane in an area that is specific to the frequency of the vibration. In this way, higher frequencies cause movement in the base of the cochlea, and deeper frequencies work at the apex. This characteristic is known as cochlear tonotopy.

Question 46

What is the stria vascularis

Answer 46

The stria vascularis, a complex epithelial structure composed of various cell types, produces endolymph and releases it into the cochlea. The basal and marginal cells are true epithelial cells, whereas the intermediate cells are ‘melanocyte-like’. Intricate vasculature provides the oxygen and nutrients needed for the stria vascularis to function correctly.

Question 47

The function of the organ of Corti, for a soft sound (such as speech), can schematically be summed up in 5 sequences:

Answer 47

(1) Sound waves, transmitted by the perilymph, make the basilar membrane vibrate up and down. Passive tonotopy mobilises the basilar membrane from the base (high sounds) to the apex (low sounds) of the cochlea (2) Stereocilia of the OHCs, embedded to the tectorial membrane, bend when the basilar membrane rises, causing the OHCs to depolarise (by the influx of K+ ions). (3) Excited (depolarised) OHCs react by contracting (= electromotility). Due to the close link between the OHCs, the basilar membrane, and the reticular lamina, this active mechanism creates energy that amplifies the initial vibration. It also plays a role in active frequency filtering (active tonotopy). (4) The IHC is excited, probably via direct contact with Hensen’s stripe within the tectorial membrane. (5) The synapse between the IHC and the auditory nerve fibre is activated, and a message is sent to the brain.