Lecture 4

Question 1

Where is the cochlea located?

Answer 1

In the bony labyrinth in the temporal lobe. Small structure. Is living in the bone - a cavity within the bone

Question 2

What are some characteristics of the cochlea?

Answer 2

2.75 turns, uncoiled would be 2mm diameter, and 35mm long.
Spiralling of cochlea - cavities.
Thin sheet that spirals up - basilar membrane.
2 spaces - endolymph and perilymph
Patched conduit for endolymph in cochlea and vestibular system (comes from lymphatic sac)

Question 3

What is the vestibular system?

Answer 3

Vestibule is connected to the three semicircular canals and contains the utricle and saccule. Also connected to the scala vestibuli of the cochlea.
Endolymph - scala media
Scala tympani - connected to media.

Question 4

What are the compartments of the cochlea?

Answer 4

Scala Vestibuli - bounded by Reissner’s membrane - thin membrane separates scala media and vestibuli
Basilar Membrane - between scala media and tympani; sheet of cells trampoline like structure - on top sits hearing sensors (stereocilia: inner hair cells and outer hair cells)
Scala Media - hearing - where our sense organ is - organ of corti
Membrane moves up and down - back/forth (stereocilia) due to pressure differences.

Question 5

What are the 2 membranes of the cochlea?

Answer 5

Two membranes separate it into three portions: Reissner’s Membrane, Basilar Membrane
Reissner’s membrane joins the basilar membrane at the helioctrema, sealing the scala media.

Question 6

What is the role of the spiral ganglion?

Answer 6

Collection of cell bodies, of auditory nerve fibres and send axon off into the brain

Question 7

What is the role of the stereocilia of the inner and outer hair cells in the tectorial membrane?

Answer 7

Stereocilia of OHC are embedded in the tectorial membrane, those of IHC are moved by fluid in the space between the hair cells and the tectorial membrane.
When pressure across the BM changes (through activity of stapes), the membrane bends and flows in this space, causing IHC stereocilia to move.

Question 8

What are the polarities of the inner ear fluids? How do they work together?

Answer 8

Endolymph (+80mV) - increase potassium, decrease sodium
Ionic composition of endolymph is maintained by activity of epithelial cells of the stria vascularis. Pumps out sodium of endolymph and maintains high potassium content.

Perilymph (+2-5mV) - decrease potassium, increase sodium

Perilymph (0mV)

• Fluid electrical environment

Question 9

What is the role of the organ of corti?

What structures does it have?

Answer 9

Sense organ; helps support cells metabolically and physically.
Houses 2 main hearing receptors: IHC (3 rows), OHC (1 row)
2 more IHC because most os hearing loss occurs here.
Most structures in organ help support ions and concentration gradient.
Tectorial membrane
Stereocilia embedded on OHC in tectorial membrane (when this moves, so do the stereocilia) - helps with transduction

Question 10

What are some dimensions of the basilar membrane?

Answer 10

Base (oval and round windows) - narrow, thicker (diameter larger)
 - high frequencies, more stiff
Apex - wider, thinner (diameter smaller)
Sits on cochlea
Tip of helioctrema = apex
 - low frequencies, less stiff

BM has different RFs depending on the stiffness and width of the membrane at that point. Changes systematically as a function of distance from the base. IHC at particular place are more stimulated when BM motion is greater at that place.

Question 11

What are the roles of the IHC and OHC?

Answer 11

OHC (12000): change mechanical properties of the tectorial membrane to sharpen frequency response. Tiny motors that amplify the mechanical movement of the BM
- 150 hairs in 3 rows per
hair cell
IHC (3500): transduce mechanical vibration into electrical activity in the auditory nerve. Turn movement of BM into changes in firing rate of the auditory nerve
- 40 hairs in 2 rows per
hair cell

Question 12

What are the structures and movements of the stereocilia?

Answer 12

Lattice structure - microtubules that keep up stereocilia.
Tip links - communicate mechanical energy (transduction)
When stereocilia tilt - they open up and ions come in and cause electrical signal - NT - afferent fibres - mechanical
IHC not embedded - pressure gradient cause by tensile strength causing stereocilia to move back and forth
Process: Mechanical - Electrical - Chemical - Electrical

Question 13

How does depolarization of IHC activate ascending afferent neurons?

Answer 13

Voltages are relative to perilymph in scala tympani and scala vestibuli: endolymph (K+ rich - re perilymph; +80mV)
Time dependent modulation of membrane potential caused by opening of ion channels on stereocilia. Are mechano-sensitive - tip links stretched, increases channel opening probability
Intra-cellular potential changes cause release of NT and firing in afferent IHC is -70mV re perilymph

Question 14

How is movement in the basilar membrane activated?

Answer 14

BM vibrates due to motion of the oval window - moves from base to apex. Energy transfer across, reissner and BM move up and down. Pressure wave across the whole cochlea.
Most of the movement is in the middle because of mass and stiffness - base is quite stiff compared to apex. Sets up an energy transfer across the membrane.
There is certain mass and stiffness that resonates to sound at the stapes.
Position along the BM at which its amplitude is highest depends on the frequency of the stimulus - change in stiffness along length of BM

Question 15

Travelling waves - what is the characteristic frequency?

Answer 15

The frequency to which a particular place on the BM is tuned. (think of frequency range at base and apex…)
Frequency of vibration at each place on the BM = frequency of pure tone

Question 16

How do travelling waves work?

Answer 16

Peak of travelling wave traces an outline/envelope that shows overall region of response on BM. There is a peak place on the BM, the region response covers portion of the total length.
Responds maximally in one region, some fluctuation depending on frequency
The wave resonates all the way through the BM. Is in incompressible fluid, with pressure on one side which is communicated across.
NOT sinusoidal motion

Question 17

How is the firing rate of a neuron in the auditory nerve determined?

Answer 17

By magnitude of BM vibration at place to which it is connected. It follows that each neuron has a particular characteristic frequency and is only sensitive to a limited range of frequencies around this frequency.

Question 18

What is the importance of the travelling wave envelope?

Where does the amplitude of the wave come from?

Answer 18

Indicates the maximum displacement of the BM at each point along the BM. Up/down vibration of each point is sinusoidal at the stimulus frequency. Amplitude and phase vary along length of BM.

Amplitude comes from the energy from the OHC departing on the BM.

Question 19

How does the brain piece together the information of the basilar membrane?

Answer 19

Basilar membrane broadband energy - separated into time and frequency, amplitude. The brain puts this all together.
Sinusoids broken down into their components - phase of each deflection relative to onset where membrane deflects the most (phase difference)

Question 20

How is tonotopy related to BM?

Answer 20

Also called cochleotopy - frequency specific tuning: low tones relatively spread out, high tones squashed together

Question 21

What are the effects of intensity changes in travelling wave amplitudes?

Answer 21

More deflection happens at higher dB SPL, amplitude at peak
With 70dB, extra deflection at 10dB…envelope, not just in one spot. The dips are the double bounces.
Narrowness and spread = spread of excitation. If you increase stimulus energy, the energy is more spread along the cochlea (important for perception)
Increase stimulus intensity to cause it to deflect.

Question 22

What happens when there is displacement at different intensities?

Answer 22

If less stiff and more massive - harder to get mass to move. Mass and stiffness determine deflection of the BM - hard to move due to intertial forces. If higher mass: stiffness ratio, the mass limits the deflection
Pressure stays the same.

Question 23

How doe the cochlea act like bandpass filters?

Answer 23

The gradient of mass and stiffness set up filtering characteristics along its’ length. Has a filtering function to perform frequency analysis. Little bands going along the cochlea.

Question 24

What is frequency selectivity?

Answer 24

Like characteristic frequency - each sound frequency causes maximal displacement/vibration in only a small region of the BM; each region of BM has its own characteristic frequency.

Question 25

What is the relevance of the resonant frequency and the mass and spring system?

Answer 25

Resonant frequency of mass-spring system. Set up difference resonant frequencies depending on stiffness and mass. Changes from base to apex.
Response to a given tone is largest near the place on the BM that has that RF.
Resonant Frequency = fr = 1/2pi x root k/m
Resonant frequency occurs when stiffness and mass reactance components are equal and cancel each other out.
MASS AND STIFFNESS DETERMINE WHERE DEFLECTION OCCURS.

Question 26

How are the OHC related to resonance on the BM?
What is the double bounce?
How is this an audiological implication?

Answer 26

OHC change stiffness of BM - sets up different aspects of function across distance - nonlinear system; add energy to the system.
OHC give the double bounce (input receives another input - input came in as sound energy activated (person)-output is a lot bigger than input given; only occurs when transduction occurs)

As an audiologist, have to overcome nonlinear to linear system - for someone with HL.

Perception gradient of frequencies changes perception - with HL changes don’t respond well.

Question 27

What happens when there is a higher amplitude and higher frequency?
Why is further from the base less stiff?

Answer 27

Less stiff; less energy

Less stiff due to depart of more energy into stiffer system. You are mass limited, further = more mass to move (think of big mass vs. small mass)

Question 28

What is the relevance of damaged hair cells?
How does this affect pitch?
What happens to someone with hearing loss as a result?

Answer 28

Damaged HC removes gain in the system, can’t hear, OHC loss.
Different frequencies on BM - perception of frequency = pitch. Pitch - fundamental frequency: if you don’t have complexity of sounds and different pitches, no complex speech. Cannot discriminate, not as dynamic (frequency) - no sound input.

We need to be able to hear discrete frequency to be able to speak them. Lose narrow sharpness, more broad spectrum. In HL, speech sounds mixed up - can’t detect differences, speak differently because can’t discriminate frequencies.