Lecture 8

Question 1

What is the basic construct for the OHC motility?

Answer 1

Activity of OHC produces vibration that adds mechanical energy back to the cochlea - leads to emission. 
OHC contract (transduction), departed into cochlear fluid
Sound comes into endolymph, causes it to have a pressure wave in it - cochlear emissions - put out cochlea through fluid

Question 2

How can the vibrations of the OHC travel outwards? (opposite direction)

Answer 2

In back propagation, travels from oval window - stapes - incus - malleus - tympanic membrane = sound is transferred out the ear canal.

Question 3

With back propagation of the OHC, what can this tell us clinically?

Answer 3

Screen for hearing loss for infants, newborns, very young children. Indicates status of the OHCs and cochlea.
Assess effects of noise/music exposure in occupational settings. Determine whether a hearing loss is cochlear or more central.
Abnormal OAEs reveal cochlear dysfunction before actual hearing loss is present.

Question 4

What is the process for performing OAEs?

Transient evoked otoacoustic emission to clicks

Answer 4

Put in multiple clicks (a transient signal), want to see responses emitted out of the cochlea.
Complicated because large sections of cochlea are stimulated simultaneously and response frequency content changes over time.
OHC near middle ear give initial high frequency content
OHC near apex give lower frequency content.
Look for responses different than normal

Question 5

What occurs with distortion product otoacoustic emissions?

What does this tell us clinically?

Answer 5

Introduce spectrum and frequency, creates the distortion product and present in the ear. The cochlea produces the distortion product. Responds at f1 and f2 and BM bounces nonlinearly.
What to see the f1, f2 and dp spikes, where dp is (2f1-f2)
You perceive the dp as lower pitch, could be a sign of hearing loss in the cochlea if the spike is not present - then you know the ear did something. Spike needs to be larger than noise
Tests OHC - if dp is due to OHC and is absent, the OHC may be damaged.

Question 6

What are some structures and characteristics of the 8th nerve?

What is its pathway?

Answer 6

Auditory and vestibular portion.

Spiral ganglion - cell bodies of the first order neurons. Type I go through the habenula perforata into the spiral ganglion. Trace along the BM bottom through the perforata
Nerve fibres from the spiral ganglia - axons trace down the inner part of the cochlea from the modiolus through the internal auditory canal

Enters the brainstem at the cerebello-pontine angle.

Internal auditory meatus gives the nerve passage through the temporal bone to brainstem

Question 7

In cochlear innervation, what are the two types of neurons in the afferent pathway?

Answer 7

Afferents: to the brain
Type I: IHC - synapse with 1 IH
(respond at high frequency)
- myelinated - rapid communication

Type II: OHC - on average synapses with 10 OHCs
- not myelinated

Clusters specific to a region of the cochlea. Cells respond to specific frequencies

Question 8

How is cochleotopy maintained in the organization of the auditory nerve?

Answer 8

Type I fibres serve the low-frequency regions of the cochlea, make up the centre of the bundle that makes up the cochlear nerve.
Apex = low frequencies
Middle = medium frequencies
Bottom = high frequencies

Question 9

How does the auditory nerve fibre response occur?

whole process of transduction

Answer 9

Mechanical energy causes stereocilia to bend, depolarize, release NT - transduction
Depolarization in nerve fibres, reaches threshold - fires. Spontaneous rate of firing increases (coding of info from hair cells to the brain).
The depolarization/hyperpolarization will increase/decrease the firing rate of the nerve fibre.

Question 10

What is the difference for the 8th nerve firing when it is spontaneous versus stimulus evoked?

Answer 10

Spontaneous rate - action potentials with no stimulus is just a neural response - channel slightly open at rest, some release of NT, natural Ca+ in cell.
Stimulus evoked - see the action potentials, the increased firing frequency.
The increased firing rate tells the brain

Question 11

What are spike rates?

Answer 11

Proportional to basilar membrane velocity or displacement. Single neuron can only fire so fast due to its refractory period. When strongly stimulated, most auditory neurons fire at rates <500 spikes/sec, with a maximum of 1000Hz (can be greater for hearing…4000Hz). That is the top frequency it can code for.
Refractory period = 1ms.
When spike rate changes = signal. 1 nerve fibre fires 1x every 1ms.

Question 12

What does auditory nerve fibre tuning mean?

Answer 12

Tuning due to BM deflecting - OHC have sharp tuning. If OHC not present - broad tuning.
The BM breaks down frequencies and the cochlea takes sound and breaks it down, parsing it out, onto the nerve fibres.
The brain needs to know how to break down and put things back together = coding to interpret frequencies of sound in the air.

Question 13

What is the characteristic frequency of a nerve fibre?

Answer 13

The frequency at which the lowest stimulus level is required to achieve neural threshold. Due to the BM having the largest response at specific frequency.
Increase amplitude = increase range of frequencies to which fibres responds.

Seen graphically when there is a change from spontaneous rate firing to something faster. (frequency vs. sound pressure required to achieve a threshold firing rate).

Depends on the energy to activate the nerve fibre, plus changing the intensity, and it will then respond best to the different frequencies with respect to intensity.

The brain sees the tuning of the nerve fibre at the specific frequency.

Question 14

What occurs to nerve fibres when the stimulus level is increased?

Answer 14

If you increase the level, more frequencies on the BM will stimulate more nerve fibres. It will be frequencies that are lower than the fibre CF.
- the width of the response gets larger.
Increase the intensity/energy of the BM, it deflects more along its length - other parts of the BM are recruited.

Question 15

How is information about the frequency/stimulus level of the auditory nerve carried?
4 ways

Answer 15

Place coding: fibres servicing a particular place increase firing
Temporal coding: produced by phase locking

Firing rate: by the extent to which low and high spontaneous rate fibres fire
Spread of excitation: more neurons will stimulate at higher levels.

Question 16

What is temporal coding?

Answer 16

BM goes up and down with stimuli of a pure tone, the frequency causes the BM deflection. The frequency is following the stimulus frequency. The action potential then happens at specific frequencies.
Action potential fires in phase with peaks of depolarization of hair cells - peaks of the sound input.
Sound pressure wave coming in from a single nerve fibre - fires at the peak of the pressure wave coming in. Combination of many nerve fibres create the wave - not one nerve fibre coding for one frequency, so all eventually filled in.

Question 17

How is auditory nerve fibre activity affects by spontaneous rate?

Answer 17

Change in spontaneous rate coming from certain places on the cochlea (brain trying to put things back together)
Nerve fibres innervate a certain part of the cochlea and the brain learns this, knows its a certain frequency.
If both low and high frequencies are firing, look at the one that is firing the most.

Question 18

What are the three theories of frequency coding?

THINK OF THE PIANO ANALOGY

Answer 18

Volley theory - fibres with characteristic frequency near to a stimulating tone's frequency will be phase locked to the tone.
Place theory - fibres whose characteristic frequency is close to stimulating tone's frequency will fire at a higher rate than those remote from it
 - which fibres tell which BM place and therefore what stimulus frequency 
#3 = combination of the above two

Question 19

Why is the cochlea/basilar membrane a non-linear system?

Answer 19

OHC are putting in energy, don’t get the same output, not linear. OHC jams out (dB ratio in and out not the same)
Nonlinear process sets up distortion product and its emissions, 2-tone suppression - put in one and it effectively suppresses the other

Question 20

What is two-tone suppression?

Answer 20

Due to nonlinearities, it suppresses the nerve fibre from firing. One tone is played and another is put in, gets confused as to which tone to respond to, so the tones suppress an area of the cochlea, suppressing the OHC from adding energy, therefore suppressing the sound.
It depends on the decibel level and the frequency of the tones - there is an area where suppression occurs if these tones overlap.
Possible that single tone will will fire on its own alright, but nerve fibres cannot respond to one tone and the other at the same time.

Lower tones tend to suppress higher tones.

Question 21

What is contralateral suppression?

Answer 21

This is like masking - put tones in each ear, and see what happens to the ears relative to the suppressor.
The nerve fibre responding to one tone, goes up (contralateral suppressor) - transfers over, the right ear goes, the left ear picks up with suppressors and inhibits the OHC. Because of crossover with auditory system, both will be suppressed?
Suppression through the neural fibres will cause reduction in the left ear, and bouncing around in right ear connections will also cause reduction. Happening through central mechanism.