Dimensions of the cochlear partitions

Cochlea: Narrower toward the apex

Basilar membrane: Narrower at the base and wider toward
the apical end

Apical end: Flaccid and under no tension

Base end: Stiff and under a small amount of tension
Cochlear microstructure

Organ of Corti

Lies on the basilar
membrane in the scala
media

Contains many different
types of specialized cells
Organ of Corti
Pillar cells or rods of Corti

Provide structure and
support

Inner and outer pillar
cells

Form a tunnel called
tunnel of Corti

Contains fluid called
cortilymph
Hair cells

On the outer side of the
outer pillar/rod cells:
Outer hair cells (OHC)
(Away from modiolus)

On the inner side of the
inner pillar/rod cells:
Inner hair cells (IHC)
(Toward the modiolus)
Supporting cells

OHC supported by
cells of Deiter and
Hensen

IHC supported by
border cells of the
inner sulcus
Reticular lamina
Formed partly by the phalangeal processes of
the Deiter’s cells.
OHC

Around 12000 in number

Test-tube shaped

Three-five rows
IHC

Around 3400 in number

Flask shaped

Single row
Stereocilia

Hairlike projections

Project from the top of
IHC and OHC

Graded in length

Have cross-bridges called
tip-links
Stereocilia of OHC and IHC

OHC:
6-7 rows per OHC
 Each row has a W shaped
arrangement


IHC:
2-4 rows per IHC
 Each row in a shallow Ushaped arrangement

Tectorial membrane

Gelatinous transparent
membrane projecting from
spiral lamina

Attaches loosely on outer edge
to the Deiter’s and Hensen’s
cells.

Longest stereocilia of OHC
embedded in inferior surface
of tectorial membrane

Stereocilia of IHC not
embedded in tectorial
membrane
Physiology of the cochlea
Mechanical response of cochlea in response to sound
Two major functions:
1.
Analysis of sound into components:
Frequency/Spectral analysis
2.
Transduction of sound:
Converting mechanical energy into
electrochemical/neural energy
http://www.neurophys.wisc.edu/animations/
Mechanical response of cochlea to sound

Basilar membrane moves in response to stapes vibration

This vibration takes the form of a ‘traveling wave’ (von
Bekesy)

Wave motion starts at the base and moves toward the
apex
http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm
Instantaneous basilar membrane pattern
and
envelope of traveling wave
Characteristics of traveling wave

Points of maximum displacement: Peak

Peak depends on the frequency of incoming
sound

Each part of basilar membrane ‘tuned’ to a
particular frequency called
critical/center/characteristic frequency (CF)
Low frequencies: Localized toward the apex
High frequencies: Localized toward the base
Frequency/Spectral analysis

Cochlea acts as a series of
bandpass filters

Incoming sound broken
down into individual
sinusoidal components

Basilar membrane vibrates
in response to each of
these components
Basilar membrane mechanics

Greater the stimulus level, greater the amount
of basilar membrane displacement

Temporal pattern of basilar membrane vibration
follows that of incoming sound
Input-output functions

At mid-intensity levels, basilar
membrane vibration increases
with intensity in a non-linear or
compressive fashion

At very low (< 30 dB SPL) and
very high (> 90 db SPL), linear
vibration

This is frequency dependent:
Only for the CF

Above and below the CF:
Linear input-output functions
Steps in transduction
Mechanical vibrations translated into neural
responses in the auditory nerve

Stapes vibration sets inner ear fluid into vibration

Basilar membrane vibrates

Shearing motion of tectorial membrane

Stereocilia of haircells bend
http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm
Bending of stereocilia
http://www.iurc.montp.inserm.fr/cric/audition/english/corti/hcells/ohc/fohc.htm
http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm
Transduction, Cont’d.

Increase in number of open ion channels at tip links

Influx of positive pottassium and calcium ions into the cell body

Resting membrane potential of endolymph: + 80 mv

Intracellular resting potential:
OHC: -70mv
IHC: - 45mv

Change in membrane potential

Release of neurotransmitter

Neuron at the base of the haircell fires
Transduction, Cont’d.

Increase in number of open ion channels at tip links

Influx of positive pottassium and calcium ions into the cell body

Resting membrane potential of endolymph: + 80 mv

Intracellular resting potential:
OHC: -70mv
IHC: - 45mv

Change in membrane potential

Release of neurotransmitter

Neuron at the base of the haircell fires
Differences between OHC and IHC

IHC: True sensory part of the cochlea
Only forward transduction (Mechanical to neural)

OHC: Both sensory and motor functions
Forward and backward transduction (Mechanical to
neural, and neural to mechanical)
Physiology of OHC and IHC

OHC display ‘motility’ (change in length) in
response to stimulation. IHC do not.

Because of muscular contractions within the cell
body and efferent connections to the OHC
bodies.

Thought to be basis of ‘cochlear amplifier’
and ‘active cochlea’
What is the cochlear amplifier?

‘Feedback loop’ within the Organ of Corti, because of OHC motility.

Forward transduction: Basilar membrane moves, stereocilia are
deflected, tranduction current flows into the OHC. This transduction
current causes a change in the receptor potential (depolarization).

Reverse transduction: Depolarization triggers the motility of the
OHC. This change in length exerts force on the basilar membrane,
deflecting the stereocilia and so on.
OHC feed energy back into the basilar membrane!!
Evidence for active versus passive cochlea

History: von Bekesy measured broad traveling waves in
dead cochleae. However, finely-tuned tuning curves
measured for auditory neurons.

Fine tuning of basilar membrane is lost when OHC die.

Compressive non-linearity (as seen in input-output
curve) lost when OHC are damaged.

Most important evidence for active cochlea: Otoacoustic emissions.
Otoacoustic emissions (OAE)

Responses that originate within the inner ear

May be spontaneous or evoked

Generated due to



The feedback system in the cochlea
Active processes within the cochlea
Non-linearities within the cochlea
Tuning in the basilar membrane

Vibration of basilar
membrane can be
described by a ‘tuning
curve’

Amplitude required for
the basilar membrane to
vibrate at a certain
constant displacement, as
a function of the
frequency of the input
sound
Neural connections in the cochlea

Afferent and efferent fibers of the VIIIth cranial nerve (auditory
nerve)

Afferent: From organ of Corti to brain

Efferent: From brain to organ of Corti

Peripheral processes of auditory nerve neurons enter the cochlea
through small openings on the edge of the osseous spiral lamina

These openings are called ‘habenulae perforata’

These fibers are then gathered in the modiolus
Organization of the auditory nerve bundle in the modiolus

Fibers from the apex in
the middle

Fibers from the base on
the outside

This nerve bundle then
goes to the cochlear
nucleus in the brain
Afferent fibers

Around 30,000 neurons in man

Only 5-15% of these innervate
the OHC


These are called Type II or
outer spiral fibers

One neuron connects to one
OHC (one-to-many)
Rest innervate the IHC

These are called Type I or
radial fibers

Many neurons connect to one
IHC (many-to-one)
Efferent fibers

Originate from the olivocochlear bundle in the
auditory brainstem

Efferent fibers synapse on the afferent nerves
innervating the IHC

Efferent fibers synapse directly on the OHC
Discharge pattern of a neuron

Neural spike has an initial large
potential shift

Following this, refractory or
rest period of around 1 msec
Other terms

Spontaneous discharge rate: Neuron’s discharge
rate without a stimulus

Threshold: Minimum stimulus level needed to
cause an increase in the discharge rate above
the spontaneous discharge rate
Spontaneous rate and thresholds

Neurons with high
spontaneous rate: Low
threshold

Neurons with low
spontaneous rate: High
threshold
Rate-Level function

Also called input-output
or intensity function

Increase the level of the
acoustic stimulus and
measuring changes in the
discharge rate of a single
neuron
Response area

Also called isolevel or
isointensity curve

Iso: “Same”

Plotting how a neuron
fires in response to
sounds of different
frequencies at a fixed
intensity
Tuning curve

Define a certain
threshold for a neuron

Plot the level of the
tone required for this
neuron to discharge at
this threshold, as a
function of the
frequency of the tone
Encoding of frequency
Two theories:
1.
Place theory
Tonotopic organization
2.
Temporal theory
Based on the periodic nature of nerve firing
Place theory
Different neurons in the nerve respond to different
frequencies
Frequency of input determined by which neuron(s) in
the auditory nerve fires at the greatest rate
Temporal theory

For lower frequencies (< 5000 Hz), discharge
rates of neurons are proportional to the period
of the input stimulus.

So for lower frequencies, rate of discharge of
the neuron also provides information about the
frequency of the stimulus.
Encoding of intensity

Increase in discharge rate with increase in stimulus
intensity

However, for most neurons, increase in discharge rate
only occurs for a limited range of input intensity

Possible that discharge rate of many neurons may be
combined to account for the 140 dB dynamic range of
the ear
Psychoacoustics

Branch of psychophysics

Relation between the physical aspects of sound
and the perception of sound
Psychoacoustic correlate of intensity:
 Psychoacoustic correlate of frequency:

Threshold

Threshold of detection

Absolute threshold
Just detectable level of sound

Threshold of discrimination

Difference limen (DL) or Just noticeable difference
(JND)
Just detectable difference between two sounds
1000 Hz tone
4000 Hz tone
Minimum audible curve
Plot of threshold as a
function of frequency

Minimum audible
pressure (MAP)
Measured using
headphones

Minimum audible field
(MAF)
Measured in free field
using loudspeakers
10 Hz
20 Hz
100 Hz
1000 Hz
2000 Hz
4000 Hz
250 Hz
8000 Hz
500 Hz
10000 Hz
Dynamic range of hearing

Range between threshold
of hearing and threshold
of pain

About 140 dB in normal
hearing listeners
Temporal integration

Signal must contain some
critical amount of energy to be
detected.

If time is short, then needs
more power to be detected
(power = energy/time)

For sounds below a certain
duration (300ms), the
threshold of detection
decreases with duration.
Ear
summates or integrates energy over time.