Download Hair Cells

George Berkeley (1710) “A Treatise Concerning the Principles
of Human Knowledge”
• “If a tree falls in the woods,
and there’s no one around to
hear it… does it make a
sound?”
George Berkeley (1710) “A Treatise Concerning the Principles
of Human Knowledge”
• “If a tree falls in the woods, and
there’s no one around to hear it…
does it make a sound?”
• Is there “objective” reality or only
“subjective” perspective?
– Objective reality defined: pressure
changes in the air or other medium
(water).
– Subjective reality defined: the
experience of hearing (perceiving).
You hear vibrations
object
vibrates
air molecules
vibrate
eardrum
vibrates
sound waves
Periodicity of
waves
The simplest sound waves – Pure Tones – a
sound that is portrayed by a single sine wave
Compression (density increase) of air
molecules
Frequency (Hz) = 1 cycle/t (unit of time or
distance)
A
Amplitude (dB) = difference between
atmospheric pressure & maximum pressure of
a sound wave
time
Depression (density decrease) of
air molecules
Pure tones can vary in frequency (Hertz
(Hz = cycles/second))
Pure tones can vary in amplitude (Decibels
(dB) – Sound Pressure Level (SPL))
the softest sounds we hear are about
1/10,000,000th the amplitude of the
loudest sounds
Sound
SPL (dB)
Barely audible sound (whisper)
Leaves rustling
Quiet residential community
Average speaking voice
Loud music from radio/heavy traffic
Express subway train
Propeller plane at takeoff
Jet engine at takeoff (pain threshold)
Spacecraft launch at close range
0+-ish
20
40
60
80
100
120
140
160
Elements of Sound
Magnitude of displacement
Intensity
Loudness: amount of sound energy
falling on a unit area (cm2)
Because air pressure changes
are periodic (i.e. they repeat a
regular pattern), one can do a
Fourier analysis
Fourier Analysis reduces sound wave to its fundamental
frequency and harmonics frequencies
“Fundamental Frequency” is the
lowest frequency of a vibrating
object
Relative Power
Fourier frequency spectrum
0
440
880
1320
1760
Frequency (Hz)
The auditory system does break down tones into
simpler components = Ohm's acoustic law
Additive synthesis & Fourier analysis
Fundamental
Frequency (“1st
harmonic”)
2nd Harmonic
3rd Harmonic
Perception of sound
Pitch:
Subjective quality of “high” or “low” tone (tone
height)
Constructed from fundamental & harmonics
Octave: multiples of pitch (200 Hz, (x2) 400 Hz,
(x4) 800 Hz, (x8) 1600 Hz, etc.)
Perception of sound
Timbre (or “tambre”):
Subjective quality of sounding different at same
pitch and loudness (flute v. bassoon)
Constructed from differential energies (loudness)
at different fundamental & harmonic frequencies,
and differences in the number of harmonics
Timbre: differences in number & relative strength of harmonics
guitar- many harmonics
400
800 1200 1600 2000 2400
Frequency (Hz)
flute - few harmonics (only
one actually)
400
800 1200 1600 2000 2400
Ear—overall view
Outer Ear
• pinna(e)
• auditory canal - amplifies sounds
in 2-5 kHz range
• tympanic membrane (eardrum)
Middle Ear
• Ossicles: malleus (hammer); incus (anvil); stapes (stirrup)
• Oval Window
• Middle ear muscles - at high intensities they dampen the
vibration of the ossicles
Inner Ear
• Cochlea
Redrawn by permission from Human Information Processing, by P. H. Lindsay and D. A. Norman,
2nd ed. 1977, pg 229. Copyright ©1977 by Academic Press Inc.
Outer ear
Airborne vibratory processing – helps capture,
direct & modulate sound
Auditory canal amplifies sounds through
Resonance (combining reflected sound in
canal with newly entering sound)
Tympanic membrane vibrates
Middle ear
Mechanical processing – vibration
from airborne processes translate into
movement of bones & muscles
-- causing a second level of vibrations on the
oval window
Inner ear
Liquid medium – Neural processing –
translation of vibrations into neural
signals
Copyright © 2002 Wadsworth Group. Wadsworth is an imprint of the
Wadsworth Group, a division of Thomson Learning
Figure 10.14, page 345
Middle ear
ossicles
tympanic
membrane
surface area of stapes
The ossicles concentrate the
vibration on a smaller surface area
which increase the pressure per
unit area (factor of 17)
The ossicles act as a lever, thereby
increasing the vibration by a factor
of 1.3
Ear—overall view
Redrawn by permission from Human Information Processing, by P. H. Lindsay and D. A. Norman,
2nd ed. 1977, pg 229. Copyright ©1977 by Academic Press Inc.
The inner ear is responsible for both:
• Balance (vestibular system)
• Transduction for hearing (Cochlea)
The outer and middle ear are
purely mechanical.
Neural transduction and
analysis begin at the inner ear.
• The cochlea is the site at which
vibrations of the stapes and
inner ear fluid are transduced
to neural responses in fibers of
the auditory nerve.
The cochlea is a coiled tube that resembles a snail
shell
• In humans, the
cochlea coils about
2.5 times
• 2 mm diameter
• 35 mm long
• A cross-section
through the coiled
cochlear tube
reveals that the
inside is divided
into three
compartments
The receptor cells are located in the Scala Media,
part of the Organ of Corti.
Organ of corti
The Organ of Corti – located on the basilar membrane in
the Scala Media
• Three rows of outer hair cells
• One row of inner hair cells – main receptor cells involved
in hearing
• Tectorial membrane covers the tops of the hair cells
• Basilar membrane sits underneath the Organ of Corti
(underneath the hair cells)
Hair Cells
3,500 inner hair cells
12,000 outer hair cells
-connected to fibres which leave the cochlea
as the auditory nerve
inner hair cells diverge - each connects to 8-30
Ganglion Cells (auditory nerve fibres)
outer hair cells converge - each auditory nerve fibre
is connected to many outer hair cells
hair cells have cilia - when they bend, the
transduction process starts
Innervation of hair cells
• Each inner hair cell innervated many spiral ganglion cells
• A single spiral ganglion cell is innervated by many outer
hair cells
Transduction in auditory receptor cells
• The tops of the cilia extend up to the tectorial
membrane
• The base of each hair cell is contacted by one or more
spiral ganglion cells
Stapes vibrates - oval window vibrates
- this causes pressure changes in the fluid
- the basillar membrane vibrates at the
same frequency as the stapes
- the cilia of the hair cells are embedded in the
tectorial membrane
-when the basillar membrane vibrates, the hair
cells bend
tectorial
membrane
basillar membrane
Release of neurotransmitter Termination of neurotransmitter
depolarization
hyperpolarization
The traveling wave of
sound transduction
across the Basilar
Membrane
Movement of the cilia results in transduction
• When pressure at the oval window increases, the basilar
membrane moves downward. When pressure decreases, it
moves upward. These movements drag the stereocilia in
opposite directions.
The auditory nerve and central auditory system
• The auditory nerve projects in an organized way so that the
map of frequency is maintained at every level of the central
auditory system.
Tonotopic
processing
Cochlear Base
Cochlear Apex
The location of the peak
of the envelope is
frequency dependent
high frequency
close to base
Hi Hz
low frequency
close to apex
Low Hz
1. Place code - von Bekesy
basilar membrane is narrower at the base than at the apex
stiffer at base
-present vibration to basilar membrane
traveling wave results
Peak
base
envelope
apex
Tonotopic Map of the Cochlea
#’s indicate fundamental frequency (& pitch)
Base
Apex
Hair cells along the basilar membrane are sharply
tuned to a best or characteristic frequency
threshold (dB SPL)
single-unit recording
70
60
50
40
30
2
5
10
Frequency (kHz)
20
Tuning frequency curve:
Specific hair cell locations respond to specific frequencies across
the Basilar Membrane
How do we code for frequency (pitch)?
1. Place coding
neurons on basilar membrane code different frequencies
oval
window
basilar
membrane
2,000 Hz
1,000 Hz
2. Rate coding
-the frequency (& amplitude) of
the sound is reflected
in the firing rates of neuron(s)
500 Hz
Sound Complexity: (i) number of peaks
in the traveling wave, and (ii) how much
“side to side” activation occurs.
The traveling wave of sound transduction
across the Basilar Membrane
Basilar Membrane’s
response to complex tones
Fourier frequency spectrum
Relative Power
Fourier Analysis reduces sound wave to its fundamental
frequency and harmonics frequencies
0
440
880
1320
1760
Frequency (Hz)
The auditory system does break down tones into
simpler components = Ohm's acoustic law
COMPLEX TONES
The Central Auditory
System
Where do we go
after the cochlea?
“What?”
information
Nuclei of
Lateral
Lemniscus
“Where?” information
The Central Auditory
System
Where do we go after the cochlea?
The cochlear nucleus is the first stage in the central
auditory pathway
• Each auditory nerve fiber diverges to three divisions of the
cochlear nucleus.
This means that there
Are three tonotopic maps
In the cochlear nucleus.
Each map provides a
separate set of pathways
& neural code patterns.
Each division of the cochlear nucleus contains specialized
cell types
• Different cell types receive different types of
synaptic endings, have different intrinsic
properties, and respond differently to the same
input.
Changes in response pattern
• The stereotyped
auditory nerve
discharge is
converted to many
different temporal
response patterns.
• Each pattern
emphasizes
different
information.
Neurons in the cochlear nucleus (and at higher levels)
transform the incoming signal in many different ways.
• The auditory nerve input is strictly excitatory
(glutamate). Some neurons in the cochlear nucleus are
inhibitory (GABA or glycine).
• Auditory nerve discharge patterns are converted to
many other types of temporal pattern.
• There is a progressive increase in the range of response
latencies.
Increase in range of latencies
• Each synapse adds approximately 1 ms latency.
• Any integrative process that requires time adds latency.
The Central Auditory
System
• There are many
parallel pathways in
the auditory
brainstem.
• The binaural system
receives input from
both ears.
“What?”
information
Nuclei of
Lateral
Lemniscus
• The monaural
system receives
input from one ear
only.
• 1st place that
binaural information
can merge: Superior
Olivary Nuclei
“Where?” information
Monaural pathways (“what?”)
Information from each ear is distributed to parallel pathways
In the cochlear nucleus and from there, the contralateral lateral
lemniscus
Binaural Pathways: (“Where?”)
The superior olivary complex receives input from both cochlear nuclei and
compares the input to the right and left ears.
Auditory information from receptors to Auditory
Association Areas:
Each set of auditory pathways has a specialized function
More Analysis &
Transduction
Route of auditory impulses from the receptors in the ear to the
auditory cortex: “SONIC MG”
SON (Superior Olivary Nucleus)
IC (Inferior Colliculus)
MG (Medial Geniculate Nucleus)
“A1”
Auditory areas in monkey cortex: lobe between (and under) the Lateral and
Superior Temporal Sulcus
Lateral Sulcus
(LS)
Information flow
within the auditory
cortex:
Received in Core
Area (A1)
Secondary area
Association area
Superior Temporal
Sulcus (STS)
Outside the primary auditory area are other specialized
areas for speech, language, and probably other purposes
Unlike vision, much less overlap of left & right hemispheres: called
“hemispheric lateralization”
Cortical Representation of Frequency: tonotopic
representation
A1
columnar organization in
cortex: neurons in same
column prefer the same
characteristic
frequency
Cochlea
Combination & integration of sound happens “above”
the cochlea (at the level of the cortex?): “periodicity
pitch”
• Pitch perception significantly influenced by the fundamental frequency
• Pitch can be processed at the level of the cochlea (by analyzing different
“peak waves” on the basilar membrane)
• However, what happens if you:
– take out the fundamental frequency (no 400 Hz)
– only play some harmonics in one ear (800, 1600)
– and other harmonics in the other ear (1200, 2000)
• Effect of the “missing fundamental:” we hear the pitch (400 Hz) from
only hearing the harmonics
• Called “periodicity pitch”
Periodicity Pitch
The effect of the missing fundamental
Merging left-right
information in the Central
Auditory System
• The monaural
system receives
input from one ear
only
• The binaural system
receives input from
both ears
• 1st place that
binaural information
can merge: Superior
Olivary Nuclei
“What?”
information
Nuclei of
Lateral
Lemniscus
“Where?” information
Combination & integration of sound happens “above”
the cochlea (at the level of the cortex?): “periodicity
pitch”
• Can’t be explained by pitch processing at cochlea
(neither cochlea has all harmonic information)
• Could be happening at the level of Superior Olivary
nucleus or Nuclei of Lateral Lemniscus (both have
first combination of information from both ears)
Combination & integration of sound happens “above”
the cochlea (at the level of the cortex?): “periodicity
pitch”
• However, patients with damage to a specific area
auditory cortex of the right hemisphere no longer can
process periodicity pitch
– Hence, use of “merged” information may be
processed at the level of the cortex
– “Hemispheric lateralization”: differences in what
left-right hemispheres process (i.e., left  language;
right  non-lang sound)
Cortical response to sound: Rhesus monkey calls
• Cells in the non-associative area (i.e., beyond A1 in right
temporal lobe) of the monkey cortex respond poorly to pure
tones, and respond best to “noisy” sounds (lots of fundamental
and harmonic frequencies combined)
• Some cells respond “best” to monkey calls (same left temporal
lobe area responsive to language in humans)