Download Auditory System Barb Rohrer (SEI614 – 2

Auditory System
Barb Rohrer (SEI614 – 2-5086)
Nature of sound
• Sounds arise from
mechanical vibration
(creating zones of compression
and rarefaction; which ripple
outwards)
• Transmitted through
gaseous, aqueous or solid
medium (vacuum?)
• Any object capable of
creating disturbance can
create sound
• 2 properties perceived by hearing
- Frequency - perceived as pitch
- Amplitude - perceived as loudness
Measures of sound intensity
• Sounds measured in decibels
(dB) (typically measured by
microphones)
• 0 dB = sound pressure at
faintest sound heard
• Increase in 20 dB is 10-fold
increase in sound pressure
and a 20-fold increase in
power (power goes with the
square of the pressure)
• Most sensitive hearing
occurs at 1-4 KHz
– Frequencies below & above
must be louder to be heard
(tuning curve)
• Above 120 dB is painful
Measures of sound intensity
• sound is white noise (a mix of
all audible frequencies)
• second sound uses ½ power as
first sound:
10 log (P2/P1) = 10 log 2 = 3 dB
• 0 dB = sound pressure at faintest
sound heard
• One decibel is close to the
Just Noticeable Difference
(JND) for sound level.
• Mechanical conduction
components:
• Outer ear
Structure of the ear
Inner ear
– Pinna - direction
sensitivity (limited)
– Ear canal - natural
resonance at 1-4 KHz;
important for speech
fricatives (amplification)
• Middle ear - overcomes
30 dB loss of energy
going from air to water
– Tympanic membrane to
ossicles (malleus,
hammer; incus, anvil;
stapes, stirrup) to oval
window membrane
– Area ratio of two
membranes (17:1)
Outer ear
-
Middle
ear
Lever action of the ossicles (1.3:1)
• Inner ear - cochlea
– Transduction of sound to neural activity
• Boney cavity in
temporal bone
Cochlea structure
– Filled with fluid
– Coiled
• divided lengthwise
by membranous tube
– basilar membrane
– cochlear duct (scala
media)
– follows cochlear
spiral
basilar membrane
• Scala vestibuli (above basilar membrane)
• Scala tympani (below basilar membrane)
– S. vestibuli & tympani filled with perilymph (like extracellular fluid; high in Na+)
– S. media filled with endolymph (like intracellular fluid; high in K+)
• Fluid movement:
– Stapes imposes vibration on scala vestibuli; transmitted across basilar membrane
to scala tympani; pressure relief by round window
Model of cochlea
• Fluid filled chamber vibrated
by stapes movement
• Divided by basilar membrane
• Receptor cells rest on basilar
membrane
• Membrane stretched out to
increase length
• Coiled to take up less space
Cochlear fluid movement
• Vibration causes a
traveling wave to
move down the
basilar membrane
– Independent of
method of stimulation
• Different frequencies
cause maximal
vibration at different
places along the
basilar membrane
Cochlea organization
• 3 tubes separated by basilar
membrane (BM)
• Organ of corti rests on BM
• Pressure diff. across s. media
cause vibration of BM
• Receptor cells (Hair cells)
rest on basilar membrane
– 1 row of inner; 3 rows of outer
• Stereocilia of hair cells
covered by stiff, gelatinous
tectorial membrane
– Afferents come from inner hair
cells (spiral nerve fibers)
– Outer hair cells regulate
frequency sensitivity (input
from superior olivary complex)
Organ of corti
Hair cell stimulation
• Vibration of basilar membrane pushes
stereocilia against tectorial membrane (TM)
• TM is stationary
• Bending of stereocilia results in
depolarization of hair cells
• Hair cells release neurotransmitter onto
afferent fiber terminals
• EMs of hair cells
– A = transmission EM
– B = scanning EM
Hair cell stimulation
• Vibration of basilar membrane
pushes stereocilia against
tectorial membrane
• Bending of stereocilia results in
depolarization of hair cells
• Cation influx depolarizes cell
(scala media contains high K + such
that K+ rather the Na+ carry current;
thus many drugs which affect kidney
also affect cochlea)
• Depol. opens voltage-gated
Ca2+ channels
• Hair cells release
neurotransmitter (glutamate) onto
afferent fiber terminals (cell
bodies in cochlear nucleus)
Mechanical properties
of basilar membrane
• Base (near oval window)
– basilar membrane: narrow, rigid
• Apex (helicotrema)
– basilar membrane: wide, loose
• Tonotopic localization: Different
frequencies cause maximal amplitude
at different places along BM
– High freq. - max vibration at base
– Low freq. - max vibration at apex
• Each afferent fiber arises from
small point on basilar membrane
cochlear implants take advantage of
this tonotopic arrangmenet
• Bone vs air conduction
(test for middle ear damage)
Noise damage to hair cells
• Many
stereocilia of
the outer and
inner hair cells
damaged
• Some hair cells
die (we only
have 16,000
hair cells)
• A = normal
B = high noise exposure
• Guinea pig cochlea - scanning EM
• Hair cells
sensitive to
high frequency
more sensitive
to noise
damage
Tonotopic representation of frequency
•
•
•
•
Cochlear afferent responses tuned to one frequency
Each frequency causes max vibration at one part of cochlea
Each frequency excites afferents from one area of cochlea
Afferents responding to each frequency project to separate areas in
ascending relay sites
• As intensity of sound is increased:
•
•
a.
b.
Firing rate of active fibers is increased
More fibers are recruited from adjacent parts of cochlea
Ascending auditory pathways
• Cochlear afferents (cochlear
component of IIX cranial nerve)
• Cochlear nucleus (brainstem)
– Ipsilateral input
• Superior olivary nuclei
– Beginning of bilateral input
•
•
•
•
Nucleus of lateral lemniscus
Inferior colliculus
Medial geniculate (thalamus)
Auditory cortex - temporal
lobe (Brodmann’s area 41 and 42)
• Tonotopically organized
• binaural interaction: intensity and time differences help localize sound
• Located on superior
temporal lobe
(Brodmann’s area 41 and 42)
• Tonotopically organized
Auditory cortex
(Cells in the same vertical
column respond to same
frequency)
• New cell properties
– Direction sensitivity
(intensity and phase delays)
(humans can discern a shift in
sound source from midline
corresponding to an
interaural delay as small as
10 µsec)
– Tone pairs
– Frequency modulation
(important for processing
speech)
QUESTIONS?
Barb Rohrer (SEI614 – 2-5086)
[email protected]