Lecture 18 - Ear Anatomy/ Auditory Transduction Flashcards
equal loudness curve
we don’t hear all frequencies equally, so we don’t have the same threshold for all frequencies
to get an idea of how this relates to perceived magnitude you create an ELC, you start with a standard stimulus and then another stimulus and try to judge whether they’re equal
physical signal (pressure waves) collected by
pinna and resonates in the inner ear canal
Middle ear muscles can dampen….
…..the ossicles’ vibrations to protect the inner ear from
potentially damaging stimuli.
Stapes is pulled away from…..
…..the oval window to
keep from transmitting all of the pressure wave.
acoustic reflex of the ossicles
can reduce transmission of loud sounds by 20 dB, protecting the inner ear.
similar to the pupillary light reflex
main structure is the cochlea
– Fluid-filled snail-shaped
structure (35 mm long) set into
vibration by the stapes
– Sound waves are transduced (changed from physical, mechanical waves) into neural signals within the cochlea.
– Signals will be transmitted out
of the cochlea via the auditory
nerve (comprised of axons called spiral ganglion cells).
transduction takes place in the
cochlea (fluid filled)
The __________ pushes against the ___________, transmitting
atmospheric vibrations to the inner ear.
stapes, oval window
Structural organization of the
cochlea
– Hard, boney structure
– Divided into chambers: the scala vestibuli (above, where it starts) and scala tympani (below) by the cochlear partition
- the round window bulges out a bit to maintain a balanced pressure as the stapes pushes on the oval window
– Cochlear partition extends from the base (stapes end) to the apex (far end - the "top")
– Organ of Corti contained by
the cochlear partition (scala media)
organ of corti
contains all of the cells that are crucial for transduction
extends all the way through the cochlear partition
sits on top of the basilar membrane
basilar membrane
the floor of the cochlear partition
as the pressure wave goes through, it’s going to make the basilar membrane bounce (vibrate) up and down
tectorial membrane
vibrates side to side as the basilar membrane vibrates up and down
makes contact with the inner hair cells
hair cells
are doing the transduction
inner and outer
in the organ of corti in the basilar membrane
inner hair cells
most responsible to changing (transducing) the physical stimulus to neural signal
for each cochlea there are _____ inner hair cells and _____outer hair cells
3,500
12,000
cilia
how transduction takes place:
- bend with the combined movement of the basilar and tectorial membrane
- the contact triggers the hair cells to fire: creating an action potential
on the end of hair cells bending in response to movement of the organ of Corti (as the basilar membrane move up and down) against the tectorial membrane.
Basilar membrane vibrates in response to….
….sound and supports the organ of Corti
Transduction and the cilia
Movement of inner hair cell cilia in
one direction opens ion channels (influx of positive ions triggers action potential).
• This process causes a release
of neurotransmitters.
• These bind to the auditory nerve fiber, causing depolarization (action potentials)
– Movement in the other direction (of the two membranes) closes the channels (closes the tip links)
this is all INNER HAIR CELL STUFF
outer hair cell
are involved in transduction by amplifying transduction,
they themselves do not create the signal
Two ways nerve fibers signal frequency:
two theories:
- Which/where nerve fibers are responding
- Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers.
- Place Theory: what place on the spiral basilar membrane is being activated in associated with which frequencies
- How/when fibers are firing
- Rate or pattern of firing of nerve impulses.
- Temporal coding
Békésy’s place theory of hearing
PLACE THEORY
Frequency of sound is indicated by the place (of hair cells) along the cochlea that has the highest firing rate.
• Békésy determined this in two ways:
– Direct observation of the vibrational properties of the basilar membrane in cadavers.
– By doing this in different locations on the basilar membrane he built a model of the cochlea using the physical properties of the basilar membrane.
Base of the membrane (by stapes) is:
place theory
- Three to four times narrower than at the apex.
* 100 times stiffer than at the apex.
Both the model (from the creep Békésy) and direct observation showed
place theory
showed that the vibrating motion of the membrane is a traveling wave.
Envelope of the traveling wave
place theory
to best capture the frequency of the wave coming in:
– Most of the basilar membrane vibrates in response to a traveling wave, but the displacement is largest in one place.
– The envelope shows the entire displacement caused by a traveling wave and its peak (maximum).
– Hair cells at this maximum point are stimulated the strongest (than any other hair cells), leading to the nerve fibers firing the most at this location.
– Position of the peak (P) is purely a function of frequency.
Envelopes at different frequencies
show different maximum vibration
points. This indicates…..
place theory
…..the place where most hair cell activity is predicted.
Low frequencies activate toward
place theory
the apex
high frequencies activate hair cells near
place theory
the base
envelope closer to the base than the apex
If I play a pure tone at 100 Hz, it will be transduced by
__________ hair cells nearest the _________ of the cochlea.
inner, apex
Cellular recordings
[implementation level] have
supported Place Theory.
recording from one spot in the apex: play a low sound and the cell responds a lot, play a high sound and the cell doesn’t respond or responds very little
– Cochlea shows an orderly map
of frequencies along its length
• Apex responds best to low
frequencies
• Base responds best to high
frequencies
– This is called a tonotopic map.
guinea pigs!
tonotopic map
place theory
responses to characteristic frequencies that activate specific places on the cochlea
similar frequencies are represented in brain
Of or being a structural arrangement, as in the auditory pathway, such that different tone frequencies are transmitted separately along specific parts of the structure.
Physiological support for place theory: Neural frequency tuning curves
Pure tones are used to determine the threshold for specific frequencies measured at single neurons.
– Record action potentials from one cochlear neuron and play tones at different frequencies.
– Plotting thresholds for frequencies results in tuning curves.
– Frequency to which the neuron is most sensitive is the characteristic frequency.
Psychophysical support for place theory:
auditory masking experiments
– First, thresholds for a number of frequencies are determined.
– Then, an intense masking frequency is presented at the same time that the thresholds for the original frequencies are re-determined.
– The masking effect (raised
threshold) is seen at the masking
tone’s frequency and spreads to
higher frequencies more than lower ones.
– greatly affects the tone closes the the mask and the ones right above it
Masking effects spread to….
higher frequencies more than
lower ones.
The masking effect matches
the predicted …
….envelope for higher frequencies tones.
