27. ear Flashcards
3 parts of outer ear?
- pinna (ear shaped part)
- auditory canal
- tympanic membrane
4 parts of middle ear?
Ossicles (mini bones):
- malleus
- incus
- stapes
–> stapes hits oval window to transmit to cochlea
- eustachian tube
2 Parts of inner ear?
- Oval window
- Cochlea
Why do we need the bones in the middle ear?
- to amplify the vibrations that come from the auditory canal
Why do we need to amplify the vibrations?
- the vibrations must be amplified in the middle ear because there is a density change from air to fluid in the inner ear
- without the amplification (approx 30 dB), everything would be really muffled
What other structure helps with the amplification?
The tympanic membrane is 15-20x larger than the oval window, so sound energy is much more concentrated when it gets to cochlea
What are the 3 chambers of the cochlea? What 2 membranes separates them?
- Vestibular canal
- Cochlear duct
- Tympanic canal
Reissner’s membrane separates Vestibular and Cochlear
Basilar membrane separates Cochlear and Tympanic
What are the 2 types of fluid that fill the cochlea? Which fluids are in which chambers?
- perilymph
–> vestibular canal
–> tympanic canal - endolymph
–> cochlear duct –> electrochemical properties that facilitate transduction
perilymph –> both the side ones
endolymph –> one important one
Basilar membrane def? What does it respond to? What does it do?
- membrane between the walls of cochlea, separating cochlear duct and tympanic canal
- thicker, narrower, and stiffer at BASE than at apex
- each section responds most strongly to different frequencies
- reflects the traveling sound waves in the perilymph
- its displacement moves the hair hairs, which starts the process of transduction
Basilar membrane:
- characteristics from base to apex?
- frq most sensitive at each point?
- memory cue?
BASE:
- thicker, narrower, stiffer
- HIGHER frq
APEX:
- thinner, wider, floppier
- LOWER frq
Memory cues?
- uptight at the start, then loosens up a bit
- guitar strings –> the tighter (stiffer) they are, the more high frequency it will be
- the high will fade over time… :)
Characteristic frequency (of basilar membrane) def?
- The frequency to which each location on the basilar membrane responds most readily
- each location has a characteristic frequency, the frq it’s most sensitive to
Displacement envelope meaning? Which side is wider for high frq? Low frq?
- the shape of the displacement of the basilar membrane from base to apex over time
- higher frq –> wider by base
- lower frq –> wider by apex
Organ of Corti:
- where is it located?
- what 3 important things does it consist of?
- within the cochlear duct, resting on the basilar membrane
Consists of:
- inner hair cells
- outer hair cells
- tectorial membrane
Inner hair cells def / purpose?
- Neurons in the organ of Corti
- responsible for auditory transduction
- connected to Type I nerve fibers
Outer hair cells def / purpose?
- Neurons in the organ of Corti
- serve to amplify and sharpen the responses of inner hair cells
- connected to Type II nerve fibers
Tectorial membrane
A membrane that lies above the hair cells in the organ of Corti
Auditory nerve def?
- Conveys signals from the hair cells in the organ of Corti to the brain
- made up of Type I and Type II auditory nerve fibers bundled together
Tip links def? On what kind of cells? What do they do?
- fibers that connect the top ends of adjacent cilia (hair cells)
- BOTH types of hair cells
- when the hair cells bend, the distance between them increases, and the tip links get really tense
- this opens up membrane channels for potassium and calcium, causing depolarization
Depolarization response in inner hair cells vs outer hair cells?
Inner:
- release NTs
- action potentials in Type I nerve fibers
- starts transductions
Outer:
- results in change of shape of proteins
- makes the cell and cilia stretch
- called motile response
Motile response def? Relationship to activity on basilar membrane?
- A response by outer hair cells that magnifies the movements of the basilar membrane, amplifying sounds and sharpening the response to particular frequencies
- fine-tunes / specifies info from basilar membrane, especially for LOW frq
What 2 ways is frequency represented on the basilar membrane?
- place coding
- temporal coding (timing)
Place coding def?
- Frequency representation based on the displacement of the basilar membrane at different locations
- knowing where the max. displacement is can tell you what the frq is
What did von Bekesy do?
- discovered the physical basis for place coding
–> ie. that the basilar membrane changes in stiffness, which determines its response to frequencies - he also determined envelopes of displacement
Characteristic frequency (of type I nerve fiber) def? Accounted for by…?
- the frequency to which the auditory nerve fiber is most sensitive
- can be accounted for by the frequency tuning of the basilar membrane
Frequency tuning curves for nerve fiber and basilar membrane. What does the graph show? What’s the important point? Why is this important?
- shows the sound level (dB SPL) needed to produce a response in that nerve / location for each frequency
–> generally higher as its farther away from characteristic frq - the lowest point is the characteristic frequency
- the 2 curves directly mirror each other, which suggests that the tuning curve for the nerve fibers can be accounted for by the tuning curve for the basilar membrane
Frequency tuning curves shape? Which direction?
- they’re ASYMMETRIC
- they extend more towards LOWER frequencies
- ie. a nerve fiber will respond to a lot of frqs lower than its characteristic frequency, but not that many that are higher than its characteristic frq
Frequency tuning curve: what shapes for lower and higher frequencies? What does this tell us?
- lower frqs have much broader curves
- higher frqs have much narrower curves
- lower frequencies need more information from the outer hair cells to specify frq
Relationship between tuning curves and masking effects?
- tuning curves are ASYMMETRIC towards LOW frqs
Therefore…
- masking effects extend MORE towards HIGHER frequencies
- a low frq masks a high frq WELL
- a high frq masks a low frq BADLY
Temporal coding def? 2 ways?
- frq representation based on firing rates of auditory nerve fibers
- firing matches frequency for some low frqs (up to a few hundred Hz)
- Phase synchronization (locking) extends the range (up to 4000-5000 Hz)
Volley principle def? For what issue?
- for phase synchronization with temporal coding for frequency
- no cell can fire action potentials at every peak, but in a population, they can match the rate of the frequency
- individual cells fire in phase with peaks of sound wave, but not at every peak
Book def:
- idea that each nerve fiber in a population of auditory nerve fibers produces action potentials in phase with the peaks in the incoming sound wave, even if not at every peak; explains how a temporal code could represent frequencies much higher than the maximum firing rate of any individual fiber
How is amplitude represented / coded?
- individual fibers are ambiguous
- info ab amp comes from which nerves and how many are responding
- look at frq tuning curves
- if you know frq, and you know which ones are responding, you can determine a RANGE of amplitudes it could be
How will a cells response to its characteristic frequency change with a change of amplitude?
- for different nerves, their response to the characteristic frq VARIES with amplitude
- if you have 2 nerves with the SAME ChFrq, but DIFFERENT thresholds and diff responses…
- they’ll have a DIFFERENT firing rate at the SAME amplitude
- so, you can know a range for the amplitude
Dynamic range def? (for a nerve fiber)
What is it’s general shape? What are the two ends of the dynamic range called?
- the range of amplitudes over which the firing rate of the fiber changes
- generally an S shape
- threshold at the beginning
–> amp. where the nerve starts to fire above baseline - SATURATION LEVEL at the end
Saturation level def?
- the amplitude where a fiber reaches its MAXIMUM firing rate
- no matter how loud the sound is, the fiber won’t fire any faster
