Hearing Science: Inner Ear
it decreases (depolarization)
After potassium enters IHCs, what happens to the intracellular potential?
the frequency that elicits the strongest displacement for that cochlear location
As the frequency of a sound is changed, the measurement location in the basilar membrane will reveal a different part of the traveling wave. What is the "best frequency?"
low to moderate only
At what intensities does OHC motility influence basilar membrane motion?
high (doesn't contribute as much at low intensities)
At what intensities does do passive cochlear mechanics dominate the basilar membrane?
high-intensity
At what intensities is the cochlea dominated by passive cochlear mechanics?
- thickest and stiffest at base - flaccid at apex
At what parts of the cochlea is the basilar membrane thickest and stiffest; where is it most flaccid?
- basilar membrane filters: broad - auditory nerve fibers: tuned
Between basilar membrane filters and auditory nerve fibers, which are broad and which are very finely tuned?
- function is nearly linear - gain is about 60 dB
For sounds played at the best frequency, the cochlear input/output function is non-linear. What is the function and gain at low levels?
- function has a shallow slope (compression) - gain gradually decreases
For sounds played at the best frequency, the cochlear input/output function is non-linear. What is the function and gain at mid-to-high levels?
the hair cell at the cuticular plate
Hair cells contain stereocilia (tiny hairs). What are they attached to?
set up to convert changes in their surface area into changes in length while maintaining cylindrical shape
How are OHCs set up for OHC motility?
as level increases, cochlear filters get broader
How are cochlear filters dependent on sound pressure level?
lateral-links: side to side, row to row, tip to side (tip links)
How are stereocilia connected?
- OHCs expand and contract - IHCs release neurotransmitters
How do hair cells react in response to influx of potassium ions following shearing of stereocilia?
it increases the wave's displacement
How does OHC motility affect a traveling wave?
it amplifies the displacement of the basilar membrane
How does OHC motility contribute to high hearing sensitivity and sharp filtering?
the motion causes an increase in the intensity of sound at the cochlea
How does OHC motion act as a cochlear amplifier?
the healthier the cochlea, the sharper the cochlear filters (and vice versa)
How does the health of the cochlea influence cochlear filters?
it changes mechanical energy from the middle ear into neural impulses
How does the middle ear act as a transducer?
low-intensity sounds are amplified more than high-intensity sounds
How is the cochlea a nonlinear amplifier?
- 1 row of IHCs - 3 rows of OHCs
How many rows of inner and outer hair cells does the organ of Corti contain?
for IHCs: 120 mV [+80mV -(-40mV)] for OHCs: 150 mV [+80mV -(-70mV)]
How much lower is the resting potential of hair cells than endolymph?
the base
In a traveling wave, where do high-frequency sounds vibrate within the cochlea?
the apex
In a traveling wave, where do low-frequency sounds vibrate within the cochlea?
- the shallower response growth of basilar membrane with increasing input level (compression) is a characteristic of normal auditory system - therefore, compression is lost when the cochlea is compromised
What is the relationship between cochlear compression and cochlear health?
-40 mV
What is the resting potential of IHCs compared to perilymph?
-70 mV
What is the resting potential of OHCs compared to perilymph?
- contiguous with vestibule at cochlea's base - connects to scala tympani at helicotrema located at cochlea's apex
What is the scala vestibuli contiguous with, and what does it connect to?
correspondence between stimulation frequency and place along the cochlea
What is tonotopic organization?
- lose sharply tuned component of traveling wave - lose sensitivity (good hearing thresholds)
What occurs in active cochlear mechanics when outer hair cells are damaged?
they release neurotransmitters to the auditory nerve fibers
What occurs when IHC sterocilia are sheared?
- neurotransmitters are released into the synaptic cleft - neurotransmitters then bind to specialized receptor sites on auditory nerve endings
What occurs when intracellular potential decreases (depolarization)?
movement of the stapes
What results in a traveling wave in the cochlea?
- mass - stiffness - damping
What three factors influence basilar membrane motion?
- hair cells - support cells
What two types of cells does the organ of Corti contain?
they open and close the ion channels
When the tectorial membrane slides back and forth across the tops of stereocilia, causing them to bend, what happens to tip-links?
traveling-wave energy "looks for" path of least resistance for a given frequency and excites low-impedance location
Describe basilar membrane tuning.
when two sounds are played to the cochlea, the sound eliciting the greatest excitation may suppress the excitation of the other sound
Describe cochlear suppression.
- stapes pushes on oval window, displacing scala media - this causes round window to bulge - when stapes pulls on oval window, process is repeated in opposite direction
Describe the movement of the scala media.
the wave begins at the base of the cochlea and gradually increases in amplitude to its maximum then abruptly decays
Describe the movement of the traveling wave when the oval window is stimulated at a specific frequency.
the resting potential (+80 mV) between endolymph in the scala media and perilymph in scala vestibuli and scala tympani
Electric potentials in the human body are called bioelectric potentials. Resting potentials are bioelectric potentials that exist normally in the human body even without stimulation. What is, then, endocochlear potential?
- frequency components that may not exist in the input signal (distortion products) - non-linear response growth - suppression of a weak sound by a strong sound - level dependent filter shapes
Nonlinear amplifiers create nonlinear distortions. What are possible examples of these distortions?
- the higher percentages mean the cochlea is healthy (99.8%) - lower percentages mean the cochlea is more compromised (0% is practically dead)
On a cochlear input/output graph, what do the low and high percentages indicate in terms of the cochlea's health?
they also move up and down
The basilar membrane moves up and down in response to vibrations from the stapes. When this occurs, what happens to the hair cells on the basilar membrane?
each frequency of stimulation produces maximal displacement at a unique location along the basilar membrane
The basilar membrane varies in stiffness and mass, so each patch of the membrane functions as a resonator. What does this mean?
tectorial membrane
Rows of stereocilia on each hair cell increase in height (tallest away from modiolus. Where do the tallest tips of the OHC stereocilia project into?
neural transduction (conversion of signal from hydromechanical form to electrical form
Shearing the stereocilia produces different results in OHCs vs IHCs. What happens through shearing of IHC stereocilia?
OHC motility is activated, allowing amplification of the basilar membrane response
Shearing the stereocilia produces different results in OHCs vs IHCs. What happens through shearing of OHC stereocilia?
endolymph --> hair cells --> perilymph --> stria vascularis --> endolymph --> hair cells ...
What is the path of potassium ions through the scalae?
it slides back and forth across the tops of the stereocilia aka "shearing motion" which makes the stereocilia bend
The attachment point of the basilar membrane is farther from the modiolus than the attachment point of the tectorial membrane. As the basilar membrane moves up and down, what does the tectorial membrane do?
created by ions in perilymph, endolymph, and hair cells
The force between two opposing charges is an electric potential. How are electric potentials created in the ear?
separating positive and negative ions (charged particles)
The inner ear requires a source of electricity. How is this created?
filters measured from the basilar membrane (act as band-pass filters)
What are "cochlear filters?"
sounds observed in the outer ear as a result of vibrations generated in the inner ear (tympanic membrane creates sound waves in ear canal)
What are otoacoustic emissions?
- Deiters cells - inner phalangeal cells - Hensen cells - Claudius cells - Boettcher cells
What are the five support cells within the organ of Corti?
- scala vestibuli - scala tympani - scala media
What are the three spiral passages of the cochlea?
- widest: base - narrowest: apex
What are the widest and narrowest parts of the cochlea called?
protein called prestin located in OHC wall
What attributes to OHC motility?
- broader cochlear filters - hearing sensitivity suffers
What can result when the cochlea is compromised (not in a healthy state)?
bony labyrinth of cochlea
What causes the cochlea to form a spiral tunnel?
properties of the basilar membrane: - graded stiffness - graded mass - graded width
What determines passive cochlear mechanics?
traveling wave within basilar membrane motion; observed cochlear filters with broad bandwidths relative to the auditory nerve
What did Georg von Bekesy measure, and what did he observe?
an active source of mechanical energy in the cochlea
What do active cochlear mechanics result from?
a broadly-tuned component of the traveling wave
What do passive cochlear mechanics produce?
reticular lamina
What forms the upper surface of the organ of Corti?
- compression - distortion products - suppression - level-dependent tuning
What four non-linear processes are a result of OHC motility?
they immediately diffuse into the perilymph
What happens to the potassium ions after entering the hair cell?
a door/channel is opened, causing potassium ions to enter the hair cell
What happens when stereocilia tip links are stretched?
potassium (K^+) ions rush from the endolymph into the hair cell
What happens when tip-links open the ion channels during shearing of the stereocilia?
- the deflection of stereocilia which cause OHCs to contract and expand - results in amplification of vibration of basilar membrane in that region, creates motion of fluids toward IHCs
What is OHC electromotility, and what results from it?
the cochlea's physiological state
What is a factor that strongly influences the cochlea's response?
- a wave motion perpendicular to the direction of the moving wave - in-and-out motion of stapes creates traveling wave
What is a traveling wave, and how does it apply to movement of the scala media?
- encoding sounds over a wide range of levels (from threshold of hearing to loudest tolerable sound) - deficits in hearing impaired listeners in common auditory tasks (e.g. speech understanding in noise)
What is cochlear compression (or lack thereof) responsible for?
a change in the length of the OHC in response to changes in the electrical potential of the cell
What is electromotility (or just motility)?
modiolus
What is the bony core of the cochlea called?
- a series of tunnels coursing through solid bone - contains cochlea for hearing; semicircular canals and vestibule for balance - also contains membranous labyrinth
What is the bony labyrinth, and what does it contain?
perilymph (similar to extracellular fluid)
What is the fluid that fills the bony labyrinth?
endolymph (similar to intracellular fluid)
What is the fluid that fills the membranous labyrinth?
- separates scala vestibuli from scala media - attaches to spiral limbus and spiral ligament
What is the function of Reissner's membrane?
- amplify traveling wave as it passes through - produces sharp tuning
What is the function of active cochlear mechanics, and what do they produce?
sensory cells that amplify basilar membrane motion (outer hair cells); transduce basilar membrane motion into electrochemical impulses (inner hair cells)
What is the function of hair cells?
- separates scala tympani from scala media - attaches to spiral lamina and spiral ligament
What is the function of the basilar membrane?
OHC electromotility
What is the key to active cochlear mechanics?
spiral lamina
What is the name of the bony shelf that projects from the modiolus (bony core of cochlea)?
outer hair cells
Where do active cochlear mechanics take place?
middle ear wall where the round window is located
Where does the scala tympani end at?
at the cochlear location that resonates best to the frequency of the stimulus
Where does the traveling wave on the basilar membrane create a crest?
in the bony labyrinth* within petrous portion of the temporal bone
Where is the inner ear located?
on the basilar membrane
Where is the organ of Corti located?
outside edge of scala media
Where is the stria vascularis located?
- attached to spiral limbus - extends out over the organ of Corti
Where is the tectorial membrane located?
- scala vestibuli and scala tympani contain perilymph - scala media contains endolymph
Which of the cochlea's three spiral passages are filled with perilymph, and which are filled with endolymph?
those higher than the best frequency
Which type of frequency will reveal the apical portion of a traveling wave during basilar membrane measurement?
those lower than the best frequency
Which type of frequency will reveal the basal portion of a traveling wave during basilar membrane measurement?
- OHC stereocilia are sheared - potassium enters OHCs and activates OHC motility
While traveling to the cochlear location of the best frequency, what occurs that eventually activates OHC motility?
