Cochlear Physiology Flashcards
What does the cochlea do when it first receives a signal/sound?
Determine frequency, intensity, and temporal cues of signal in order to transmit it correctly to the higher auditory pathway.
What is the pathway of the acoustic signal?
Sound is found in the air, with compressions and rarefactions.
Compressions and rarefactions travel through the outer ear, which cause the TM to move.
Acoustic signal is altered to a mechanical signal by the impedance matching system of the middle ear.
The movement is translated to the oval window.
This movement is directly related to the frequency, intensity, and timing of the original signal.
What does compression cause the oval window to do? Rarefaction?
Compression causes the stapes footplate to move inward. Rarefaction causes it to move outward.
What is the Traveling Wave by George von Bekesy?
The movement of the cochlear partition, with the key player being the basilar membrane, in relation to the input arriving at the oval window.
What does the vibration of the stapes result in?
Displacement of non compressible cochlear fluids along with a compensatory displacement or bulge of the round window.
What does the pressure wave that originates at the oval window create?
Temporary pressure differences between the scala vestibuli and scala tympani- which cause displacement of the basilar membrane.
What do compression waves do to the basilar membrane? Rarefaction waves?
Compression drives the BM down, and rarefaction drives it up.
Where does the BM have greater displacement? Why?
Greater displacement longitudinally than radially, because of it’s attachments and characteristics.
In which direction does the wave travel?
Always from the base to the apex.
What is the cochlear operation of frequency analysis dependent on?
Graded width, graded stiffness, and graded mass.
Explain the graded width of the BM
The width of the BM increases from base to apex- greater at apex than at base
Explain the stiffness of the BM
The stiffness of the BM decreases from base to apex- apex is flaccid, base is stiff
Explain the graded mass of the BM
Increases in mass from base to apex- apex is more massive than the base.
As stiffness of a vibrating object increases…
Resonant frequency increases.
As mass of a vibrating object increases…
Resonant frequency decreases.
How does the traveling wave represent the frequency and intensity of the stimulus from the oval window?
Frequency- location of maximum displacement of the BM
Intensity- amount of deflection of the BM
What does tonotopic arrangement mean?
Frequency analysis
Where do different frequencies reach their maximum deflections along the BM?
High frequencies cause maximal displacement in the basal region of cochlea, low frequencies cause maximal displacement of the BM in the apical region of the cochlea. Low frequency will simulate both base and apex, but the max displacement will be in the apex.
What does the amplitude of the traveling wave look like? What does it result in?
Builds in amplitude as it moves away from base of cochlea, then decays quickly after reaching it’s maximum. Results in “envelope” of activity that is asymmetric in shape.
Explain the excitation of the OHC’s.
Cilia are embedded in the tectorial membrane- shearing action is produced between the two when the wave moves along the BM. Shearing action is greatest at the point of maximum displacement of the basilar membrane.
Explain shearing force in excitation of OHCs
Shearing force is greater than vertical force would provide on its own due to pivot points for tectorial and basilar membranes being hinged at different points.
What is the cochlear amplifier action/electromotility of OHC?
The OHC contracts when BM moves upward and expands when BM moves downward. Results in greater deflection of cilia at the point of maximum displacement.
Explain depolarization/excitation of IHCs
Cross links hold the stereocilia together, causing bundle to move in unison when shearing force is applied. Tip links course from shaft of one stereocilia to the tip of shorter stereocilia in the next row. As stereocilia bundle is deflected in direction of tallest row, tension increases in the tip link, which pulls open the mechanically gated ion channels located near the tip of the cilia.
What does a rarefaction wave result in in IHC excitation?
Will displace the BM upward and result in deflection of stereocilia bundle toward the tallest stereocilia/away from modiolus- which will cause transduction channels to open and K+ to flow into the cell, initiating depolarization. This will lead to CA 2+ channels opening which aids in repolarizing cell and releasing neurotransmitters from base of hair cells.
What neurotransmitter is released by the IHC? What does it do?
Glutamate- it diffuses across synaptic cleft, activates receptors on postsynaptic membrane of afferent nerve fibers and depolarizes fibers to generate an action potential.
What happens in hyperpolarization/inhibition of hair cells?
A compression wave, displacing the BM downward, will stretch the tectorial membrane and cilia are pulled toward the modiolus. Tip links become slack, disallowing any ion flow into the cell. Increases number of transduction channels that are closed and results in hyperpolarization of the hair cell.
What does hyperpolarization result in?
Decreases neurotransmitters from hair cell and leads to a decreased frequency of firing of the auditory nerve fibers.
What is the first system to perform analysis of an incoming signal/sound?
Cochlea
