Section 1 Flashcards
What is the angle that the pinnae project outwards at?
15-35 degrees
How big is the ear canal?
25-35mm long
5-7mm in diameter
What is the function of the OHC?
To increase fluid disturbance and add energy into the cochlea. The IHC can respond to soft sounds if the OHC disturb the fluid enough
What are OHC responsible for?
OHC are tuned for low sound thresholds (<40dB) and responsible for sharp tuning/fine frequency discrimination
What is the effect of Prestin Motility?
It increases the fluid disturbance inside scala media and increases the displacement of the BM to create a shearing force for inner hair cells
What is Prestin Motility?
When the OHC is depolarized, it activates the prestin in the cell walls and causes the cell to expand and contract, stretching at the rate the stereocilia are moving
What is Place Theory?
The way that the auditory system codes frequency corresponds to the place on the BM where it is moving/resonating the most
What is the Pars Tensa?
-The “effective area” of the tympanic membrane
-It vibrates more when sound hits it and is a better transmitter of sound
-Radial and spiral fibers create its stiffness
-Middle layer of the TM exists in Pars Tensa, but Pars Tensa exists outside of it
What is the area of the Pars Tensa?
55mm^2
What 3 things do the middle ear structures do to amplify sound?
1.) Area difference between the TM and the stapes footplate17:1 ratio of pressure (High Heel Shoe Effect) (25dB)
2.) The level action of the ossicles (2dB)
3.) The buckling action of the TM (6dB)
What are the 5 structures of the Head Related Transfer Function (HRTF)?
1.) Head
2.) Neck/Torso
3.) Pinnae
4.) Concha
5.) External Auditory Canal
What are 3 important structural/mechanical characteristics of the basilar membrane?
1.) Narrower at the base and wider at the apex
2.) Stiffer at the base and less stiff at the apex
3.) Attached differently at the base (spiral ligament) compared to the apex (osseous lamina)
What does the tensor tympani do?
-Negligible role in activating with loud sounds
-The muscle contracts and impacts the ability of the malleus to make movements to the incus and stapes
What is the acoustic reflex?
-Triggered after 100ms
-At 80dB, the stapedius muscle contracts and pulls the stapes footplate away from the oval window to create a less intense fluid disturbance
-At 140-150dB, the footplate starts to rock rather than have a piston-like movement to create uneven/less pressured fluid disturbance
What are the 3 layers of the tympanic membrane and what are they made of?
1.) Outer layer»_space; skin
2.) Middle layer»_space; connective tissue
3.) Inner layer»_space;mucus membrane
How are stereocilia shaped or grouped together on OHC?
~150 on OHC
V-shaped
3 rows (short, medium, tall)
Tip links and cross links
How are stereocilia shaped or grouped together on IHC?
~50 on IHC
Bowl shaped
1-2 rows
Where does Prestin Motility take place?
OHC
How are afferent and efferent fibers synapsed?
-Both afferent and efferent synapse directly to OHC
-Afferent fibers synapse directly to IHC
-Efferent fibers synapse to the afferent fibers in IHC
What were the 2 problems with von Bekesy’s experiments with hair cells?
1.) Limited microscopy and he had to put 90dB in to see any movement of the IHC
2.) He used cadavers and would only be able to see the cochlea without OHC.
What is a tuning curve?
-The least amount of energy to give to a cell to make it start firing
-The level of sound against the frequency of the sound to find the cell’s favorite frequency
How is an OHC shaped?
~25 microns
Cylindrical Shape
Nucleus displaced more toward the bottom
Mitochondria on cell walls and at the bottom of the cell
~150 stereocilia
How is an IHC shaped?
~35 microns
Flask/Beaker shape
Nucleus in the middle
Mitochondria spread throughout
~50 stereocilia
What is the role of the Stria Vascularis?
-The blood and metabolic supply for the cochlea
-Provides K+ and Ca2+ ions that makes the endolymph super highly charged (+80mV)
What cells hold OHC and IHC in place?
OHC: Hensen’s and Deiter’s cells
IHC: Border and Phalangeal cells
How many hair cells are in each ear?
15,000 total per ear
12,000 OHC
3,000 IHC
What is the cochlea?
A long tube that spirals up and inward toward your head
2 5/8 turns
35mm if unrolled
What is the importance of the helicotrema?
-Apex of the cochlea where the BM and Reissner’s membrane join and seals off scala media from the other 2 chambers
-Where scala vestibuli turns around and becomes scala tympani
What is the total amount of amplification from the ME and OE structures?
OE: 20dB (HRTF)
ME: 33dB (25+2+6)
Total: 53dB of amplification
What are 3 ways that sound can get to the ME?
1.) Mechanical motion of the ossicles
2.) Air moving through the ME
3.) Bone conduction
How much does the tympanic membrane tilt?
20-30 degrees
What is the Umbo?
The maximum point of concavity in the TM
What is the total area of the TM?
90mm^2
What is the fundamental challenge that must be overcome in getting sound energy from its source into the inner ear?
Impedance mismatch: sound changing media from air to fluid, which causes the sound to lose 99% of energy
What is the pars flaccida?
Looser part of the TM that is made of skin and doesn’t transmit sound very well
What is one reason why otitis media episodes are more common in children?
The Eustachian tube is more horizontal in childhood, so it is easier for bacteria to build up
What happens when disturbance of the basilar membrane causes deflection of hair cell stereocilia in the direction of the tallest row?
The shearing force occurs and the tallest row opens up K+ channels into the OHC, which makes the fluid inside more positively charged.This then changes the cell wall permeability to allow Ca2+ ions in making the fluid even more positive. This creates the action potential, and the cell depolarizes and fires through the synapse to the afferent fibers
Why do we are about the difference in electrical charge between endolymph and the fluid inside an OHC? How large is this difference?
The endolymph is highly positively charged at +80mV and the fluid inside the OHC is negatively charged at -70mV, creating a difference of 150mV. We care about this because it causes the cell to depolarize and release neurotransmitters and create the electrochemical signal to the brain
If otoacoustic emissions are absent in a particular frequency region, what does that tell us about structures in the cochlea?
It tells us that the OHC are not firing or sending an electrochemical signal and the hair cells will not react at all
What structures are affected when we administer kanamycin in the cochlea? What functions of hearing are lost
Kanamycin selectively kills outer hair cells. When the OHC die, we lose the ability of sharp tuning and fine frequency discrimination. It is harder to differentiate between sounds and you cannot hear under 40dB
What is the resting potential of an OHC?
-70mV
What is the resting potential of an IHC?
-40mV
Why do auditory nerve fibers exhibit a spontaneous firing rate?
They exhibit a spontaneous firing rate because neurotransmitter can remain in the synapse, so if enough build up from not having enough previously to fire, then that buildup can cause spontaneous firing.
What are the 2 types of afferent fibers in the cochlea? What structures do they synapse with? Which type is myelinated?
Type 1: Inner hair cells; myelinated
Type 2: Outer hair cells; unmyelinated
