Cochlear Anatomy & Mechanics Flashcards
What does the inner ear consist of?
- Cochlea
- Semicircular canals
- Initial portion of auditory nerve
What does the inner ear do?
- Translates mechanical vibrations into neural impulses
- Provides initial “decomposition” of sound by frequency
- Balance function
What is the size of the inner ear?
Roughly the size of pea
Where is the inner ear located?
It is contained in the petrous portion of the temporal bone
What does the bony labyrinth form?
The outer shell for the cochlea, three semicircular canals, utricle, and saccule (balance organs).
What is the inside of the cochlea made out of?
Membranous labyrinth
What does the membranous labyrinth aid in?
It keeps in all the fluids needed for proper functioning
Why is the cochlea encased in bone?
- The cochlea is an extremely delicate organ that needs protection from trauma
Temporal bone fractures normally affect?
Hearing and balance
The apex of the cochlea refers to?
The top end of the spiral
The base of the cochlea refers to?
The bottom, beginning of the spiral
The beginning or base of the spiral connects?
The round and oval windows to the middle ear
What is the average cochlea length from base to apex uncoiled?
3cm, just over an inch
What are the bony structures of the inner ear?
- Modiolus
2. Osseus Spiral Lamina
What is the modiolus?
The bony center of the coiled cochlea
What nerve runs through the modiolus?
The cochlear nerve runs through the middle of modiolus.
What is the osseous spiral lamina?
The bony ridge that extends from the modiolus toward the internal structure of the cochlea.
What type of neural projection does the osseous spiral lamina have?
Neural projections from the cochlear nerve to various parts of the cochlea
What are the internal three fluid-filled cavities structure of the inner ear?
- Scala Vestibuli
- Scala Media
- Scala Tympani
How are the cavities of the internal structure of the inner ear separated?
By two membranes:
- Reissner’s & Basilar
What membranes makes up the membranous labyrinth?
- Reissner’s Membrane
2. Basilar Membrane
The Reissner’s Membrane separates
Scala vestibuli from scala media
The Basilar Membrane separates
Scala media from scala tympani
How are the fluids of the cochlea concentrated?
With charged ions
What are the charged ions of the fluids of the cochlea?
- Sodium (Na+)
2. Potassim (K+)
What creates endocochlear potential
A balance of sodium and potassium
What are the scala vestibuli & scala tympani filled with?
Perilymph
What is the perilymph low and high in?
Low in potassium and high in sodium
What is the scala media filled with?
Endolymph
What is the endolymph high and low in?
High in potassium and low in sodium
Where does the footplate of the stapes fit in?
The oval window
The footplate of the stapes transduces?
Vibrations into scala vestibuli
The round window is…
A circular membrane leading to the basal turn of the scala tympani
How does the round window vibrate?
The round window vibrates in opposite phase with the oval window
Where is the helicotrema located?
At the top of the apex
What structures are connected at the apex of the cochlea by the helicotrema?
Oval and round windows lead to the scala vestibuli and tympani
Which fluids are continuous between the two sections of the helicotrema?
Scala vestibuli and tympani (perilymph)
What causes the movement of the oval and round window movement?
The stapes push into the cochlea, displace the fluid in the scala vestibuli. The pressure wave travels up through scala vestibuli, through the helicotrema at the apex, and back down through scala tympani. Then the pressure wave pushes on the round window. Thus, the round window vibrates in opposite phase with the oval window.
What initially causes the opposite phases of the oval and round window movement?
When the stapes pulls outward on the oval window
What is the organ of corti?
Is the organ of hearing that translates mechanical vibrations in the cochlea into neural impulses
Where is the organ of corti located?
- Within the scale media
- Sits on top of the basilar membrane
- Runs the length of the cochlear spiral, from base to apex
What two types of sensory hair cells are located in the organ of corti?
OHCs & IHCs
Where are the OHCs located?
Arranged in three rows on the lateral side of the cochlea
Where are the IHCs located?
Arranged in a single row on the medial side of the cochlea
Where is the tectorial membrane located?
- Superior to the organ of Corti
- Runs parallel to the basilar membrane
T/F: The tectorial membrane touches the hairs of the hair cells?
True
How are the OHCs arranged?
In three rows
What does the end of each OHC have?
A bundle of stereocilia directly connected to the tectorial membrane
What causes the shearing of the stereocilia bundles?
Sound vibrations cause these stereocilia bundles to push against the tectorial membrane to stimulate the hair cell.
What is electromotility?
It’s when the stimulation of the OHCs causes contraction that pulls the basilar membrane closer to the tectorial membrane
What is the function of electromotility?
Provide amplification of soft sounds in the cochlea
T/F: The OHCs are attached to many nerve fibers?
F: They are not
How are the inner hair cells arranged?
In a single row
What’s at the end of the IHCs?
A bundle of stereocilia close to the tectorial membrane
When would the IHC touch the tectorial membrane?
When there’s sufficient movement of the basilar membrane
How are the IHC stimulated?
When the stereocilia are pushed against the tectorial membrane
- same as OHCs
What is mechanotransduction?
The stimulation of IHC’s that creates a pattern of electric impulses that are sent through the auditory nerve
What is the function of mechanotransduction?
It’s responsible for translating acoustic vibration into electrical signal sent to the brain
T/F: IHC’s have lots of nerve fiber connections?
T: They do because when it’s stimulated, they send the signal to the brain.
What is the cochlear physiology of the OHC & IHCs?
- OHCs amplify motion of the basilar membrane through electromotility
- IHCs send the signal to the brain through mechanotransduction
What are the OHCs ONLY responsible for? What does a complete loss of OHCs cause?
- Amplifying the vibrations
- 60dB of hearing loss
Which hair cells is a big contributor to dynamic range of the auditory system?
OHCs
What makes the IHCs so successful at send most of the neural impulses to the brain?
The bony labyrinth allows the auditory nerve to touch all points along the spiral lamina
Where does the basilar membrane run and what does it provide?
- Runs the length of the cochlea
- Structural foundation for the organ of Corti
Is the basilar membrane compliant or stiff?
Compliant
What is the function of the basilar membrane?
It moves in response to changes in pressure in the cochlear fluids
What exerts force onto the basilar membrane?
When the pressure waves travel through the cochlea
What does pressure waves in the cochlea create?
A traveling wave that ripples down the basilar membrane
How would you describe the base and apex of the basilar membrane?
Base = thing and tight
Apex = Thick & loose
How does the width of stiffness of the basilar membrane change?
Linearly from the base to the apex
Describe what happens at the base of the cochlea at the basilar membrane
- BM is thin and tight
- Base vibrates the most in response to high frequency sounds
Describe what happens at the apex of the cochlea at the basilar membrane
- BM is wide and loose
- Apex vibrates the most in response to LFs
Tonotopic organization pertains to the?
Basilar membrane
What is tonotopic organization?
Where sounds of different frequencies will maximally stimulate the basilar membrane at different locations
How are tonotopic organizations created within the cochlea?
Through resonant properties of the basilar membrane
What does rach spot of the basilar membrane have?
A “characteristic frequency” that it responds to the best
T/F: Tonotopic organization is maintained throughout the central system too.
T: Because each point of the cochlea connects to a different set of nerve fibers
What is sensory characteristics and what is it based on?
- Our auditory system having extraordinarily precise resolution for frequency and intensity
- Based on how the BM changes from base - apex
We can distinguish between …….
1kHz different pitches of pure tone
With tonotopic organization, can multiple places on the basilar membrane be stimulated?
YES! This is what happens with speech
How does anatomy helps the auditory system organize sound by frequency?
- Nerve fibers connect to each spot on the basilar membrane
2. Tonotopic organization is maintained throughout the central system
Since tonotopic organization is maintained throughout the central system, state where these nerve fibers are connected and what information they send out
- Nerve fibers at the base carry high-frequency sound information
- Nerve fibers at the apex carry low-frequency sound information
List out the process of sound traveling through the inner ear:
- Sound vibrations enter the cochlea through the movement of the stapes in the oval window.
- This causes displacement of the cochlear fluids, and a pressure wave travels through the scale vestibuli, the helicotrema, and back through scala tympani to the round window.
- The displaced fluid becomes a traveling wave on the basilar membrane that runs from base to apex
- Stimulation of the organ of Corti occurs. When the basilar membrane moves, it presses on the OHCs and this activation of OHCs facilitates stimulation of IHCs
- 1-4 leads to tonotopic organization where different frequencies vibrate at different places along the basilar membrane
What causes the traveling wave to grow on the basilar membrane?
When the frequency of vibration is close to the characteristic frequency of the point on the basilar membrane
How does the endocochlear potential support the cochlear amplifier and the role of the stria vascularis in potassium recycling?
- Since the scala media is filled with endolymph, there is high potassium and low sodium, the high concentration of potassium creates an endocochlear potential
- The tip links on the ends of the stereocilia controls the opening and closing of K+ ion channels.
- This opening and closing of channels, allows recharging of the battery to occur. Potassium is recycled back into the endolymph through a series of ion transport and pumps in the spiral ligament and stria vascularis
- Transportation of K+ and recycling requires ATP and these goes back to the scala media, which is our cochlear amplifier, and allows boots signals with low intensity at higher frequencies.
What were the findings of Schulte & Schmedit (1992) as they relate to the effects of age on the lateral wall and endocochlear potential?
- Advancing age affects the stria vascularis and the ion transport proteins associated with potassium recycling, because reduction of endocochlear potential means less K+ rushing into hair cells when channels open, which causes larger deflections of stereocilia to depolarize hair cells.
