Hearing Flashcards
Main function of the External Ear
To gather sound energy and focus it onto tympanic membrane.
Structural components of the External Ear
External Auditory Meatus (EAM)
The Pinna
The Concha
Role of the Pinna and the Concha
Involved in elevation detection as selectively filter diff. frequencies to provide cues on elevation
How is the structure of the Pinna related to its function?
The vertically asymmetrical convolutions of the pinna allow the external ear to transmit higher frequency components from elevated sources rather than those at ear level.
Structure and Function of the External Auditory Meatus
- Open-ended tube of length 2-3 cm
- Specific length allows it to selectively boost sound pressure 30-100x at 2-5kH via passive resonance effects
- Makes humans esp. sensitive to this frequency range.
Action of tympanic membrane in response to collision with sound waves specifically directed by external ear
As the sound wave exists as a cycle of increasing and decreasing air pressure, the membrane vibrates and moves inwards and outwards matching the timing of the wave.
Main function of the middle ear
To ensure the efficient transmission of sound from the air into the fluid-filled chambers of the inner ear through amplification.
What are the three inner ear bones each called and referred to as collectively?
The ossicles: the malleus, incus and stapes
How do the ossicles respond to the movement of the tympanic membrane?
The vibrations of the tympanic membrane displaces the malleus, which in turn causes the incus and the stapes to move also.
Stapes acts like a piston, alternately pushing against and retracting from the oval window.
Define impedance
A medium’s resistance to movement.
What happens when sound travels from a low to high impedance?
Much of the acoustic energy is reflected and thus not transmitted effectively.
Define impedance matching
A process that matches low-impedance airborne sounds to the higher-impedance fluid of the inner ear.
Achieved by boosting the pressure at the tympanic membrane by 200 fold by the time it reaches the inner ear.
How does the pressure boosting necessary for impedance matching occur?
Force impinging on large-diameter TYP membrane is focused onto much smaller window of oval window. This buckling motion combined with lever action of the ossicles increases the pressure of the sound waves by the time they reach the inner ear.
Name and describe the middle ear muscles
The tensor tympani that connects to the neck of the malleus, and the stapedius that connects to the neck of the stapes
Role of the middle ear muscles
Protect the inner ear from loud noises and own vocalisation.
How do the middle ear musscles carry out their function?
Contract automatically in response to loud noises (100ms after) or self-generated vocalisations (before they occur). Counteracts the mvmt of the ossicles THF reducing amount of sound energy transmitted to cochlea.
Main function of the cochlea
Site at which energy from sonically generated sound waves is amplified and transformed into neural impulses. Also acts as mechanical frequency analyser, decomposing complex acoustic signals into component frequencies.
Length of cochlea coiled vs uncoiled
Coiled: ~10mm wide
Uncoiled: ~35mm long
Oval and round windows
Located at the basal end of the tube
What are the fluid-filled chambres on each side of the cochlear partition called?
The scala vestibuli
The scala tympani
What is the name of the chamber running within the cochlear partition?
Scala media
Describe the cochlear partition
A flexible structure that bisects the tube and supports the basilar and tympanic membranes. Is narrower and stiffer near the base & vibrates maximally for high frequency sounds near the base and at the apex for lower frequency sounds.
Define the helicotrema
The opening that joins the scala vestibuli to the scala tympani, allowing their fluid to mix.
What fluid is contained within the scala vestibuli?
Perilymph
What fluid is contained within the scala tympani
Perilymph
What fluid is contained within the scala media?
Endolymph
Which window directly leads to the scala tympani?
Round window
Which window directly leads to the scala vestibuli?
Oval window
How does the cochlear respond to the movement of the oval window?
Causes the incompressible fluids in the cochlea to vibrate and displaces them
How does the inner ear apparatus respond to the (inward) movement of the oval window?
The (inward) movement of the window displaces the perilymph inside the scala vestibuli.
As the fluid is incompressible, the vibrating fluid stretches the cochlea partition (BM) deflecting it downwards.
This in turn increases the pressure in the scala tympani which displaces the tympani perilymph.
This movement of the fluid pushes the round window out.
THF, each cycle of sound stimulus evokes a complete cycle of up and down movement of the basilar membrane.
Physical properties of the basilar membrane and the resulting effects on its movement.
Thinner at the base and grows wider as it reaches the apex.
Stiff at the base but decreases in stiffness as it reaches the apex.
The non-uniform stiffness and width of the basilar membrane means that for each tone of sound, different parts of the membrane do not oscillate in phase - some sections will be moving up whilst others are moving down etc.
Define a travelling wave
A unique moving waveform whose point of maximal displacement traces out a specific set of locations.
Define the organ of corti
The receptor organ of the inner ear. A ridge of epithelium extending along the basilar membrane that contains roughly 16,000 hair cells and is innervated by 30,000 afferent fibres. Single row of inner hair cells (3,500) and 3 rows of outer hair cells (12,000). Supported by phalangeal cells.
How are auditory stimuli with different frequencies represented by the movement of the basilar membrane?
Different auditory stimuli that have different frequencies will cause a peak in the wave at different locations along the basilar membrane.
High frequency sounds will peak nearer the base.
Lower frequency sounds will peak nearer the apex.
Basilar membrane is THF tuned to different frequencies at each point along its length.
Describe the tectorial membrane
Covers the organ of corti. Is anchored at its base by interdental cells and its distal edge forms a fragile connection with the organ of corti.
How does the movement of the travelling wave affect the movement of the basilar and tectorial membranes and what structural features allow this to occur?
The movement of the wave displaces the basilar membrane causing it to move up and down. As it runs parallel to the organ of corti and the tectorial membrane, these structures move as well. However, as the basilar and tectorial membrane are anchored at different points, they pivot about different positions of insertion. This causes a shearing motion between the two membranes.
Describe the shearing motion observed between the basilar and tectorial membranes and how this affects the hair cells
Occurs when the membrane on top (tectorial) moves more than that on the bottom (basilar). This force occurs btw the hair cell stereocilia embedded in the tectorial membrane and the hair cells themselves embedded in the basilar membrane. It is this shearing force that transduces mechanical energy into electrical energy.
