Flashcards in Week 10 - Audition - finished Deck (17)
What is the outer portion of the ear?
The auricle and the external auditory canal/meatus
What are the 3 ossicles?
The incus, malleus and stapes
How does vibration translate to sound through the middle ear?
Vibration of the tympanic membrane travels through the ossicles & pushes the foot of the stapes into the oval window, creating a perilymph wave in the scala vestibuli.
What does the inner ear contain?
A membranous labyrinth within the boney labyrinth, and a cochlea.
What are the 3 key regions of the cochlea?
The scala vestibuli, the scala media, and the scala tympani
What does the middle ear contain?
The 3 ossicles.
What divides the scala vestibuli from the scala media
The Reissner's membrane
What divides the scala media from the scala tympani?
The basilar membrane
What sits on the basilar membrane? What is it made of?
The organ of corti sits on the basilar membrane within the scala media and is made up of the hair cells and tectorial membrane
Describe the mechanical process of audition:
Sound waves are funneled to the tympanic membrane by the auricle
Sound wave hits membrane
Compressed air causes bulging into the cochlea
Rarefied air allows for recoil of the membrane
This back & forth movement of the tympanic membrane along its pressure gradient generates a concurrent vibration in the auditory ossicles.
The footplate of the stapes binds to the cochlea at the membrane of the oval window, thus vibration of the stapes has a piston like effect on the oval window.
This creates a fluid wave in the perilymph of the scala vestibuli & the scala tympani (via the helicotremor)
At the basal end of the scala tympani is the round window
Like the oval window it is covered by an elastic membrane
As the stapes drives into the oval window, it propels the perilymph toward the apical end of the cochlea, through the helicotremor and back toward the round window
The membrane of the round window is forced to deform and stretch back into the middle ear cavity before recoiling to push the fluid back toward the scala vestibuli & oval window
Thus the fluctuations in air pressure are mimicked by fluctuations in fluid pressure within the cochlea.
This process is known as impedance matching.
What is the scala membrane filled with?
How does the perilymph created by the stapes create sound?
The perilymph wave created by the stapes will deform the basilar membrane and it is this deformation that activates the sensory afferents of the auditory pathway.
What is the hearing range of humans?
20Hz to 20,000Hz
How does the basilar membrane turn perilymph waves into electrical signals?
Gradual changes in the properties of the basilar membrane along its length makes different areas most responsive to different frequencies
When the basilar membrane deforms the stereocilia above are forced to bend
It is this bending of the stereocilia which causes them to fire changing the mechanical energy into electrical energy
It is this preferential bending of the basilar membrane and stereocilia in response to specific frequencies, that gives rise to tonotopy within the auditory system
At the base of the membrane this degree of tonotopy is enough to enable us to determine sound frequency based solely on the location of hair cells which deform most
This spatial code for frequency analysis is known as the ‘place theory’
The apex of the membrane deforms with decreased specificity and as such the place theory no longer holds here
Lower frequencies are analysed by cells which are specific to a particular phase of the wave
Thus frequency distinction at the apex of the membrane relies on the rate of change of the wave
This temporal code for frequency analysis is known as ‘phase locking’
Information about auditory analysis:
Initial auditory analysis should reveal 3 important aspects of any sound, the frequency, intensity and location of the stimulus.
For the most part the auditory system uses two codes for deciphering the exact frequency of a sound:
The first and most obvious is the spatial code known as the place theory.
The place theory simply states that the degree of tonotopy is enough to enable us to determine sound frequency based solely on the location of the hair cells that deform most. The problem with this theory being that frequencies from 200Hz and below are no longer distributed tonotopically along the basilar membrane.
The system used for frequencies below 200Hz is then our temporal code, also known as the frequency code or more commonly, phase locking.
This simply means that the hair cells bend in response to a specific phase of the wave and as such a generator potential will be produced every time that specific part of the fluid wave passes under the basilar membrane, at that point.
Thus the frequency of the original sound can be calculated. The problem with phase locking is that high frequency sounds can easily cause saturation of the hair cells and bipolar cells.
Put simply, the hair cells at the far basal end of the membrane will miss entire cycles while the bipolar neuron undergoes its refractory period.
What is the volume or intensity of sound determined by?
The volume or intensity of a sound is determined by the degree of deformity of the membrane and hair cells, this is reliant upon the amplitude of the wave. This rule holds across the entirety of the basilar membrane.