12 - Auditory System 1: structure and function of the Middle and Inner Ear Flashcards
Define sound?
Longitudinal waves passing through a medium.
When a tuning fork moves towards air molecules, it’s called compression.
When the tuning fork moves away from the air molecules, the space left between them is called rarefaction.
What is the function of the pinna?
Channels and filters sound to the external canal.
The external canal has resonance properties and amplifies frequencies ~3000 Hz.
What is acoustic gain and why is it important?
Amplification you get from the external ear. With the pinna and the canal, there’s a huge acoustic boost that allows you to hear high frequency sounds which is important for speech.
What is located in the inner ear?
The eardrum (tympanic membrane) makes up the border of the inner ear.
the malleus attaches to the TM, then the incus attaches to the malleus, and the stapes attaches to the incus at one end and the oval window at the other end.
What are the characteristics of the TM?
3 layer translucent membrane about 1 sq. cm vibratory surface.
Cone shaped, which is why when we look at it we can see a reflection of light.
What is the function of the middle ear?
Transmit sound pressure waves from air to fluid, but need to overcome the impedance midmatch between media because things can pass more easily through air than fluid.
How does the middle ear do impedance matching?
Area ratio, lever action, and bucking of the TM.
This allows for a total gain of the middle ear of ~31 dB.
How does the middle ear use area to overcome impedance?
The TM has a large area, while the oval window has a small area.
This difference allows a greater force to be transmitted to the oval window, resulting in a gain of ~23 dB.
How does the middle ear use lever action to overcome impedance?
Ossicular chain: the length of the malleus is longer than the process of the incus.
This allows a greater force to be generated and creates a gain of ~2dB.
How does the bucking of the TM help us combat impedance?
It allows it to transmit almost twice as much force to the malleus, resulting in a gain of ~6 dB.
What are the three chambers found in the segments of the cochlea?
Scala vestibuli, the scala media, and the scala tympani.
The scala media houses the important structures such as the organ of corti.
The _____ membrane separates the scala vestibuli from the scala media, while the ______ membrane separates the scala media from the scala tympani.
Reissner’s membrane between the vestibuli and media.
Basilar membrane between media and tympani.
Where is the organ of corti located?
It sits on the basilar membrane.
Where and what is the stria vascularis?
The blood supply for the cochlea that provides nourishment of structures and ions to maintain the charge of the scala media.
Located on the outer wall of the scala media.
Describe the structure of the basilar membrane and its function?
The basilar membrane produces a continuum of encoding frequencies via a traveling wave.
It gets wider and less stiff as you to towards the apex - this part near the apex is for low pitches.
It’s tight and short near the base and results in a higher pitch.
Describe the tonotopic organization of the cochlea?
Different frequencies maximally displace the basilar membrane at different points.
High frequencies at base and low frequencies at apex.
Describe the hair cells near the tectorial membrane on the basilar membrane?
Hair cells on either side of the .jdahsb.dkjashdlasjdn
Describe the shear force that occurs when sound-induced vibration occurs at the basilar membrane.
In the downward phase of the basilar membrane: shear force is inward.
Upward phase: shear force is outward and pushed stereocilia laterally.
What is the mechanism by which the cochlea increases the basilar membrane deflection?
There’s an active mechanism other than just the mechanical up and down of the basilar membrane.
This is caused by hair cell motility.
Outer hair cells have ____ properties. How does this work?
Motile.
Dynamic mts and microfilaments alone the membrane allow for contractile behaviors.
What is the function of the inner hair cells?
Acoustic (mechanical) enegery to electrochemical energy- ie mechanical to neural transduction.
Afferent fibers are 95% innervated by inner hair cells (5% outer), and have 20 contacts per inner hair cell.
What is the composition of the endolymph in the scala media? What about the perilymph of the scala vestibuli and scala tympani?
Endolymph is high in K+ and is at about 80 mV.
Scala tympani and vestibuli perilymph is low in K+ and have a charge of 0 mV.
What is the charge of inner hair cells?
-45 mV.
What occurs when stereocilia are deflected?
Tiplinks are pores on top of stereocilia that open and close.
When they open, K+ flow from the endolymph into the depolarizes the hair cell.
Hair cells release nts.
Which deflection phase of the basilar membrane results in depolarization?
The upward phase results in deflection of stereocilia laterally and tip links open, resulting in K+ flowing in to depolarize the cell. Vesicles dock and release nts.
During the downward phase channels close and the hair cells are hyperpolarized.
What is the normal intensity of sound that healthy people can hear on an audiogram?
20 dB or softer.
What are the types of hearing loss?
Conductive or sensorineural.
Sensorineural can be congenital, due to noise exposure/trauma, medications, or age (presbycusis).
What are ways that hearing loss can be treated?
Amplification devices that sit behind the ear, in-ear devices, in-canal devices, completely in canal, and invisible in the canal.
These can be modified to give people the frequencies that they need.
What is done when people have ear damage that’s too severe for them to be treated with amplification devices?
Cochlear implant.
These have electrodes with contacts based on where they are in the cochlea and they mimic the frequency range of the normal cochlea.
