Physiology of Hearing Flashcards
(38 cards)
the ‘cochlear amplifier’
The outer hair cells (OHCs) are considered to be “cochlear amplifiers.” They pull down on the tectorial membrane in response to movement from the basilar membrane and cause a greater disturbance of the fluid in the cochlea (specifically perilymph in the scala vestibuli and scala tympani). They make our frequency perception sharper because specific OHCs will pull down harder on the tectorial membrane in response to specific frequencies depending on where they are located on the basilar membrane (tonotopic organization).
cochlear frequency tuning
Cochlear frequency tuning is dependent on tonotopic organization of the basilar membrane and the movement of the outer hair cells.
active mechanism for cochlear frequency tuning
The active mechanism for cochlear frequency tuning involves the OHCs contracting and pulling down on the tectorial membrane at where the basilar membrane is moving. This improves our frequency perception and discrimination.
Bekesy’s Tuning Curve
Bekesy measured tuning curves from cadavers. His tuning curves from the basilar membrane were done in 1960, and were the first of their kind. These early tuning curves were broad and shallow, as they only demonstrated the frequency selectivity of the cochlea with the passive mechanism. Since the OHCs were dead, the frequency curves generated by Bekesy from cadavers only showed the response of the basilar membrane to sound. OHCs amplify the movement of the fluid inside of the scala media and allow for more defined frequency perception (active mechanism), therefore resulting in sharper points on a tuning curve for normal hearing.
Otoacoustic Emissions (OAEs)
Otoacoustic emissions (OAEs) are the result of the cochlear amplifiers (outer hair cells) working. The basilar membrane will move in response to any input (passive mechanism), however the OHCs work to amplify that movement (active mechanism), meaning that the basilar membrane will end up moving with more energy than the original input. This amplification generates sound—OAEs—that can be measured in two ways: distortion product OAEs, and transient evoked OAEs.
OAEs in expected levels indicate healthy function of the outer ear, middle ear, and outer hair cells.
No OAEs means that there is either a conductive issue or an issue with the outer hair cells.
A person could still have OAEs and be deaf as a result of an issue with their inner hair cells, auditory nerve, etc.
Distortion Product OAEs
Distortion product OAEs (DPOAEs) involve playing a pair of sounds into the ear canal.
- distortion products are tones generated in the cochlea that were not initially present in the input
- the sounds “echoed back” by the cochlea will be slightly distorted by virtue of the OHCs moving (nearby OHCs will affect ear other)
- DPOAEs will return in predictable patterns, and if they do not, then this is evidence for a hearing deficit
Transient Evoked OAEs
Transient evoked OAEs involve playing a click sound (a transient) into the ear canal. Transient noises are broadband, meaning that they will stimulate a large portion of the basilar membrane. The responses from the cochlea are then measured.
bone conduction
Bone conduction is the ability to hear when sound waves are transmitted through the skull and bone of the cochlea. So long as the cochlea is working, we can use bone conduction because it will cause the basilar membrane to move.
* Bone conduction can be helpful for individuals with a conductive hearing loss (i.e. tympanic membrane or ossicular chain is damaged), but not for individuals with a sensorineural hearing loss
endocochlear potential
The endocochlear potential is the resting electrical potential within the endolymph (of the scala media). The endolymph has a positive endocochlear potential due to a high concentration of K+ ions of +80 mV, which is the highest resting electrical potential in the human body.
The endocochlear potential is essential for the function of stereocilia. IHCs have an intracellular potential of -40 mV and OHCs have an intracellular potential of -60 mV.
When the stereocilia bend in the excitatory direction, the tip links open the K+ channels of the stereocilia and allow K+ ions to flood inside, and when the stereocilia bend in the inhibitory direction, the tip links close the K+ channels and reduce K+ flow into the cell.
stereocilia tip links
Stereocilia are connected via tip links, which are essentially like “trap doors” on the cell. When the stereocilia bend in the excitatory direction, the tip links open K+ channels of the stereocilia. When the stereocilia bend in the inhibitory direction, the tip links close the K+ channels of the stereocilia.
place theory (frequency encoding)
A theory of frequency encoding that relies on the idea of tonotopic organization.
The idea that frequency encoding begins where the basilar membrane distorts—that auditory neurons fire correspondingly to different frequencies.
—Essentially, we can detect frequency just based on where the basilar membrane moves.
drawbacks:
- Psychoacoustic experiments have shown that the frequencies people perceive are not always those that they are presented with (ie. do not always reflect the specific movement of the basilar membrane)
- “missing fundamental”: people will hear fundamental frequencies in a harmony even if they’re only hearing the upper parts of the harmony
- is problematic for lower frequencies because lower frequencies have less representation in the basilar membrane
frequency theory (frequency encoding)
aka temporal theory
A theory of frequency encoding that involves encoding the frequency via firing patterns; the firing patterns of neurons along the basilar membrane code frequency
drawbacks
- the maximum firing rate for neurons is 1000 Hz, meaning that we cannot code frequency directly with one neuron firing (or else we’d only be able to encode up to 1000 Hz)
volley theory (frequency encoding)
A theory of frequency encoding that involves alternating nerve firings in order to encode frequency
- based on the idea that neurons can only fire so often, so they must “trade off”
—the collective pattern of neurons firing encodes for frequency
—For example, let’s say that neurons can only fire once per millisecond. This means that, if neural firings code for frequency, neurons can only code for 1000 Hz. However, let’s say that instead of neurons firing all at the same time, multiple neurons can fire at offset intervals, such that 3 neurons fire in a staggered pattern to code for frequency. Collectively, these 3 neurons can fire 3 times per millisecond, or at 3000 Hz.
—the total response of neurons together codes for frequency
phase-locking (frequency encoding)
In tandem with volley theory, the idea that neurons will only fire during the excitatory phase of the sound stimulus (i.e. peaks) and not the inhibitory phase of the sound stimulus (i.e. rarefaction).
- “neurons tend to fire in the positive phase of the sound”
- one drawback: phase locking stops at around 5000 Hz (but humans can hear past 5000 Hz)
—above 5000 Hz, sound wave compressions and rarefactions occur in such small intervals that it is not possible for neurons to specifically encode highs and lows
recruitment (amplitude encoding)
The theory of amplitude encoding that the basilar membrane will move according to the strength of the sound signal. This results in the “recruitment” of more neurons firing in response to more of the BM moving.
- “As the signal gets stronger, more of the BM moves”
—the basilar membrane requires smaller areas to move for smaller responses, and larger areas to move for larger responses
—more neurons fire because more the BM is moving
differential sensitivity (amplitude encoding)
The theory of amplitude encoding that relies on the idea of different discharge rates for different neurons, such that some neurons are more sensitive than others; less sensitive neurons respond to stronger (i.e. louder) stimuli.
- highly sensitive neurons will fire to any sound stimuli at all, and less sensitive neurons will only fire at stronger stimuli
ampulla
A chamber at the end of a semicircular canal which contains the crista ampullaris
cupula
A gelatinous structure that is attached to the ampulla using stereocilia (crista ampullaris).
—When the ampullae move, the stereocilia move, opening tip links and allowing us to detect motion
—the stereocilia are connected to the vestibular nerve
- Has a specific density, so the cupula is naturally buoyant within the endolymph
—gravity does not affect the cupula
otoliths/otoconia
Calcium carbonate crystals that lie on top of the macula of the utricle or saccule. The otoliths/otoconia help us detect acceleration, static tilt, and gravity, via inertia.
What triggers OHCs to contract?
OHCs contract when there is movement of the basilar membrane. When OHCs contract, they pull down on the tectorial membrane, thus amplifying the motion of the basilar membrane and causing a larger disruption of the fluid inside of the scala media.
Why would OHC death reduce our frequency perception?
OHCs respond depending on the movement of the basilar membrane. Remember that the basilar membrane deforms according to the frequency of the signal. This means that OHCs in a specific part of the basilar membrane will respond more strongly when input causes that part of the BM to move; we have more defined frequency perception by virtue of the OHCs working.
With OHC death, this frequency discrimination is dampened; this doesn’t mean that a person cannot perceive frequency, but they might struggle with hearing differences in pitch and with vowel perception.
What are the elements of the ‘cochlear amplifier’?
Outer hair cells are the cochlear amplifier.
Why do we have both endolymph and perilymph, and why are they separate?
Endolymph and perilymph are fluids of the cochlea. They are separate because they have different ion concentrations:
- endolymph is in the scala media, and it has a high K+ concentration but a low Na+ concentration
- perilymph is in the scala vestibuli and scala tympani, and it has a high Na+ concentration but a low K+ concentration
The stria vascularis transfers K+ ions from the perilymph into the endolymph. The K+ ions are essential for the function of stereocilia in the scala media that are attached to the basilar membrane.
The perilymph and endolymph are separate because they have different functions, and because the endocochlear potential must be maintained.
Why would changes in the density of fluid within the semicircular canals cause disruption to balance?
The cupula is naturally buoyant in the endolymph fluid inside of the ampullae. Alcohol alters the specific gravity of the endolymph inside of the semicircular canals, such that the cupula becomes lighter than the endolymph and floats more. This gives us a sensation of movement (when there is none), thus disrupting our balance perception.
