Term
| Describe the basic physical properties of sound frequency, wavelength, amplitude and Sound Pressure Level (SPL)/"Phons" |
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Definition
1) Frequency is cycles per second (Hz) and determines pitch
2) Wavelength is distance between equivalent points in a sound wave (c/f= wavelength)
3) Amplitude is proportional to amount of pressure (P) produced by displacement of molecules
4) SPL is a 20(logarithm) of a measured pressure over a reference pressure (70 dB is normal conversation)
5) "Perceived" loudness is different from SPL, and is defined in "phons" |
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Term
| What is the basic structure of the outer/middle/ear canal? |
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Definition
1) Outer ear consists of pinna (auricle) with resonance chamber (concha) and external auditory meatus, ending at the tympanic membrane.
2) Middle ear is airway filled with ossicles (malleus, incus and stapes). The eustachian tube is found here.
3) Inner ear, or Cochlea is fluid filled chamber within temporal bone that contains cellular elements of sound transduction. |
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Term
| What are the auditory functions of the outer ear? |
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Definition
Outer ear performs the first step in the localization of sound (Band-pass filtering and Gain is adjusted by angle sound hits ear)
1) Auditory meatus acts as Band-Pass filter, only allowing certain frequency bands to reach the tympanic membrane.
2) Resonance Chamber to sum sound waves at some frequencies, yielding waves of greater amplitude (increases gain for 1-4KHz/frequency of human voice!) |
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Term
| What are the auditory functions of the middle ear? |
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Definition
1) Overcome differences in impedences (i.e. resistance to movement) of air and water (inner ear)
- Malleus (attached to tympanic membrane), Incus and Stapes (inserts into Oval window of inner ear) act as leverage system to increase sound-induced pressure changes at inner ear.
- Movement of ossicles is modulates by Tensor tympani muscle (V), which inserts on the malleus and Stapedius muscle, which inserts on the stapes (VII). Contraction of these muscles dampens movement.
2) Connected to pharynx by Eustachian tube |
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Term
| How do you distinguish between Conductive and Sensory-neural hearing loss? |
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Definition
1) Conductive hearing loss results from disruption of the balance of the ossicles in the middle ear.
- Positive Rinne test (longer at mastoid process than in ear)
2) Sensorineural damage occurs in the neural portion of the ear. |
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Term
| What is the basic anatomy of the inner ear? |
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Definition
1) Cochlea acts as a channel within temporal bone that spirals around the modiolus bone and is continuous with the osseous labyrinth surrounding the vestibular apparatus
2) consists of three fluid-filled tubules : Scala vestibuli, media and tympani.
- Scala vestibuli is connected to tympani by helicotrema (both filled with Perilymph, which is high in Na and low in K)
- Scala media is filled with endolymph (high K and low Na) and is separated from S. vestibuli by Reissner's membrane and from S. tympani by Basilar membrane. |
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Term
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Definition
This fluid (high K and low Na) is made in the Stria Vascularis within the Scala Media of the cochlea.
Remember, the S. media is separated from the S. vestibuli by Reissner's membrane and from S. tympani by the Basilar membrane, each of which are filled with Perilymph (high NA and low K) |
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Term
| How is sound transduced in the inner ear? |
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Definition
Mechanoelectrical transduction occurs in Organ of Corti, which sits on basilar membrane and is covered by ribbon-like, movable Tectorial membrane.
1) SIgnal transduction occurs in hair cells (basal parts are attached to basilar membrane, and apical processes to tectorial membrane)
- 1 inner row innervated by efferent fibers for transmission to brain
- 3 outer rows
2) Sound-evoked vibration of tympanum is transmitted to vibrations of the oval window (articulating with Stapes), which travel down the scala vestibuli, causing a deflection of the basilar membrane including the organ of Corti.
- Displacement of the organ of Corti and the basilar membrane activates hair cell stereocilia (deflection towards longest stereocilium opens mechano-gated ion channels at tips) leading to depolarization and glutamate release onto peripheral processes of VIII nerve fibers. |
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Term
| What is the origin and structure of Inner ear Hair Cells? |
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Definition
1) Originate from surface ectoderm and retain epithelial character (NOT NEURONS)
2) Most hair cells have stereocilia and a single kinocilium with tapered ends (but hair cells in the cochlea DO NOT have kinocilium)
3) Deflection of stereocilia towards longest stereocila, opening mechano-gated ion channels at tip links, which leads to depolarization, calcium influx and glutamate release
4) Deflection away from longest stereocilia closes these channels that are open at rest, hyper-polarizing the cell.
5) Form synapses with bipolar neurons whose axons form VIII |
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Term
| What does it mean for hair cells in the organ of corti to exhibit "Graded" responsiveness? |
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Definition
The stronger the deflection, the larger the membrane potential changes.
** 0.3nm deflection is sufficient (atom of gold)! Overcomes Brownian motion of hair cells (3nm) by averaging many cycles and the response of many hair cells. |
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Term
| How does deflection of hair cells open mechanosensitive channels? |
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Definition
| Neighboring stereocilia are connected by "gating springs," which tug on mechano-gated ion channels enabling FAST signal transduction. |
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Term
| Why should you be careful when prescribing Aminoglycocides? |
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Definition
| These antibiotics enter hair cells at mechano-gated ion channels and cause death and irreversible sensorineural hearing loss. |
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Term
| Explain how an "Endocochlear potential" is generated and how it regulates membrane potential fluctuations.? |
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Definition
1) Basal end of hair cells are surrounded by Perilymph in scala tympani (high Na/ low K)
2) Stereocilia of hair cells are surrounded by Endolymph in scala media (high K/ low K)
- Ionic composition is maintained by transport mechanisms in Stria Vascular (epithelium) of Scala media
3) Perilymph:Endolymph differences create electrical potential of +80mV in endolymph, which increases the membrane potential at stereocilia to 125mV
4) This negative potential causes K+ from endolymph to enter the stereocilia, opening mechanosensitive channels and leading to depolarization.
5) Opening of calcium-activated K-channels at basal part of hair cell causes efflux of K into the perilymph and a re-polarization (hyperpolarization of hair cells).
** allows rapid fluctuations in membrane potential and minimizes energy hair cells have to spend on ionic transport. |
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Term
| How do Loop diuretics modulate the Endocochlear potential (EP)? |
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Definition
1) Ethacrynic acid and Furosemide (Lasix) are blockers of Na/K/2Cl transporters, which are heavily expressed by Stria Vascularis (responsible for maintaining ionic composition of endolymph where stereocilia of hair cells insert)
Breakdown of EP leads to hearing loss that can be permanent if cells die. |
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Term
| How is Frequency Analysis accomplished by the Cochlea? |
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Definition
1) Passive physical properties of basilar membrane (cochlea sits on the basilar membrane) act as frequency analyzer (different frequencies of sound vibrate different regions of basilar membrane more strongly)
2) Basilar membrane is wider and more elastic at Apical (where low frequency sounds peak) end than at Basal end (where high frequency sounds peak). |
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Term
| What is the role of Outer hair cells in sound transduction? |
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Definition
Sharpen frequency tuning.
- OHCs are mobile cells that change their length according to their membrane potential (which is determined by hair bundle deflection)-
- They shorten during depolarization and lengthen during hyper-polarization, and are responsible for amplification of low intensities of basal membrane motion. |
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Term
| What are Otoacoustic Emissions (OAEs) and how are they utilized clinically? |
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Definition
- Cochlea acts as a "loudspeaker" and OAEs arise from outer hair cell activity.
- Measuring OAEs in children provides a measure of cochlear function.
**remember, outer hair cells amplify basilar membrane motion at low low intensities** |
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Term
| Explain the innervation pattern of the cochlea. |
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Definition
1) Cell bodies of afferent fibers arise from spiral ganglia within the body modiolus
- 95% of primary fibers synapse "convergently" (each fiber only goes to 1 hair cell, but many fibers per hair cell) on base of inner hair cells (thick myelinated type 1 fibers) and release glutamate
- other 5% are thin, unmyelinated Type II fibers that synapse on outer hair cells (each fiber innervates many cells).
2) Cell bodies of efferent fibers reside in superior olivary complex (SOC) in the brainstem (Olivocochlear System)
- efferent fibers innervate basal part of outer hair cells and use Ach to hyperpolarize and increase the length of these cells (Noise reduction in hair cell-induced damage via Gain reduction).
- few efferents that synapse on inner hair cells, make synapses on processes of afferent fibers |
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Term
| How is "gain" adjusted in the case of hair cell-induced hearing damage? |
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Definition
Olivocochlear pathway-mediated hyper-polarization of outer hair cells.
1) Efferent fibers from the SOC synapse ont he basal part of outer hair cells and release Ach
2) Ach binding leads to hyper-polarization and lengthening of outer hair cells, thereby reducing the extent of cochlear amplification via basal membrane vibration alteration. |
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Term
| How does the auditory system differ from the visual and somatic sensory systems in terms of stimulus detection and localization? |
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Definition
1) Visual and somatic systems utilize a homogeneously organized receptive surface with multiple receptor subtypes (e.g in the retina). The brain then determines the source of these stimuli via topographic connections typical of axonal pathways.
2) The auditory pathway utilizes a tonotopically organized basal membrane, where hair cells responding to high frequencies are located at the base, and those responding to low frequencies are located at the apex (extract frequency, amplitude and time).
These hair cells are activated by their respective frequencies, irrespective of the stimulus source! |
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Term
| What does it mean that afferent fibers from the spiral ganglion process a "spatial" and a "temporal" code? |
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Definition
-Sound-evoked information is carried by Type 1 fibers of CN VIII from spiral ganglian in the body modiolus to the cochlear nucleus.
1) Spatial Code- These fibers have characteristic frequencies (CF) and are organized tonotopically within the VIII nerve.
2) Temporal Code- These fibers are "phase-locked" and discharge busts of action potential "in phase" with the frequency of the stimuli. |
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Term
| How is "loudness" processed by sensory afferents of VIII? |
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Definition
1) The amplitude of a sound determines the amount of NT release from hair cells in the basilar membrane to Type 1 fibers on CN VIII in the spiral ganglion.
2) The amount of NT binding to VIII fibers determines firing rate, which codes for Loudness. |
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Term
| How are humans able to localize sounds in space, despite the fact that the structure of the basilar membrane only affords information about frequency, time and amplitude? |
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Definition
Extract Interaural time differences (ITDs) and Interaural Intensity differences (IIDs) between ear) in the MSO and LSO, respectively.
The pathway is as follows:
1) Hair cells in basal membrane respond to sound and lead to glutamate release to afferant fibers of VIII in the spiral ganglian (spatial and temporal coding).
2) VIII afferants synapse in the ipsilateral cochlear nuclei (CN), bifurcating into the ventral, intermediate and dorsal CN (CN determines frequency, stimulus onset and loudness).
3) Second order cells from CN synapse project in dorsal, intermediate and ventral acoustic striae (Trapezoid body), with many fibers crossing the midline.
4) Fibers in these striae synapse on MSO (ITD) and LSO (IID) forming SOC within the tegmentum of the mid-pons (first cells that respond to bi-aural information).
-MSO receives contralateral excitatory fibers from medial CN and ipsilateral excitatory fibers from the medial CN
- LSO receives excitatory inputs from ipsilateral CN and inhibitory inputs from contralateral CN through the MNTB (Calyx of held synapse)
5) Neurons from the SOC go on to the Central Nucleus of the Inferior Colliculus (IC) in the midbrain via the lateral lemniscus pathway (IC is also a source of descending fibers)
6) From IC, fibers go on to the ipsilateral medial geniculate nucleus of the thalamus and then on to the auditory cortex. |
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Term
| What type of information is present in the central nucleus of the ipsilateral inferior colliculus? |
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Definition
1) Binaural stimulus cues
2) highly processed information
3) Information about spatial location of sound source.
- This nucleus has received information from the SOC via the lateral lemniscus.
- The information from the SOC contains binaural auditory information that has been processed int the MSO (ITD) and LSO (IID) |
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Term
| What is the functional importance of the Calyces of Held in the MNTB? |
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Definition
Remember that sound intensity information from the contralateral Cochlear nucleus to the LSO travels via the MNTB, but the same information from the ipsilateral cochlear nucleus goes directly to the LSO.
This differences creates a synaptic delay, which is compensates for by the extremely fast transmission that occurs in the Calyces of Held. |
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Term
| Where can you find neurons with both visual and auditory receptive fields? |
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Definition
| In the deep layers of the superior colliculus in the midbrain. They project via the tectospinal tract. |
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Term
| What clinical evidence supports the role of abnormal auditory experience in developmental language disorders? |
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Definition
| Children with prolonged OM (affecting sound transduction at round window) have significantly increased risk for developing dyslexia. |
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Term
What different types of Auditory evoke potential (AEPs) exist?
How are they used clinically? |
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Definition
To detect hearing loss in children and infants.
1) Auditory Brainstem Response (ABR) occurs 1.5-15ms post stimulus and originates in 8th nerve (waves 1 and 2) and auditory brainstem (wave V; region of lateral lemniscus and IC)
2) Middle Latency Response (MLR) occurs 25-50 ms post stimulus and includes Na waves (negative following wave V from ABR) from upper brainstem and/or auditory cortex and Pa waves (positive ate 30ms) originating from the auditory cortex bilaterally. |
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Term
| What are the 2 major types of Auditory Dysfunction and how do they present? |
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Definition
Hearing Loss and Tinnitus
1) Hearing Loss
- Conductive (sounds don't reach inner ear and caused by OM)
- Sensorineural (loss of hair cells or 8th nerve fibers and caused by acoustic trauma, ototoxicity and infection/aging)
- Central (diminished hearing due to damage of central neural pathways by tumors or strokes (DOES NOT RESULT IN MONOAURAL)
2) Tinnitus
- Objective (heard by examiner using stethoscope) caused by patent eustacian tube, aneurisms, aterio-venous malformations
- Subjective (heard only by patient) may be caused by subacute damage to ear following trauma to brainstem or inner ear (refractory to treatment) or use of Aspirin |
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Term
| What are "more or less" successful approaches to treating Tinnitus? |
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Definition
1) Hearing aids,
2) Cochlear implants/electrical stimulation
3) Cognitive therapy
4) Anti-anxiety med (Xanax benzodiazepine). |
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