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Nervous System III > Auditory System > Flashcards

Auditory System Flashcards

1
Q

Sounds above what SPL can result in permanent hearing loss?

A

120dB

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2
Q

What is the frequency range of human hearing?

A

2,000 - 20,000 Hz

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3
Q

Auditory threshold

A

The smallest dB sound that a subject can just detect

Different for each ear and at each frequency

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4
Q

Impedence mismatch

A

Because fluid is much more resistant to movement than air, most of the sound energy reaching an air-water interface will be reflected back (99.9%)

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5
Q

How does the middle ear alleviate impedence mismatch?

A
  1. Decreased area of the stapes footplate relative to the tympanic membrane - augments force transmitted to the inner ear
  2. Orientation of the middle ear bones confers a levering action resulting in greater force
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6
Q

Conductive hearing loss

A

Caused by degradation of mechanical transmission of sound energy through the middle ear; due to:

Filling of middle ear with fluid 
Impaction of ear canal with wax 
Otosclerosis - arthritic bone growth impeding movement of ossicles 
Malformations of ear canal 
Perforation of TM
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7
Q

Sensorineural Hearing Loss

A

Caused by damage to or loss of hair cells and/or auditory nerve fibers; due to:

Exposure to excessively loud sounds
Ototoxic drugs
Age

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8
Q

Presbycusis

A

Gradual, bilateral, high frequency sensorineural hearing loss due to age

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9
Q

What do excessively loud sounds and ototoxic antibiotics have in common?

A

Damage to outer hair cells causing sensorineural hearing loss

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10
Q

Cochlear amplifier

A

OHCs receive efferent innervation from the central auditory system; prestin, a voltage-sensitive motor protein, responds by changing the length of the hair cell which pulls the BM toward or away from the tectorial membrane thus changing the mechanical frequency selectivity of the BM;

Contributes up to 50dB of cochlear sensitivity to sound

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11
Q

Type I ANFs

A

95% of all ANFs
10-30 Type I ANFs innervate a single IHC
Myelinated

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12
Q

Type II ANFs

A

5% of all ANFs
Each Type II ANF innervates ~10 different OHCs
Not myelinated

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13
Q

What is the most common form of congenital hearing loss?

A

Mutation in gap junction protein Connexin-32, found in the stria vascularis

Decreased K+ concentration of the endolymph causes reduced driving force on K+ to enter the IHCs; diminished depolarization leads to less effective sound signal transduction

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14
Q

Otoacoustic Emissions (OAEs)

A

Spontaneous movements of the OHCs which set the BM in motion, causing the tympanic membrane to act as a loudspeaker

Lack of normal OAEs in infants can indicate sensorineural hearing loss

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15
Q

Medial Olivocochlear neurons (MOC)

A

Efferent neurons which innervate OHCs; MOCs sense the context of the sound environment (frequency and intensity) and provide feed-back control to change cochlear sensitivity via the OHCs

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16
Q

How is sound intensity encoded?

A

Rate of firing of IHCs (frequency specific)
Recruitment of neighboring IHCs and ANFs

Important for perception of loudness

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17
Q

Phase-locking

A

The property by which auditory nerve fibers fire action potentials in response to a particular phase of the sound waveform; this temporal pattern of action potentials in ANFs is used to code the “pitch” of sounds with frequencies below ~1,000Hz

Most important for low frequency sounds which cause phasic release of NT from low frequency sensitive IHCs

Important for perception of pitch

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18
Q

Interaural Time Differences (ITDs)

A

Caused by the direction-dependent differences in path length that sounds must travel to reach each ear from a source, generating different times of arrival of the sound at the two ears

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19
Q

Interaural Level Differences (ILDs)

A

For high frequency sounds with wavelengths on the order of the diameter of the head, the head creates an “acoustic shadow” for the far ear as sound waves are reflected off of the near side of the head; therefore, the sound arising at the far ear is relatively attenuated, creating a direction-dependent difference in amplitude

ILDs are small in magnitude for low frequency sounds and increase in magnitude for high frequency sounds

20
Q

Monaural spectral shape

A

Results from direction and frequency dependent reflection and diffraction of sound pressure waveforms by the pinna, causing spectral “shapes” that encode information about location (including elevation, front/behind)

21
Q

What is the tonotopic arrangement of the basilar membrane?

A

The basilar membrane is thinner, narrower, and more rigid at the base of the cochlea near the oval and round windows; this region vibrates in response to higher frequency sounds

The BM is wider and more flexible at the apex; here it vibrates in response to lower frequency sounds

22
Q

Where are inner hair cells located?

A

In the Organ of Corti, which sits in the scala media, on top of the basilar membrane

23
Q

How do IHCs participate in signal transduction?

A

The apical surface of hair cells project an array of stereocilia of various lengths; bending of the stereocilia toward the longest fiber opens a voltage insensitive, non-specific cation channel which allows influx of depolarizing K+ from the endolymph into the IHC

24
Q

Endolymph

A

K+-rich fluid bathing the IHC stereocilia within the scala media

25
Q

How is endolymph produced?

A

Endolymph is produced by the stria vascularis, a specialized epithelium found within the scala media that actively pumps K+ into the the fluid

26
Q

What is the endocochlear potential?

A

The endolymphatic fluid of the scala media has a potential of +80mV relative to the perilymph

27
Q

Perilymph

A

Na+ rich / K+ poor fluid bathing the basal end of the hair cell within the scala vestibuli and scala tympani

28
Q

What is the total driving force on K+ to enter the IHC?

A

Endocochlear potential (+80mV) + membrane potential (-50mV) = - 130mV

29
Q

How is auditory information relayed from IHC to brainstem?

A

Depolarization of the hair cell allows influx of Ca2+ through VSCSs, triggering glutamate release onto afferent auditory nerve fibers whose cell bodies are located in the spiral ganglion; centrally, spiral ganglion cells send axons to end in the cochlear nucleus of the brainstem

30
Q

What is the characteristic frequency of an ANF

A

The frequency at which a single ANF fires the maximum number of action potentials per second; this is the frequency to which that fiber is maximally sensitive

31
Q

Auditory neuropathy

A

Sensorineural hearing disorder caused by dysfunction in the neural transmission from IHC to ANFs

Patients present with normal OAEs and normal tone thresholes but ANFs have lost the ability to phase lock, causing deficits in discriminating or understanding speech

32
Q

Where do the auditory fibers of CN VIII end?

A

ANFs with cell bodies in the spiral ganglion project centrally into the brainstem and bifurcate; one branch innervates the ventral cochlear nucleus (VCN) and the other branch innervates the dorsal cochlear nucleus (DCN) of the inferior cerebellar peduncle

33
Q

What is the ascending pathway of auditory information from the cochlear nucleus?

A

Some axons from cells in the cochlear nucleus of the inferior cerebellar peduncle cross midline to the opposite side of the brain; fibers travel via the dorsal acoustic stria (from DCN) and trapezoid body (from VCN); these tracts regroup as the lateral lemniscus an scend to the inferior colliculus of the midbrain

*This is why unilateral lesions rostral to the cochlear nuclei do not produce unilateral leafness, while lesions caudal to and including the cochlear nucleus do produce unilateral deafness

34
Q

Where does ascending auditory information travel from the inferior colliculus?

A

From the inferior colliculus, afferent projections travel to the ipsilateral medial geniculate nucleus of the thalamus, which sends projections to the primary auditory cortex (A1) in the superior temporal gyrus via the auditory radiations

35
Q

How are ITDs processed?

A

ANFs from both ears project to the anteroventral cochlear nucleus (AVCN) on both sides, carrying information in the form of phase-locked neural responses; the AVCN projects to the medial superior olive (MSO), which acts like a ‘coincidence detector,’ responding maximally only when APs from the AVNC cells from the left and right ears arrive simultaneously at the MSO; MSO neurons send excitatory afferents to the ipsilateral inferior colliculus

MSO neurons are sensitive to low frequency sounds <1.5kHz where phase-locking is best, and also to sounds from the contralateral ear where the ITD compensates for the longer delay line pathway

36
Q

Where are ILDs processed?

A

LSO neurons receive afferent inputs from the ipsilateral ear (via ANF synapses with AVCN cells, which project to the ipsilateral LSO) and from the contralateral ear (via AVCN fibers which project across the midline to the medial nucleus of the trapezoid body [MNTB] which project inhibitory input onto the LSO

LSO neurons compute the difference between excitatory (ipsilateral) and inhibitory (contralateral) input intensity by rate code

LSO neurons then send projections to the contralateral DNLL and inferior colliculus; this achieves contralateral representation of space

37
Q

Calyx of Held

A

The synapse between the AVCN cells and MNTB neurons

This is the largest synapse in the CNS, resulting in very fast and secure synaptic transmission

38
Q

Role of the inferior colliculus in central auditory processing

A

Neural representations of cues to location reconverge at the contralateral IC; therefore, unilateral lesions in the IC or above do not cause unilateral deafness but do result in deficits in sound source localization for sources contralateral to the lesion

39
Q

Where does ascending auditory information travel from the IC?

A

The IC projects ipsilaterally to to the medial geniculate body of the thalamus, which projects to the primary auditory cortex (A1) deep in the lateral sulcus

40
Q

What is the mechanism of aminoglycoside ototoxicity?

A

Direct toxicity - AGs are taken up by hair cells, where they are converted into ROS that damage DNA and mitochondria, leading to cytochrome c release and apoptosis

41
Q

Ototoxic agents - examples

A 
Aminoglycosides 
Cancer agents (cis-platinum)
Macrolides
Aspirin
Loop diuretics
Hydrocodone
Viagra
42
Q

Endolymphatic hydrops

A

Expansion of the endolymphatic compartment of the inner ear causing swelling of the scala media

Associated with recurrent episodes of vertigo, sensorineural hearing loss, tinnitus, aural fullness (Meneire’s disease)

43
Q

What is the breakdown of causes of congenital hearing loss?

A

50% environmental (CMV) / idiopathic

50% genetic - of these, 70% are non-syndromic with the most common cause being a connexin-26 mutation

44
Q

Neural hearing loss

A

Presents as asymmetry of hearing between the two ears; caused by 8th nerve tumors, auditory neuropathy, multiple sclerosis

Treated by hearing aids or cochlear implants

45
Q

Vestibular schwannoma

A

Tumor of CN VIII; accounts for 6% of all intracranial tumors

Characterized by asymmetric hearing loss on audiogram with asymmetric word recognition scores

Nervous System III (9 decks)

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