Inner Ear and Auditory Nerve

__**Sounds**__ are generated by displacement of *particles. Sound vary in their* __*, , and , . S*__*ounds intearct with other sound surfaces in the environment. Most sounds we care about are COMPLEX rather than pure tones.*

air

frequency, intensity and phase

Pinna captures sound and BOOTS the intensity of frequencies that are important for speech. The pinna also assists with sound localization. Sound is ___ from the pinna into the ear canal.

Funneled

__**Sound hits the and vibrates the in the middle ear.**__ The middle ear boots the intensity of the incoming sound to overcome the impedance mismatch between air (outer ear) and fluid (inner ear). It accomplishes this via the lever action of the ossicles and the surface area difference between the tympanic membrane and the oval window.

tympanic membrane

ocsicles

the __ rocks against the oval window to transfer sound into the fluid0filled inner ear.

malleus , incus , stapes

stapes

Cochlea

twisting bony shell of temporal bone

the cochlea converts the *signal from the stapes footplate to a signal and then to an signal that is passed to the auditory nerve*

mechanical, hydrochemical, electrical signal

the inner ear consists of a __ and an auditory portion.

vestibular portion

the bony cochlea houses the __ ,which contains many delicate structures that are important for hearing.

Membranous labyrinth

we can look more closely at those structures by taking a *or* __ , of the cochlea.

slice or cross-section

which of the following houses the __**organ corti?**__

scala vestibuli , scala media , scala tympani

scala media

Where is Perilmph located?

scala vestibuli , scala media , scala tympani

in the scala vestibuli and scala media

where is Endolymph located?

scala vestibuli , scala media, scala tympani

scala media

Perilymph: Is _ in potassium

is __ in sodium

low

high

Endolymph: is _ in potassium

is __ in sodium

high

low

when the hair cell are at rest what is the millivolt?

\-40 to -70 mV

the outer hair cell contains how many rows?

3-5 parallel rows of 12,000 - 15,000

the inner hair cells contains how many rows?

one row of 3,000

Stereocilia

hair like projectios located on __**top of each hair cell**__

outer hair cells are attached to the __

tectorial membrane

what is happening in the Physiology of the cochlea: Traveling wave theory?

when the spates pushes in & out on the __**oval window**__, the perilymph fluid moves back and forth , __**propagating a wave of fluid**__ through the cochlea.

How is energy converted in the Physiology of the cochlea: Traveling wave theory?

from mechanical to a __**hydrochemical signal**__

Physiology of the cochlea: Traveling wave theory

what are the components?

passive and active

passive

as __**sound waves travel in the fluid**__, the basilar membrane moves up and down , shearing (twisting) the hair cells and stereocilia.

active

the outer hair cells can also exhibit a motility of their own, which can serve to __**better “tune” the cochlea to sounds.**__

first step:

when the stereocilia tilt __ toward the tallest stereocilia, ions channels on top of the hair cells open.

laterally

second step:

__ ions (Potassium K+) from the endolymph fluid flow into the negatively charged hair cells, causing the hair cells to become MORE positively charged (depolarized)

Positive

Depolarizations(positive charged) allows ..

the hair cells to __release neurotransmitter__ and pass a s__**ignal to the auditory nerve**__

tonotopic organization

spatial representation of frequencies along the basilar membrane (BM)

maximal displacement

each area of the basilar membrane shows greater displacement for some frequencies than for others (resonance)

Low frequency tones produce maximum displacement near the __

apex or base

apex

High frequency tones produce maximum displacement near the __

apex or base

base

neurons

s specialized cell designed to conduct nerve impulses

Dendrites

smalls branches that receive nerve impulses from other nerve cells

cell body

contains the cells nucleus and connects to the dendrites

axon

transmits impulses along the neuron

neurotransmitters

chemical that cause activation or inhibition of adjacent neurons; transmitted from neuron to neuron across synapses

synapses

spaces between the terminal buttons of one neuron and the dendrites of the next neuron

the human cochlea contains about __afferent (sensory) neurons and about ___ efferent (motor) neurons__

30,000

1,800

afferent neurons

carry impulses from the cochlea (up to the brain) to the central auditory system & have their cell bodies in the spiral ganglion in the modiolus

efferent neurons

project from the superior olivary complex in the brainstem to the hair cells in the cochlea

The path of the afferent auditory nerve fibers:


1. Auditory nerve fibers form the auditory (cochlear) branch of the __ cranial nerve

VIIIth

The path of the afferent auditory nerve fibers:


2. the nerve fibers exit the cochlea and travel medially toward the modiolus, where the cell bodies group together to form the __

spiral ganglion (a nerve)

The path of the afferent auditory nerve fibers:


3. the nerve fibers pass from the modiolus through a space called the __*and attach to the brainstem at the*__ __

internal auditory canal (IAC)

Cerebellopontine angle (CPA)

internal auditory canal (IAC) also contains the vestibular portion of the __*nerve, the internal auditory artery, and fibers of the*__ ____ nerve (facial nerve)__

VIIIth

VIIth

action potential

A change in electrical potential that occurs on the surface of each neuron when it is stimulated by the hair cells.

* neurotransmitters stimulate activity in the next neuron

what does the action potential causes?

__neurotransmitters to be released, travel across the synapse__, and lock onto receptor sites at the __**dendrites of the next neuron.**__

in the action potential the neurons changes from __(resting potential) to__ _ ?

\-70mV to + 40-50 mV

Frequency coding in the auditory nerve is achieved through two mechanisms :

place coding and temporal coding

Define Place coding

the auditory nerve carries over the __**tonotopicity of cochlea**__

Place Coding

* Nerve fibers from a cylindrical bundle


* Fibers that arise from the __**basal**__ ( __ ) part of the cochlea form the __**Outer portion of the Nerve bundle**__

High or low frequency

high frequency

Place Coding

* Nerve fibers from a cylindrical bundle
* Fibers that arise from the __**Apical/apex**__ ( ___ _) part of the cochlea form the __**center of the nerve bundle**__

High or low frequency

low frequency

Define temporal coding

timing, looking to a specific timing

Part of the temporal coding

Define phase-locking:

an auditory nerve fiber fires at the same phase in each sound wave cycle (fire an action potential)

* the number of action potentials in a given time period corresponds to the frequency of the incoming auditory signal

Part of the temporal coding

Caveat:

a single neuron cannot encode frequencies higher than 1,000 Hz

What is caveat solution?

Volley Theory

Part of the temporal coding

Define Volley theory:

multiple neurons work together to encode high frequencies (> 1,000 Hz) by phase looking onto some phase & alternating their firing

How is sound intensity represented by the auditory nerve?

\
some nerve fibers code low intensities, some code moderate intensities, and some code high intensities.

* this happens because nerve fibers vary in their __**spontaneous discharge rate.**__ ( how many neurotransmitters they release at rest, when there is no sound
* different spontaneous discharge rates lead to different __**neural firing thresholds**__.

How is sound intensity represented by the auditory nerve?

Nerve fibers with __spontaneous discharge rates have__ __ neural thresholds

* These fibers code low intensities because low intensities, or relatively weak sounds, will bring the neuron above its spontaneous discharge rate

\

high

low

How is sound intensity represented by the auditory nerve?

Nerve fibers with __spontaneous discharge rates have__ _ __**neural thresholds**__

* These fibers code __**high intensities**__ because having a high neural threshold means that high intensities, or relatively strong sounds, are necessary to bring the neuron above its spontaneous firing rate

low

high

Damage to the inner ear, we expect to see what type of hearing loss?

A. Conductive

B. Sensorineural

C. Mixed

sensorineural

Otoacoustic emissions (OAEs)

the cochlea not only responds to sounds entering the ear but also GENERATES sounds of its own.

Otoacoustic emissions (OAEs)

* Generated by __
* typically absent when hearing loss > ______ - good for screening.

Outer or inner hair cell

* outer hair cell
* > 30- 40 dB HL

Spontaneous Otoacoustic emissions (OAEs):

occurs in 40-60 of healthy ears; can be detected without external stimulation

Transient-evoked Otoacoustic emissions (TEOOAEs)

a low intensity (\~30 dB SPL) signal that emanates from the cochlea 5-20 ms after presentation of a sound to the external ear.

Distortion-product Otoacoustic emissions (DPOAEs)

emissions generated when two tones of different frequencies are introduced into the ear.

Define Otoacoustic emissions (OAEs)

A test that checks part of the inner ear response to sound.

sounds given off by one small part of the cochlea when it is stimulated by soft clicking sounds. When the sound stimulates the cochlea and outer hair cell vibrate.

Speech Rollover Testing

a speech recognition test that is conducted at increasingly loud levels, which can measure whether speech becomes distorted at high intensity levels.

* This helps differentiate between a cochlear and a “retrocochlear” pathology.

Speech Rollover Testing

If the auditory nerve is damage what happens?

Increasing the intensity of a speech sound does not always improve the clarity of the sound.

When speech recognition score decreases (gets poorer) as __**the intensity level of the speech is increased,**__ this can be a sign of __*, rather than a*__ _______

* when this happens, we call it _”__”

retrocochlear pathology , cochlear pathology

“rollover”

What are some disorders of the cochlea during congenital (present at birth)?

* Genetic disorders
* infections or toxins during pregnancy

What are some disorders of the cochlea that developed after birth (acquired)?

* genetic disorders with delayed onset
* infections after birth
* ototoxicity
* sudden sensorineural hearing loss
* meniere’s disease
* head injury
* presbycusis
* noise- induced hearing loss

What are some common physical problems for a cochlea disorder?

* malformation of the shape/length of the cochlea
* missing or damaged hair cells
* missing or damaged proteins/neurotransmitter production in the inner ear
* abnormal balance of the fluids in the inner ear
* missing or damaged auditory nerve fiber connections

Prenatal causes of inner ear damage

* causes have an adverse effect on the normal development of the cochlea

Hereditary (genetic) : hearing loss can be present at birth or progressive.

\~

* 80% of profound genetic hearing impairment are autosomal recessive
* each parent carries the recessive gene, resulting in a 25% chance of the condition beign passed onto the child
* Connexin-26 mutation (affects gap junction proteins on GJB2 gene)

Name the maternal disease/virus early in pregnancy

* first trimester worst
* Rubella (German measles), Zika, syphilis, genital herpes, cytomegalovirus (CMV)

What are some syndromes during prenatal, that could experience damage to the inner ear.

Cerebral palsy

Usher

Pendred

Charcot Marie Tooth

Prenatal causes of inner ear

* often result in multiple handicaps
* Anoxia:
* Prematurity:
* Head trauma

* deprivation of oxygen
* determined by the weight of the child at birth (

Acquired (developed after birth)

Name symptoms of infections (meningitis)

* mild fever
* stiff neck
* headache with sensitivity to light/sound
* lack of appetite
* nausea and vomiting
* muscle aches

Ototoxicity

this happens when someone is on a high dose of a drug that treats cancer, infection and illness.

Ototoxicity

Treatment for cancer:

* Radiation treatment can lead to hair cell loss

• Radiation can also damage the conductive mechanism (middle ear)
* Chemotherapeutic agents – platinum-based – can lead to hair cell loss

Ototoxicity

Treatments for systemic infections:

* Strong, intravenous antibiotics, often ending in –mycin

• Streptomycin – used for tuberculosis infections, originally

• Gentamycin

• Neomycin

\

Ototoxicity

Treatments for malaria

Quinine

Ototoxicity

Salicylates:

Aspirin! Mostly reversible

Sudden idiopathic sensorineural hearing loss:

Hearing loss that may develop over the course of a few days or seemingly instantaneously

* most common in adults

* __**Often UNILATERAL**__
* Decrease in hearing thresholds of at least 30 dB over at least three octave frequencies within a 3- day period
* __**Cause usually unknown**__, but possible etiologies include autoimmune diseases, viral or other infections, rupture of the BM, vascular disorders, tumors, or other neurological disorders

Sudden idiopathic sensorineural hearing loss:

* should always be treated as a __

\~ Steroids may be attempted as treatment
* Symptoms may disappear spontaneously in some patients, whereas others may experience permanent sensorineural hearing loss

medical emergency

Meniere’s disease

Symptoms:

sudden attacks of vertigo, tinnitus, vomiting, __**unilateral hearing loss**__, sound sensitivity, aural fullness, reduction in __**word recognition scores (poor)**__

• Fluctuating HL (hearing may improve in-between attacks)

• __**Bilateral in 5-10% of cases**__

Meniere’s disease

Leading __**theory**__ is that it is caused by ___

__**endolymphatic hydrops**__, the over-secretion or under-absorption of endolymph.

Meniere’s disease

Treatment:

ensure patient is adequately hydrated; avoid alcohol, smoking, and caffeine (chocolate, soda, coffee, tea)

Meniere’s disease

Extremely debilitating:

sudden (without warning) attacks can interfere with driving, working, etc.

Presbycusis

* Hearing loss due to __**advancing age**__
* Aging leads to several changes in the __**peripheral and central auditory system**__
* May be exacerbated by environmental factors (e.g., noise exposure) or genetics

Proposed Types of Presbycusis (Schuknecht, 1993)

Sensory, Neural , Strial, Cochlear conductive/mechanical:

Noise-induced hearing loss (NIHL)

Can be caused by brief or repeated exposure to high-intensity sounds (one time or often)

Noise-induced hearing loss (NIHL)

* temporary threshold shift (TTS):
* permanent threshold shift (PTS):

* cases in which hearing thresholds improve after an initial impairment following noise exposure Permanent threshold shift (example: when at a concert, after days goes back to normal)
* irreversible elevations in hearing thresholds

Noise-induced hearing loss (NIHL)

Possible causes:

Loss of hair cells and supporting structures in the base of the cochlea and nerve degeneration; biological changes in the sensory cells, physical dislodging or hair cells during high-level stimulation, changes in cochlear blood supply through alterations in stria vascularis, loss of OHCs, rupture of Reissner’s membrane; detachment of organ of Corti from BM, or some combination

Noise-induced hearing loss (NIHL)

Noise notch

elevated threshold between __*3000 and 6000 H*__z that recover around 8000 Hz are strongly indicative of noise exposure

Disorder of the auditory nerve

__**Sensorineural hearing loss**__

The audiogram __cannot distinguish between__ cochlear (sensory) and VIIIth nerve (neural) disorders

\

Disorder of the auditory nerve

Common early symptoms:

tinnitus, high-frequency SNHL, discrepancy between amount of hearing loss and scores on speech-recognition tests (__rollover test__)

Unilateral or asymmetric SNHL is a red flag

Acoustic neuroma

• Growths (usually benign) that arise from Schwann cells on the vestibular branch of the VIIIth nerve

• Most cases occur in adults

• 9__**5% are unilateral**__

Acoustic neuroma

Possible causes: